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Fifteen Years of Verticillium Wilt of Lettuce in America's Salad Bowl: A Tale of Immigration, Subjugation, and Abatement
Abstract
The Near East and the Mediterranean basin are hypothesized to be the center of origin of Lactuca sativa, the cultivated lettuce. Currently, lettuce is ubiquitously cultivated as a leafy salad vegetable. Globally, the United States ranks second in lettuce production, and the central coast of California produces nearly half of that total. Until the mid-1990s, lettuce was considered resistant to Verticillium wilt, caused by Verticillium dahliae. However, in 1994, several fields on a farm in southern Santa Cruz County, Pajaro Valley, reported a loss of the entire lettuce crop to an unknown disease. Verticillium wilt was dismissed as the potential causal agent of the disease, although it was the only pathogen isolated from infected plants and V. dahliae microsclerotia were recovered from soil samples. In 1995, V. dahliae was isolated from infected plants from the same fields and Koch's postulates were completed, proving V. dahliae to be the causal agent of wilt on lettuce. Since 1995, an increasing number of lettuce fields have shown varying levels of Verticillium wilt incidence. Following the initial appearance of Verticillium wilt, it wasn't until 1999 that it was first observed on lettuce in the neighboring Salinas Valley (Monterey County), where the majority of lettuce production in the United States occurs. Verticillium wilt was first observed to the north of Salinas City in 1999; and in 2003, the disease appeared on the south end of Salinas City. By 2006, the disease was recorded at the southern end of the Salinas Valley, more than 100 km south of Salinas in San Ardo, CA. While disease reports were confined to a small number of fields, by 2009 and 2010 most disease foci coalesced, and at the preparation of this manuscript, fields in an ~50 km stretch of the prime lettuce production area had developed Verticillium wilt.
... Verticillium wilt, caused by the soilborne fungus Verticillium dahliae, is an economically significant disease of many highvalue crops, including lettuce, tomato, artichoke, strawberry, and spinach in the Salinas and Pajaro Valleys of coastal Central California (Atallah et al. 2011;Subbarao et al. 2017) and many other hosts worldwide (Inderbitzin and Subbarao 2014;Klosterman et al. 2009;Ligoxigakis et al. 2002;Pegg and Brady 2002). Isolates of V. dahliae are classified as either race 1 or race 2 on tomato and lettuce (Fradin et al. 2009;Maruthachalam et al. 2010;Vallad et al. 2006). ...
... Overall, the Verticillium wilt management in many crops or cropping systems has primarily relied on an integrated approach of cultural, biological, and chemical methods and host resistance (Atallah et al. 2011;Hayes et al. 2011aHayes et al. , 2011bPuri et al. 2021;Sandoya et al. 2017;Subbarao et al. 2017). However, factors such as the broader host range and long-lived microsclerotia of V. dahliae in soil (Atallah et al. 2011), ineffective cultural practices (Inderbitzin et al. 2018), prohibition of methyl bromide with the unavailability of equally effective fumigant alternatives (Duniway 2002;Meadows 2013), and unavailability of resistant cultivars (Hayes et al. 2007Sandoya et al. 2017Sandoya et al. , 2021 have stymied the successful management of this devastating disease. ...
... Overall, the Verticillium wilt management in many crops or cropping systems has primarily relied on an integrated approach of cultural, biological, and chemical methods and host resistance (Atallah et al. 2011;Hayes et al. 2011aHayes et al. , 2011bPuri et al. 2021;Sandoya et al. 2017;Subbarao et al. 2017). However, factors such as the broader host range and long-lived microsclerotia of V. dahliae in soil (Atallah et al. 2011), ineffective cultural practices (Inderbitzin et al. 2018), prohibition of methyl bromide with the unavailability of equally effective fumigant alternatives (Duniway 2002;Meadows 2013), and unavailability of resistant cultivars (Hayes et al. 2007Sandoya et al. 2017Sandoya et al. , 2021 have stymied the successful management of this devastating disease. ...
Verticillium wilt is one of the most devastating soilborne diseases in lettuce and the use of host resistance is the most optimal choice for its management. This study focused on identifying and mapping the genetic loci for resistance against Verticillium dahliae race 3 in a mapping population of 200 F2:3 families developed from a cross between moderately resistant red-leaf lettuce 'Sentry' and susceptible green leaf lettuce 'La Brillante'. The population was genotyped using the tunable Genotyping by Sequencing (tGBS) approach. An ultra-high-density genetic linkage map containing 34,838 single nucleotide polymorphism (SNP) markers grouped into 1,734 bins was constructed using F2 progeny and a sliding window approach. Three quantitative trait loci (QTLs) for resistance to V. dahliae race 3 were located on linkage groups (LGs) LG 2 (qVR3-2.1), and LG 4 (qVR3-4.1 and qVR3-4.2). Each of these QTLs explained up to ~10% of the total phenotypic variation for the trait. At each locus, the resistance alleles were derived from cultivar ‘Sentry’ that is partially resistant to the pathogen. Additional resistance loci to the disease are expected in this population, and transgressive segregation indicates that some of those loci could originate from the susceptible cultivar ‘La Brillante’. Candidate genes linked to some of the QTLs for V. dahliae race 3 resistance, plant development, and leaf color were identified. The QTLs for resistance identified in Sentry may diversify the resistance gene pool and provide an alternative tool to manage a newly emerged V. dahliae race 3.
... Prior to 1995, lettuce was believed to be immune. By 2010, more than 150 fields were known to be infected with Verticillium wilt (Atallah et al. 2011), amounting to more than 4,000 acres. As not all the fields that were infected by 2010 were known at the time Atallah et al. (2011) was published, the number of fields affected by 2010 is actually even higher, numbering over 175 fields (Subbarao 2011). ...
... By 2010, more than 150 fields were known to be infected with Verticillium wilt (Atallah et al. 2011), amounting to more than 4,000 acres. As not all the fields that were infected by 2010 were known at the time Atallah et al. (2011) was published, the number of fields affected by 2010 is actually even higher, numbering over 175 fields (Subbarao 2011). Although growers have resisted reporting the extent of the disease since 2010, it is likely that the number of affected acres has increased since then. ...
... A second way in which Verticillium wilt is introduced to the soil is via infested lettuce seeds. However, studies of commercial lettuce seed lots from around the world show that fewer than 18% tested positive for V. dahliae and, of those, the maximum incidence of infection was less than 5% (Atallah et al. 2011). These relatively low levels do not cause Verticillium wilt in lettuce at an epidemic level. ...
Verticillium dahliae is a soilborne fungus that is introduced to the soil via infested spinach seeds and that causes lettuce to be afflicted with Verticillium wilt. This disease has spread rapidly through the Salinas Valley, the prime lettuce production region of California. Verticillium wilt can be prevented or controlled by the grower by fumigating, planting broccoli, or not planting spinach. Because these control options require long-term investment for future gain, renters might not take the steps needed to control Verticillium wilt. Verticillium wilt can also be prevented or controlled by a spinach seed company through testing and cleaning the spinach seeds. However, seed companies are unwilling to test or clean spinach seeds, as they are not affected by this disease. We discuss our research on the externalities that arise with renters, and between seed companies and growers, due to Verticillium wilt. These externalities have important implications for the management of Verticillium wilt in particular, and for the management of diseases in agriculture in general.
... Even though lettuce has been grown in California since the early 1900s and Verticillium dahliae, a pathogen with a broad host range, was present in coastal California for most of this time (Atallah et al. 2011), lettuce had remained immune to Verticillium wilt caused by this pathogen. Remarkably, other crops grown in rotation with lettuce were highly susceptible to Verticillium wilt and the pathogen was present in these agricultural soils. ...
... Since then, the disease has spread rapidly through the Salinas Valley. By 2010, more than 150 fields with Verticillium wilt on lettuce were identified (Atallah et al. 2011), amounting to more than 1,620 ha. Although growers have resisted reporting the extent of the disease since 2010, it is likely that the number of affected production fields and hectares have expanded since. ...
... From a sample of 54 lettuce fields in which Verticillium wilt occurred in coastal California, the level of disease incidence in the field was assessed and soil from these fields assayed for the density of microsclerotia (Atallah et al. 2011). As shown in Figure 3, the relationship between inoculum density (microsclerotia per gram of soil) and disease incidence (percentage of infected plants) is highly nonlinear. ...
Plant pathogens migrate to new regions through human activities such as trade, where they may establish themselves and cause disease on agriculturally important crops. Verticillium wilt of lettuce, caused by Verticillium dahliae, is a soilborne fungus that was introduced to coastal California via infested spinach seeds. It has caused significant losses for lettuce growers. Once introduced, Verticillium wilt could be managed by fumigating with methyl bromide and chloropicrin, but this option is no longer available. Growers can also manage the disease by planting broccoli or not planting spinach. These control options require long-term investments for future gain. Verticillium wilt can also be prevented or controlled by testing and providing spinach seeds with little or no V. dahliae infestation. However, seed companies have been reluctant to test or clean spinach seeds, as spinach crops are not affected by Verticillium wilt. Thus, available control options are affected by externalities. Renters and other producers with short time horizons will not undertake long-term investments and seed companies do not take into account the effect of their decision not to test on lettuce producers. We review the literature on the economics of managing crop disease; discuss the economics of managing Verticillium wilt; and review the recent research on the externalities that arise with short-term growers, and between seed companies and growers due to Verticillium wilt. An externality arises whenever the actions of one individual or firm affects the payoffs to another individual or firm not involved in a specific transaction. These externalities have important implications for the management of Verticillium wilt and, more broadly, for the management of migratory pathogens and the diseases they cause in agriculture in general. This review of interest to policy-makers, the producers, marketers, seed companies, and researchers.
... are amongst the most devastating and challenging diseases to manage in agricultural production worldwide (European Food Safety Authority Panel on Plant Health, 2014). These diseases can reach high incidences and cause yield losses of 50% or more in high-value crops such as artichoke, cotton, lettuce, olive, potato and strawberry (Friebertshauser & DeVay, 1982;Cirulli et al., 2010;Johnson & Dung, 2010;Atallah et al., 2011;Jiménez-Díaz et al., 2012). V. dahliae is a vascular-colonizing, soil-and seed-borne, mitosporic Ascomycete that is found worldwide (du Toit et al., 2005;Atallah et al., 2011;Inderbitzin et al., 2011;Göre et al., 2014) (www.mycobank.org), ...
... These diseases can reach high incidences and cause yield losses of 50% or more in high-value crops such as artichoke, cotton, lettuce, olive, potato and strawberry (Friebertshauser & DeVay, 1982;Cirulli et al., 2010;Johnson & Dung, 2010;Atallah et al., 2011;Jiménez-Díaz et al., 2012). V. dahliae is a vascular-colonizing, soil-and seed-borne, mitosporic Ascomycete that is found worldwide (du Toit et al., 2005;Atallah et al., 2011;Inderbitzin et al., 2011;Göre et al., 2014) (www.mycobank.org), and has one of the broadest host ranges of any fungal plant pathogen (European Food Safety Authority Panel on Plant Health, 2014). ...
... The second type of pathogenic variation in V. dahliae is the presence of two pathogenic races. Race 1 is defined by the presence of the effector gene, Ave1, which confers avirulence on hosts that carry the resistance gene Ve1 or its homolog Vr1 in resistant cultivars of tomato or lettuce, respectively (Kawchuk et al., 2001;Hayes et al., 2011;de Jonge et al., 2012). These resistance genes are conserved across many plant species, including other members of the Solanaceae (Song et al., 2016). ...
Understanding pathogenic variation in plant pathogen populations is key for the development and use of host resistance for managing Verticillium wilt diseases. A highly virulent defoliating (D) pathotype in Verticillium dahliae has previously been shown to occur only in one clonal lineage (lineage 1A). By contrast, no clear association has yet been shown for race 1 with clonal lineages. Race 1 carries the effector gene Ave1 and is avirulent on hosts that carry resistance gene Ve1 or its homologs. The hypothesis that race 1 arose once in a single clonal lineage was tested, which might be expected if V. dahliae acquired Ave1 by horizontal gene transfer from plants, as hypothesized previously (de Jonge et al., 2012). In a diverse sample of 195 V. dahliae isolates from nine clonal lineages, all race 1 isolates were present only in lineage 2A. Conversely, all lineage 2A isolates displayed the race 1 phenotype. Moreover, 900-bp nucleotide sequences from Ave1 were identical among 27 lineage 2A isolates and identical to sequences from other V. dahliae race 1 isolates in GenBank. The finding of race 1 in a single clonal lineage, with identical Ave1 sequences, is consistent with the hypothesis that race 1 arose once in V. dahliae. Molecular markers and virulence assays also confirmed the well-established finding that the D pathotype is found only in lineage 1A. Pathogenicity assays indicated that cotton and olive isolates of the D pathotype (lineage 1A) were highly virulent on cotton and olive, but had low virulence on tomato. This article is protected by copyright. All rights reserved.
... Soil steaming disinfection kills pests and soilborne pathogens without the toxic side effects because water, without chemicals or other imputs, is turned into steam and applied to the soil at high temperatures (~70-90°C) (Katan, 2000;Fennimore and Goodhue, 2016;Arancibia, 2020). Recently, soil steaming for plant disease management has increased in demand and provides a promising management approach in agriculture (Atallah et al., 2011;Mihajlovićet al., 2017). Steam is used to kill plant pathogens mainly in indoor settings, although the method is growing traction for use in high tunnel farming and is in the process of being tested for field crops (Schweigkofler et al., 2014;Arancibia, 2020). ...
... Due to its capacity to parasitize F. solani, we hypothesized that T. harzianum would decrease the abundance of F. solani regardless of when it was applied. The usefulness of steam in managing soil pathogens has been validated through other research studies, (Atallah et al., 2011;Schweigkofler et al., 2014;Mihajlovićet al., 2017), but the impacts on the microbial community remain unclear. Our work provides insight into how growers and land managers can best use beneficial fungi such as T. harzianum post-steaming. ...
Soil disinfection using high temperatures via steam is a promising approach to manage plant pathogens, pests, and weeds. Soil steaming is a viable option for growers who are moving away from dependence on chemical soil fumigants, especially in plant nursery or high tunnel environments. However, there are few studies that investigate how soil steaming causes substantial disturbance to the soil by killing both target pathogens and other soil biota. Steaming treatments also change the trajectory of the soil microbiome as it reassembles over time. Growers are interested in the health of soils after using steam-disinfection, especially if a virulent pathogen colonizes the soil and then flourishes in a situation where there are very few microbes to suppress its growth. Should recruitment of a virulent pathogen occur in the soil, this could have devasting effects on seed germination, seedling establishment and survival. Beneficial microbes are often used to prevent the colonization of plant pathogens, especially after a soil-steaming event. Here, we experimentally test how soil fungal communities assemble after steaming disinfection. We introduce to steam-treated soil Fusarium solani, an important fungal pathogen of soybean and Trichoderma harzianum, a known beneficial fungus used for soilborne pathogen suppression. Results show that F. solani significantly affects the relative abundance and diversity of the soil fungal microbiome, however, T. harzianum does not mitigate the amount of F. solani in the steam treated soil. Within the T. harzianum microbial addition, the soil fungal communities were similar to the control (steaming only). This result suggests inoculating the soil with T. harzianum does not drastically alter the assembly trajectory of the soil fungal microbiome. Other soil amendments such as a combination of Trichoderma spp. or other genera could suppress F. solani growth and shift soil microbiome composition and function post-steaming, however, more experimental research is needed.
... Further, economic losses of over 50% have been reported in cotton (Friebertshauser and DeVay 1982), lettuce (Atallah et al. 2011), olive (Jimenez-Diaz et al. 2012, potato (Rowe and Powelson 2002), and strawberries (Wilhelm and Paulus 1980). ...
... (Bhat and Subbaro 1999). Crop losses due to Verticillium wilt have been diverse and abundant-economic losses of over 50% have been found in cotton (Friebertshauser and DeVay 1982), lettuce (Atallah et al. 2011), olive (Jimenez-Diaz et al. 2012, potato (Rowe and Powelson 2002), and strawberry (Wilhelm and Paulus 1980). ...
Strawberry (Fragaria x ananassa, Duch.) is an important crop in California, with more than 35,000 acres planted in 2018 resulting in a farm gate value of $3.1 billion. In 2020, California strawberry production accounted for more than 85% of national strawberry production and faces serious threats to production due to various soil-borne diseases. One such disease, Verticillium wilt, is caused by the fungal pathogen Verticillium dahliae and is commonly found in temperate zones around the world where strawberries are grown. Due to the phase-out of efficacious fumigants like methyl bromide, alternative disease management methods have become necessary to alleviate threats to production. Alternative fumigation practices such as crop termination have recently been investigated, and the integration of crop termination with bed fumigation and host resistance can play an integral role in control of Verticillium wilt. A field trial was established at California Polytechnic State University, San Luis Obispo to examine the efficacy of integrative management solutions for control of Verticillium wilt of strawberry in a naturally infested field. The efficacy of sequential fumigation applications of crop termination and bed fumigation was examined. Further, the integration of a resistant cultivar was also implemented in hopes of further decreasing plant mortality and increasing yield. Different fumigant products such as metam potassium, metam sodium, and chloropicrin were used to assess their performance in different fumigation applications. Metam potassium and metam sodium were used for crop termination. When used for crop termination, both products delivered significant reduction in soil inoculum density and adequate crop injury. Metam potassium, metam sodium, and chloropicrin were used for bed fumigation. All products reduced soil inoculum density. Lower plant mortality and higher yield resulted from sequential applications of crop termination and bed fumigation, with average plant mortality for non-treated control plots and sequentially fumigated plots being 67.2% and 24.1%, respectively. There were no significant increases in yield for plots bed fumigated and sequentially crop terminated and bed fumigated, but significant increases in yield for all plots treated versus the non-treated plots were found. The integration of a moderately resistant cultivar Valiant after the fumigation series showed lower mortality and higher yield versus a susceptible cultivar Seascape. A two-year study was also conducted in order to evaluate host resistance to Verticillium wilt in 74 cultivars and elite breeding lines from five strawberry breeding programs. Genotypes were established in a field naturally infested with V. dahliae on the campus farm at California Polytechnic State University, San Luis Obispo. All five breeding programs had a wide range of susceptibility to Verticillium wilt, ranging from 1.5% to 100% mortality for both years of the trial. Twenty-three cultivars and elite breeding lines were common to both years of the trial; of these, five cultivars showed vastly different results between the two years. For example, ‘Monterey’ showed 78.8% mortality in 2021 and 11.5% mortality in 2022. This demonstrates the importance of evaluating host resistance over multiple years under different environmental conditions and field locations.
... The disease is caused by species of Verticillium (Inderbitzin et al. 2011), of which Verticillium dahliae has the widest host range and geographic distribution, but other species are also important (Inderbitzin and Subbarao 2014). Verticillium wilt causes heavy losses of high-value crops, including cotton (Friebertshauser and DeVay 1982), lettuce (Atallah et al. 2011;Subbarao et al. 1997), potato (Rowe and Powelson 2002), and strawberry (Wilhelm and Paulus 1980). Once established, V. dahliae is difficult to eliminate due to microsclerotia, which are heavily melanized resting structures that can lay dormant in the soil for more than 10 years (Wilhelm 1955). ...
... In the Pajaro and Salinas Valleys of coastal California, Verticillium wilt seriously affects the production of the two most economically important crops, strawberry and lettuce (Anonymous 2016a,b). Disease management has relied principally upon soil fumigation (Atallah et al. 2011;Wilhelm and Paulus 1980). However, the most effective fumigant, methyl bromide, is no longer available and other fumigants face increasing regulatory and public scrutiny due to human health and environmental concerns (Duniway 2002;White 2013). ...
Two naturally infested Verticillium wilt conducive soils from the Salinas Valley of coastal California were amended with disease-suppressive broccoli residue and/or crab meal amendments, and changes to the soil prokaryote community were monitored using Illumina sequencing of a 16S rRNA gene library generated from 160 bulk soil samples. The experiment was run in a greenhouse twice with eggplant as the Verticillium wilt susceptible host. Disease suppression, plant height, soil microsclerotia density and soil chitinase activity were assessed at the conclusion of each experiment. In high microsclerotia density soil, all amendments significantly reduced Verticillium wilt severity and microsclerotia density, and increased soil chitinase activity. Plant height was increased only in the broccoli-containing treatments. A total of 8790 error-corrected sequence variants representing 1,917,893 different sequences were included in the analyses. The treatments had a significant impact on the soil microbiome community structure, but measures of alpha diversity did not vary between treatments. Community structure correlated with disease score, plant height, microsclerotia density and soil chitinase activity, suggesting that the prokaryote community may impact the disease-related response variables or vice versa. Similarly, the abundance of 107 sequence variants correlated with disease-related response variables, which included variants from genera with known antagonists of filamentous fungal plant pathogens, such as Pseudomonas and Streptomyces. Overall, genera with anti-fungal antagonists were more abundant in amended soils than unamended soils, and constituted up to 8.9% of all sequences in broccoli+crabmeal amended soil. This study demonstrates that substrate-mediated shifts in soil prokaryote communities are associated with the transition of Verticillium wilt conducive soils to Verticillium wilt suppressive soils, and suggests that soils likely harbor numerous additional antagonists of fungal plant pathogens that contribute to the biological suppression of plant disease.
... Verticillium wilts caused by Verticillium dahliae are amongst the most devastating and challenging diseases to manage in agricultural production worldwide (European Food Safety Authority Panel on Plant Health, 2014). These diseases can reach high incidences and cause yield losses of 50% or more in high-value crops such as artichoke, cotton, lettuce, olive, potato and strawberry (Friebertshauser & DeVay, 1982;Cirulli et al., 2010;Johnson & Dung, 2010;Atallah et al., 2011;Jim enez-D ıaz et al., 2012). Verticillium dahliae is a vascular-colonizing, soiland seedborne, mitosporic ascomycete that is found worldwide (du Toit et al., 2005;Atallah et al., 2011;Inderbitzin et al., 2011;G€ ore et al., 2014;www.mycobank.org), ...
... These diseases can reach high incidences and cause yield losses of 50% or more in high-value crops such as artichoke, cotton, lettuce, olive, potato and strawberry (Friebertshauser & DeVay, 1982;Cirulli et al., 2010;Johnson & Dung, 2010;Atallah et al., 2011;Jim enez-D ıaz et al., 2012). Verticillium dahliae is a vascular-colonizing, soiland seedborne, mitosporic ascomycete that is found worldwide (du Toit et al., 2005;Atallah et al., 2011;Inderbitzin et al., 2011;G€ ore et al., 2014;www.mycobank.org), and has one of the broadest host ranges of any fungal plant pathogen (European Food Safety Authority Panel on Plant Health, 2014). ...
Understanding pathogenic variation in plant pathogen populations is key for the development and use of host resistance for managing verticillium wilt diseases. A highly virulent defoliating (D) pathotype in Verticillium dahliae has previously been shown to occur only in one clonal lineage (lineage 1A). By contrast, no clear association has yet been shown for race 1 with clonal lineages. Race 1 carries the effector gene Ave1 and is avirulent on hosts that carry resistance gene Ve1 or its homologues. The hypothesis tested was that race 1 arose once in a single clonal lineage, which might be expected if V. dahliae acquired Ave1 by horizontal gene transfer from plants, as hypothesized previously. In a diverse sample of 195 V. dahliae isolates from nine clonal lineages, all race 1 isolates were present only in lineage 2A. Conversely, all lineage 2A isolates displayed the race 1 phenotype. Moreover, 900-bp nucleotide sequences from Ave1 were identical among 27 lineage 2A isolates and identical to sequences from other V. dahliae race 1 isolates in Gen-Bank. The finding of race 1 in a single clonal lineage, with identical Ave1 sequences, is consistent with the hypothesis that race 1 arose once in V. dahliae. Molecular markers and virulence assays also confirmed the well-established finding that the D pathotype is found only in lineage 1A. Pathogenicity assays indicated that cotton and olive isolates of the D pathotype (lineage 1A) were highly virulent on cotton and olive, but had low virulence on tomato.
... Verticillium wilts caused by Verticillium dahliae are amongst the most devastating and challenging diseases to manage in agricultural production worldwide (European Food Safety Authority Panel on Plant Health, 2014). These diseases can reach high incidences and cause yield losses of 50% or more in high-value crops such as artichoke, cotton, lettuce, olive, potato and strawberry (Friebertshauser & DeVay, 1982;Cirulli et al., 2010;Johnson & Dung, 2010;Atallah et al., 2011;Jim enez-D ıaz et al., 2012). Verticillium dahliae is a vascular-colonizing, soil-and seedborne, mitosporic ascomycete that is found worldwide (du Toit et al., 2005;Atallah et al., 2011;Inderbitzin et al., 2011;G€ ore et al., 2014;www.mycobank.org), ...
... These diseases can reach high incidences and cause yield losses of 50% or more in high-value crops such as artichoke, cotton, lettuce, olive, potato and strawberry (Friebertshauser & DeVay, 1982;Cirulli et al., 2010;Johnson & Dung, 2010;Atallah et al., 2011;Jim enez-D ıaz et al., 2012). Verticillium dahliae is a vascular-colonizing, soil-and seedborne, mitosporic ascomycete that is found worldwide (du Toit et al., 2005;Atallah et al., 2011;Inderbitzin et al., 2011;G€ ore et al., 2014;www.mycobank.org), and has one of the broadest host ranges of any fungal plant pathogen (European Food Safety Authority Panel on Plant Health, 2014). ...
Understanding pathogenic variation in plant pathogen populations is key for the development and use of host resistance for managing Verticillium wilt diseases. A highly virulent defoliating (D) pathotype in Verticillium dahliae has previously been shown to occur only in one clonal lineage (lineage 1A). By contrast, no clear association has yet been shown for race 1 with clonal lineages. Race 1 carries the effector gene Ave1 and is avirulent on hosts that carry resistance gene Ve1 or its homologs. The hypothesis that race 1 arose once in a single clonal lineage was tested, which might be expected if V. dahliae acquired Ave1 by horizontal gene transfer from plants, as hypothesized previously (de Jonge et al., 2012). In a diverse sample of 195 V. dahliae isolates from nine clonal lineages, all race 1 isolates were present only in lineage 2A. Conversely, all lineage 2A isolates displayed the race 1 phenotype. Moreover, 900-bp nucleotide sequences from Ave1 were identical among 27 lineage 2A isolates and identical to sequences from other V. dahliae race 1 isolates in GenBank. The finding of race 1 in a single clonal lineage, with identical Ave1 sequences, is consistent with the hypothesis that race 1 arose once in V. dahliae. Molecular markers and virulence assays also confirmed the well-established finding that the D pathotype is found only in lineage 1A. Pathogenicity assays indicated that cotton and olive isolates of the D pathotype (lineage 1A) were highly virulent on cotton and olive, but had low virulence on tomato.
... V erticillium dahliae is a widespread, soilborne fungus that causes vascular wilt on over 200 dicotyledonous plant species, resulting in billions of dollars in agricultural losses annually (1)(2)(3). At the conclusion of its infection cycle, V. dahliae forms microsclerotia, darkly pigmented resting structures (4), on senescent and dead host tissue. ...
Deciphering the gene-for-gene relationships during host-pathogen interactions is the basis of modern plant resistance breeding. In the Verticillium dahliae -tomato pathosystem, two races (races 1 and 2) and their corresponding avirulence ( Avr ) genes have been identified, but strains that lacked these two Avr genes exist in nature.
... Verticillium dahliae is a destructive soilborne fungal pathogen that causes vascular wilt in more than 400 plant species, including lettuce, strawberry, tomato, eggplant, and other economically important crops (Atallah et al. 2011;Berlanger and Powelson 2000;Pegg and Brady 2002;Subbarao et al. 2017). Despite the broad host range, many isolates display differential virulence on some of these hosts . ...
Verticillium dahliae, the soilborne fungal pathogen, causes vascular wilt on many economically important crops, resulting in significant yield losses. V. klebahnii (isolate PD659) and V. isaacii (isolate PD660), two related species that cause few or no symptoms in some hosts, were evaluated as potential biocontrol agents (BCAs) in eggplant, lettuce, and tomato by pre-, post-, and coinoculation with a virulent race 1 isolate of V. dahliae (VdLs16). Initial studies demonstrated that the biocontrol efficacy of both BCAs was similar to reference BCA Talaromyces flavus (NRRL15936) across all hosts (α = 0.05). Subsequent experiments with PD659 against V. dahliae isolate Sm113 from eggplant, VdLs16 and VdLs17 isolates from lettuce, and Le1811 isolate from tomato demonstrated a significant biocontrol efficacy in eggplant and tomato but not in lettuce (at 95% confidence interval), suggesting host-dependent effectiveness of V. klebahnii. Confocal microscopy using green fluorescent protein-tagged tomato V. dahliae isolate Le1811 indicated delayed xylem colonization or lack of pathogen progression into the vascular system in a host-dependent manner on BCA-treated plants. Quantitative analyses of the expression of defense-related genes PR1a, PR5, acidic extracellular b-1,3-glucanase (GlucA), basic intracellular b-1,3-glucanase (GlucB), acidic extracellular chitinase (Chi3), basic intracellular chitinase (Chi9), and cysteine proteases (cysProreases) in tomato in the presence or absence of PD659 suggested an elevated expression of defense-related genes in compatible interaction of V. dahliae–tomato cultivar Early Pak. V. klebahnii (PD659) may delay the entry of V. dahliae by competing for space or nutrients during the initial stages of root colonization.
[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .
... Management practices today still aim to prevent the introduction of V. dahliae to a field, reduce the amount of initial inoculum in the soil, primarily through soil fumigation, or to reduce the impact of infection through resistant cultivars (Johnson & Dung, 2010;Taylor et al., 2005;Wheeler et al., 2012). Soil fumigation can reduce the density of microsclerotia in soil (Atallah et al., 2011;Duniway, 2002;Stromberger et al., 2005) but currently available soil fumigants such as 1,3-dichloropropene + chloropicrin (Telone C-35, Corteva Agriscience) and metam sodium (Vapam HL, AMVAC Chemical Corp.) are not always effective for controlling V. dahliae (Davis et al., 2008b;Duniway, 2002;Klosterman et al., 2009;Woodward et al., 2011). Although resistant cultivars are available for a limited number of crops, including tomato (Fradin et al., 2009), potato (Jansky & Miller, 2010;Jansky & Rouse, 2000;Li et al., 2019), and cotton (Bolek et al., 2005;Li et al., 2017), there is no known genetic resistance against V. dahliae in watermelon. ...
Vegetable grafting for disease management was first used successfully when watermelon grafted onto a Cucurbita moschata rootstock overcame Fusarium wilt. Interspecific grafting has since been used effectively to mitigate several soilborne pathogens in a variety of solanaceous and cucurbitaceous cropping systems. Verticillium wilt caused by Verticillium dahliae is a significant disease in watermelon crops and is difficult to manage. Current management practices, including crop rotation, soil fumigation, and host resistance, are insufficient due to the ability of microsclerotia to persist in absence of a host, lack of efficacy of soil fumigants, and limited availability of resistant cultivars. Watermelon grafted onto commercial cucurbit rootstocks have increased tolerance to Verticillium wilt, although no cucurbit rootstocks are known to be completely resistant. Verticillium wilt incidence decreased on grafted plants grown in artificially and naturally infested soils, while scion health and growth as well as rootstock root mass and vigour increased. Commonly used rootstocks are Lagenaria siceraria , C. moschata , and C. maxima × C . moschata ; of these, only C . maxima × C . moschata ‘Tetsukabuto’ reduced severity of Verticillium wilt across several scion cultivars, locations, years, and soil densities of V . dahliae . Although studies on Verticillium wilt resistance of grafted watermelon are few, their combined results suggest the threshold of V . dahliae soil density for watermelon may be around 5–12 cfu/g. This review summarizes available information on Verticillium wilt of watermelon and effects of different rootstock × scion combinations, assisting growers and breeding programmes in decisions to adopt watermelon grafting for management of Verticillium wilt.
... The disease is especially severe on lettuce (Lactuca sativa L.), commercially the most valuable leafy vegetable in the United States, with a revenue of approximately $3.5 billion in 2019 (California Department of Food and Agriculture 2019; U.S. Department of Agriculture 2019). Verticillium wilt often results in near-total yield loss on lettuce in the Salinas Valley of California, which produces approximately 50% of the lettuce consumed in the United States (Atallah et al. 2011). ...
Lettuce (Lactuca sativa L.) is one of the most economically important vegetables in the USA with approximately 50% of the domestic production concentrated in the Salinas Valley of California. Verticillium wilt, caused by races 1 and 2 of the fungal pathogen Verticillium dahliae Kleb., poses a major threat to lettuce production in this area. While resistance governed by a single dominant gene against race 1 has previously been identified and is currently being incorporated into commercial cultivars, identification of resistance against race 2 has been challenging and no lines with complete resistance have been identified. In this study, we screened germplasm for resistance and investigated the genetics of partial resistance against race 2 using three mapping populations derived from crosses involving Lactuca sativa × L. sativa and L. serriola × L. sativa. The inheritance of resistance in Lactuca species against race 2 is complex but a common quantitative trait locus (QTL) on linkage group 6, designated qVERT6.1 (quantitative Verticillium dahliae resistance on LG 6, first QTL), was detected in multiple populations. Additional race 2 resistance QTLs located in several linkage groups were detected in individual populations and environments. Because resistance in lettuce against race 2 is polygenic with a large genotype by environment interaction, breeding programs to incorporate these resistance genes should be aware of this complexity as they implement strategies to control race 2.
... Of the different species, Verticillium albo-atrum (Reinke & Berthold) and V. dahliae (Kleb) are the two most agriculturally burdensome species, both having the capacity to inhabit the soil and survive for years (Goldberg, 2010;Klosterman et al., 2009). These species are causing chronic economic problems in the production of crops such as peppers (Vasileva & Todorova, 2020), olive (Jiménez-Díaz et al., 2011;López-Escudero & Mercado-Blanco, 2011), cotton (Friebertshauser, 1982), lettuce (Atallah et al., 2011;Carroll et al., 2017) and potato (Rowe & Powelson, 2002). Verticillium wilt is a serious threat to chilli production across the globe (Gurung et al., 2015). ...
Chilli (Capsicum annum L.) constitutes the most important ingredient of the Bhutanese daily cuisine. Although widely cultivated, there is a need for increase in production indicated by huge imports. Further, chilli production in the country is affected by wilts which results in huge yield losses, besides other factors. With Bhutan aiming for Organic Bhutan, there is a need to identify or develop biocontrol agents (BCAs) for wilt management.
This study was experimented under field condition in Verticillium infested soil at Bajo (Wangduephodrang, West-central Bhutan). We used Trichoderma viride and Trichoderma harzianum commercial products (Indian manufactured). The Trichoderma species were mass multiplied as fermented rice bran mixture formulations and water diluted solutions for application. We assessed the efficacy of Trichoderma species and its application methods against Chilli Verticillium wilt. Both formulations of Trichoderma application enhanced survival rate, growth, and yield of chilli. The fermented rice bran mixture propagated formulation outperformed diluted solution formulations. For reasonable control of Verticillium wilt, Trichoderma application should be applied starting nursery and to field.
... Clear pathogen genotype by host interactions have not been observed from isolates in this clade (Cockerton et al., 2019;Fan et al., 2018;Zeise & von Tiedemann, 2002). VCG4B isolates are frequently recovered from diseased strawberry plants (Bhat & Subbarao, 1999;Zeise & von Tiedemann, 2002), especially in coastal California (Atallah, Hayes, & Subbarao, 2011). The species-specific sequence for the Vd9602 isolate was deposited in GenBank (MN813047). ...
Verticillium wilt, a soil‐borne disease caused by the fungal pathogen Verticillium dahliae, threatens strawberry (Fragaria × ananassa) production worldwide. The development of resistant cultivars has been a persistent challenge, in part because the genetics of resistance is complex. The heritability of resistance and genetic gains in breeding for resistance to this pathogen have not been well documented. To elucidate the genetics, assess long‐term genetic gains, and estimate the accuracy of genomic selection for resistance to Verticillium wilt, we analyzed a genetically diverse population of elite and exotic germplasm accessions (n = 984), including 245 cultivars developed since 1854. We observed a full range of phenotypes, from highly susceptible to highly resistant: < 3% were classified as highly resistant, whereas > 50% were classified as moderately to highly susceptible. Broad‐sense heritability estimates ranged from 0.70–0.76, whereas narrow‐sense genomic heritability estimates ranged from 0.33–0.45. We found that genetic gains in breeding for resistance to Verticillium wilt have been negative over the last 165 years (mean resistance has decreased over time). We identified several highly resistant accessions that might harbor favorable alleles that are either rare or non‐existent in modern populations. We did not observe the segregation of large‐effect loci. The accuracy of genomic predictions ranged from 0.38–0.53 among years and whole‐genome regression methods. We show that genomic selection has promise for increasing genetic gains and accelerating the development of resistant cultivars in strawberry by shortening selection cycles and enabling selection in early developmental stages without phenotyping.
... Verticillium dahliae is a soilborne fungus that causes vascular wilt diseases on more than 200 plant species, including many agriculturally important crops as well as ornamental, horticultural and woody plants (Pegg & Brady, 2002;Fradin & Thomma, 2006;Klosterman et al., 2009). New host species susceptible to Verticillium wilt are continually identified (Atallah et al., 2011), such as smoke tree (Cotinus coggygria) . The disease has resulted in high mortality in stands of smoke trees in China . ...
The fungus Verticillium dahliae causes vascular wilt disease on hundreds of plant species. Homologs of the bZIP transcription factor Atf1 are required for virulence in most pathogenic fungi, but the molecular basis for their involvement is largely unknown.
We performed targeted gene deletion, expression analysis, biochemistry and pathogenicity assays to demonstrate that VdAtf1 governs pathogenesis via the regulation of nitrosative resistance and nitrogen metabolism in V. dahliae.
VdAtf1 controls pathogenesis via the regulation of nitric oxide (NO) resistance and inorganic nitrogen metabolism rather than oxidative resistance and is important for penetration peg formation in V. dahliae. VdAtf1 affects ammonium and nitrate assimilation in response to various nitrogen sources. VdAtf1 may be involved in regulating the expression of VdNut1. VdAtf1 responds to NO stress by strengthening the fungal cell wall, and by causing over‐accumulation of methylglyoxal and glycerol, which in turn impacts NO detoxification. We also verified that the VdAtf1 ortholog in Fusarium graminearum mediates nitrogen metabolism, suggesting conservation of this function in related plant pathogenic fungi.
Our findings revealed new functions of VdAtf1 in pathogenesis, response to nitrosative stress and nitrogen metabolism in V. dahliae. The results provide novel insights into the regulatory mechanisms of the transcription factor VdAtf1 in virulence.
... The fungal pathogen Verticillium dahliae is considered as seed-borne and has been recorded to cause verticillium wilt in over 200 dicotyledonous plant species, including potato and sunflower [19]. This is due to a wide host range of Verticillium dahliae that includes lettuce, potato, cauliflower, eggplant, and cotton, thus making it difficult to control it with crop rotation [20][21][22][23]. ...
: Seed-borne fungi in 69 sunflower cultivars were evaluated which comprised 52 confectionery and 17 oilseed types. Seed coats were placed on both NP-10 (Nonylphenol Ethoxylate based surfacant -10) and potato dextrose agar (PDA) media to culture fungi. The rate of contamination among the different varieties was calculated by counting seed coats with fungal colonies. The rate of contamination in the confectionary group (88%) was significantly (p ≤ 0.05) higher than in the oilseed group (71%). Of the 52 confectionery varieties, the dominant fungi recovered were Verticillium dahliae along with Alternaria spp., Fusarium spp., and Rhizopus spp., whereas the oilseed type varieties were contaminated with only V. dahliae. Molecular identification of fungal species via BLAST (Basic Alignment Search Tool) was performed on fungal sequences obtained from PCR (Polymerase Chain Reaction) analysis. The results included five Alternaria spp. that included Alternaria tenuissima, Alternaria alternata, Alternaria helianthiinficiens, Alternaria longipes, and Alternaria tamaricis, three Fusarium spp. such as Fusarium oxysporum, Fusarium incarnatum, and Fusarium proliferatum, and V. dahliae and Cladosporium cladosporioides. These were identified from pure fungal cultures recovered from seed coats. To efficiently control seed-borne fungi, four broad spectrum fungicides (carbendazim, triadimefon, caprio F-500, and flusilazole) were screened against V. dahliae isolate Gn3, which was isolated from a diseased LD 5009 sunflower plant. Flusilazole was selected based on its low half-maximal effective concentration value (EC50), 78.7 µg/mL. Seeds of diseased LD 5009 plants obtained from two different locations treated with formulated flusilazole fungicide at optimum parameters showed a significant (p ≤ 0.05) increase in seed germination and a decrease in contamination rate from 98% to less than 10%. The results affirmed that confectionery cultivars are much more susceptible to fungal contamination than oilseeds, and also that seed pretreatment is a suitable way to prevent the spread of soil- and seed-borne fungi in sunflower production.
... V. dahliae cause Verticillium wilt in broad range of economic important crops. It is a polyphagous pathogen and host list is persistently expanding with diagnosis of new hosts [1][2][3]. Thus, the sustainable management of Verticillium wilt is vital. To date, the synthetic agrochemicals and pesticides has been employed for the management of wilt diseases. ...
Plant diseases pose threat to global food security. The excessive use of synthetic agro-chemical engender pesticide resistance. The exploration of alternative sustainable diseases management practices are crucial to overcome the devastative plant diseases. In this study, a facile innocuous approach was adopted for biogenic synthesis of Fe2O3-CsNPs via Crocus sativus corm aqueous extract and was evaluated for their antifungal efficacy against the Verticillium wilt pathogen Verticillium dahliae. The physico-chemical characterization of biosynthesized nanoparticles were performed through UV-visible Spectroscopy, Fourier Transform Infrared Spectroscopy, Energy Dispersive X-ray Spectroscopy, X-Ray Diffraction, and Scanning Electron Microscopy. The fungus mycelium growth was significantly inhibited in the media containing 3mg/mL Fe2O3-CsNPs. Degenerated, concentrated and shriveled hyphae were revealed in Scanning Electron Microscopy. The overall results demonstrated that the biogenic Fe2O3-CsNPs have the efficacy to control devastative phytopathogens.
... Low inoculum densities of 2 cfu per gram of soil can result in complete strawberry crop losses (Harris and Yang, 1996), indicating that strawberry exhibits a very high susceptibility to Verticillium alongside the crops cotton (Paplomatas et al., 1992) and olive when artificially inoculated (López-Escudero and Blanco-López, 2007). Verticillium infects over 200 different dicotyledonous plant species including many horticultural crops and weeds (Woolliams, 1966;Bhat and Subbarao, 1999) meaning that crop rotation is an ineffective form of disease control (Atallah et al., 2011). Effective disease control is also hampered by the absence of curative fungicides and restriction of preventative chemical fumigants due to European regulations (e.g., 91/414/EEC; Colla et al., 2012). ...
Verticillium dahliae is a highly detrimental pathogen of soil cultivated strawberry (Fragaria x ananassa). Breeding of Verticillium wilt resistance into commercially viable strawberry cultivars can help mitigate the impact of the disease. In this study we describe novel sources of resistance identified in multiple strawberry populations, creating a wealth of data for breeders to exploit. Pathogen-informed experiments have allowed the differentiation of subclade-specific resistance responses, through studying V. dahliae subclade II-1 specific resistance in the cultivar “Redgauntlet” and subclade II-2 specific resistance in “Fenella” and “Chandler.” A large-scale low-cost phenotyping platform was developed utilizing automated unmanned vehicles and near infrared imaging cameras to assess field-based disease trials. The images were used to calculate disease susceptibility for infected plants through the normalized difference vegetation index score. The automated disease scores showed a strong correlation with the manual scores. A co-dominant resistant QTL; FaRVd3D, present in both “Redgauntlet” and “Hapil” cultivars exhibited a major effect of 18.3% when the two resistance alleles were combined. Another allele, FaRVd5D, identified in the “Emily” cultivar was associated with an increase in Verticillium wilt susceptibility of 17.2%, though whether this allele truly represents a susceptibility factor requires further research, due to the nature of the F1 mapping population. Markers identified in populations were validated across a set of 92 accessions to determine whether they remained closely linked to resistance genes in the wider germplasm. The resistant markers FaRVd2B from “Redgauntlet” and FaRVd6D from “Chandler” were associated with resistance across the wider germplasm. Furthermore, comparison of imaging versus manual phenotyping revealed the automated platform could identify three out of four disease resistance markers. As such, this automated wilt disease phenotyping platform is considered to be a good, time saving, substitute for manual assessment.
... The Salinas and Pajaro Valleys of coastal central California are among the most important lettuce-producing regions in the United States [1]. One of the top disease concerns for lettuce in the area is Verticillium wilt caused by the fungus Verticillium dahliae [2,3], which is a soilborne pathogen with a wide host range that also includes artichoke, cotton, eggplant, hops, potato, sunflower, tobacco, and tomato [4,5]. Two races of V. dahliae occur in coastal central California based on their differential virulence on cultivar La Brillante [6]; however, race 1 is more prevalent and economically important than race 2 [7]. ...
Background
Verticillium wilt caused by the fungus Verticillium dahliae race 1 is among the top disease concerns for lettuce in the Salinas and Pajaro Valleys of coastal central California. Resistance of lettuce against V. dahliae race 1 was previously mapped to the single dominant Verticillium resistance 1 (Vr1) locus. Lines of tomato resistant to race 1 are known to contain the closely linked Ve1 and Ve2 genes that encode receptor-like proteins with extracellular leucine-rich repeats; the Ve1 and Ve2 proteins act antagonistically to provide resistance against V. dahliae race 1. The Vr1 locus in lettuce contains a cluster of several genes with sequence similarity to the tomato Ve genes. We used genome sequencing and/or PCR screening along with pathogenicity assays of 152 accessions of lettuce to investigate allelic diversity and its relationship to race 1 resistance in lettuce.
Results
This approach identified a total of four Ve genes: LsVe1, LsVe2, LsVe3, and LsVe4. The majority of accessions, however, contained a combination of only three of these LsVe genes clustered on chromosomal linkage group 9 (within ~ 25 kb in the resistant cultivar La Brillante and within ~ 127 kb in the susceptible cultivar Salinas).
Conclusions
A single allele, LsVe1L, was present in all resistant accessions and absent in all susceptible accessions. This allele can be used as a molecular marker for V. dahliae race 1 resistance in lettuce. A PCR assay for rapid detection of race 1 resistance in lettuce was designed based on nucleotide polymorphisms. Application of this assay allows identification of resistant genotypes in early stages of plant development or at seed-level without time- and labor-intensive testing in the field.
... Fungos do gênero Verticillium são altamente destrutivos e causadores de murcha vascular, causando severas perdas na produção e qualidade de diferentes culturas economicamente importantes, além de apresentar distribuição mundial (DAAYF, 2015). Os isolados apresentam grande plasticidade na virulência e especificidade do hospedeiro, e seu impacto, geralmente é prejudicial tanto para o rendimento econômico quanto para a qualidade do uso final (ATALLAH et al., 2011). Além disso, as condições climáticas são desfavoráveis ao desenvolvimento da doença, que é mais severa em temperaturas que variam de 18-22º C. No entanto, essa doença é muito importante nos EUA, México, Peru, Rússia, Argentina e Índia. ...
... Since its first identification on lettuce near Watsonville in 1995, the disease has spread to other prime lettuce production regions of the Salinas Valley (Atallah et al. 2012;Subbarao et al. 1997), the salad capital of the United States. Lettuce (Lactuca sativa L.), which was previously considered immune to the wilt disease, is now significantly compromised by the introduction of host-adapted isolates of V. dahliae (Atallah et al. 2011;Cirulli et al. 2010). Verticillium wiltaffected fields might suffer near total crop losses. ...
Verticillium wilt of lettuce, caused by the soilborne pathogen Verticillium dahliae, poses a serious threat to the California lettuce industry. Knowledge of disease development and its impact on postharvest marketability would facilitate better management of the affected fields. This study investigated postharvest marketability of 22 lettuce varieties harvested from two Verticillium-infested commercial lettuce fields in Salinas and Watsonville, CA, in 2005 using a randomized complete block design. Periodic sampling to monitor disease in several crisphead varieties in the field demonstrated that root symptoms developed quickly at later stages of heading, followed by the onset of foliar symptoms as the crop reached harvest maturity. Harvested marketable heads were vacuum cooled soon after harvest to about 4°C and maintained at this temperature in commercial coolers. The impact of V. dahliae on postharvest marketability was assessed based on the percentage of heads per case deemed marketable following 1, 2, and 3 weeks of refrigerated storage. Across both field experiments, the average disease incidence and postharvest marketability ranged from 4.2 to 87.5% and from 69.4 to 100.0%, respectively, among lettuce types and varieties. The Pearson correlation analysis detected no significant relationship between disease incidence and postharvest marketability across all varieties tested (r = 0.041, P = 0.727), or within lettuce types, even though V. dahliae was recovered from 34% of the plants harvested, and recovery ranged from 0 to 73.3% for V. dahliae and from 10 to 91.7% for non-V. dahliae (V. isaacii or V. klebahnii) species. These findings demonstrate that growers can harvest lettuce from an infested field before foliar symptoms develop with negligible impact by Verticillium spp. on postharvest marketability or quality.
... As expected, P. tracheiphilum was not found in fields where lettuce had never been grown and inoculum density was generally higher with increasing number of lettuce crops per year. Similarly, studies on Verticillium dahliae showed that consecutive lettuce production significantly increases soilborne V. dahliae microsclerotia concentrations (Atallah et al. 2011;Short et al. 2015). Pythium root rot incidence was also positively correlated with P. tracheiphilum inoculum density. ...
In Canada, head lettuce (Lactuca sativa capitata) is extensively produced in the muck soils of southwestern Québec. However, yields are increasingly affected by various soilborne pathogens, including Pythium spp., which cause wilt and damping off. In a survey conducted in Québec muck soils in 2010 and 2011, Pythium tracheiphilum Matta was identified as the predominant Pythium sp. in the root of head lettuce showing Pythium stunt symptoms. Therefore, to improve risk assessment and help further understanding of disease epidemiology, a specific and sensitive real-time quantitative polymerase chain reaction (qPCR) assay based on TaqMan-minor groove binder (MGB) technology was developed for P. tracheiphilum. The PCR primers along with a TaqMan-MGB probe were designed from the ribosomal internal transcribed spacer 2 region. A 100-bp product was amplified by PCR from all P. tracheiphilum isolates tested while no PCR product was obtained from 38 other Pythium spp. or from a selection of additional lettuce pathogens tested. In addition to P. tracheiphilum, the assay was multiplexed with an internal control allowing for the individual validation of each PCR. In artificially infested soils, the sensitivity of the qPCR assay was established as 10 oospores/g of dry soil. P. tracheiphilum was not detected in soils in which lettuce has never been grown; however, inoculum ranged from 0 to more than 200,000 oospores/g of dry soil in commercial lettuce fields. Also, disease incidence was positively correlated with inoculum concentration (r = 0.764). The results suggest that inoculum concentration should be considered when making Pythium stunt management decisions. The developed qPCR assay will facilitate reliable detection and quantification of P. tracheiphilum from field soil.
... The severity of disease varied greatly among the fields in Shaanxi and Inner Mongolia. While this study provides a snapshot of the current status of the disease, as observed in other systems (Atallah et al. 2011), once introduced, the pathogen not only establishes itself in individual fields but rapidly spreads from these foci to new fields with the planting of infested seeds in new fields and/or movement of soil and equipment between fields. Thus, the disease is likely to emerge as a major threat to potato production in China. ...
Potato (Solanum tuberosum L.) is one of the most important staple foods in many parts of the world including China. In recent years, Verticillium wilt has become a severe threat to potato production in China. During 2015 to 2016, 287 samples of symptomatic potato plants were collected from 15 counties in five provinces from northern China. One hundred and eighty-seven Verticillium-like colonies were isolated from these samples and identified to species based on cultural and morphological characteristics, and multigene phylogeny based on the partial sequences of actin (ACT), elongation factor 1-alpha (EF1α), glyceraldehyde-3-phosphate dehydrogenase (GPD), and tryptophan synthase (TS) genes. A consensus-rooted most parsimonious phylogenetic tree was generated from the data. One hundred and fifteen isolates comprising 61.5% of the total were identified as Verticillium dahliae, and the remaining 38.5% of the isolates were identified as V. nonalfalfae. V. dahliae was widely distributed in Shaanxi (84.1%), Inner Mongolia (76.7%), Gansu (12.8%), and Qinghai (100%, representing a single isolate). V. dahliae was not recovered from the samples in Ningxia. V. nonalfalfae dominated the collections from Gansu (87.2%) and Ningxia (100%) but was also recovered from Shaanxi (15.9%) and Inner Mongolia (23.3%) at lower frequencies. Neither V. albo-atrum nor V. alfalfae was recovered from the sampled areas. The V. nonalfalfae isolates were predominantly isolated from the samples collected from altitudes above 1,800 m, and in contrast, V. dahliae isolates were mainly recovered from fields sampled below 1,800 m. The optimum temperature for the colony growth of V. nonalfalfae was lower (20°C) than that for V. dahliae (25°C). Pathogenicity tests demonstrated that V. dahliae and V. nonalfalfae were both pathogens of potato Verticillium wilt, with V. dahliae isolates exhibiting higher virulence than V. nonalfalfae isolates regardless of the collection area of the species. This is the first documentation of V. nonalfalfae infecting S. tuberosum in China and the higher altitudes associated with infections of V. nonalfalfae anywhere in the world.
... Verticillium dahliae Kleb. is a soilborne plant pathogenic fungus which can cause wilting on over 200 plant species, including many agricultural and horticultural crops [1]. This pathogen has a rapidly expanding host range, most notably on the crops lettuce and pepper where it has become a major pathogen within these industries [2][3][4]. V. dahliae cannot act as an effective saprophyte and thus must produce resting structures to persist within the soil between infection events. The resting structures are microsclerotia, these are robust melanised hyphae which are very difficult to eradicate and can persist in the soil for up to 20 years post-formation [1]. ...
Verticillium dahliae infection of strawberry (Fragaria x ananassa) is a major cause of disease-induced wilting in soil-grown strawberries across the world. To understand what components of the pathogen are affecting disease expression, the presence of the known effector VdAve1 was screened in a sample of Verticillium dahliae isolates. Isolates from strawberry were found to contain VdAve1 and were divided into two major clades, based upon their vegetative compatibility groups (VCG); no UK strawberry isolates contained VdAve1. VC clade was strongly related to their virulence levels. VdAve1-containing isolates pathogenic on strawberry were found in both clades, in contrast to some recently published findings. On strawberry, VdAve1-containing isolates had significantly higher virulence during early infection, which diminished in significance as the infection progressed. Transformation of a virulent non-VdAve1 containing isolate, with VdAve1 was found neither to increase nor decrease virulence when inoculated on a susceptible strawberry cultivar. There are therefore virulence factors that are epistatic to VdAve1 and potentially multiple independent routes to high virulence on strawberry in V. dahliae lineages. Genome sequencing a subset of isolates across the two VCGs revealed that isolates were differentiated at the whole genome level and contained multiple changes in putative effector content, indicating that different clonal VCGs may have evolved different strategies for infecting strawberry, leading to different virulence levels in pathogenicity tests. It is therefore important to consider both clonal lineage and effector complement as the adaptive potential of each lineage will differ, even if they contain the same race determining effector.
... Verticillium dahliae Kleb. is a soilborne plant pathogenic fungus which can cause wilting on over 200 plant species, including many agricultural and horticultural crops [1]. This pathogen has a rapidly expanding host range, most notably on the crops lettuce and cauliflower where it has become a major pathogen within these industries [2]; [3,4]. V. dahliae ...
Verticillium dahliae infection of strawberry ( Fragaria × ananassa ) is a major cause of disease-induced wilting in soil-grown strawberries across the world. To understand what components of the pathogen are affecting disease expression, the presence of the known effector VdAve1 was screened in a sample of Verticillium dahliae isolates. Isolates from strawberry were found to contain VdAve1 and were divided into two major clades, based upon their vegetative compatibility groups (VCG); no UK strawberry isolates contained VdAve1 . VC clade was strongly related to their virulence levels. VdAve1 -containing isolates pathogenic on strawberry were found in both clades, in contrast to some recently published findings.
On strawberry, VdAve1 -containing isolates had significantly higher virulence during early infection, which diminished in significance as the infection progressed. Transformation of a virulent non- VdAve1 containing isolate, with VdAve1 was found neither to increase nor decrease virulence when inoculated on a susceptible strawberry cultivar. There are therefore virulence factors that are epistatic to VdAve1 and potentially multiple independent routes to high virulence on strawberry in V. dahliae lineages.
Genome sequencing a subset of isolates across the two VCGs revealed that isolates were differentiated at the whole genome level and contained multiple changes in putative effector content, indicating that different clonal VCGs may have evolved different strategies for infecting strawberry, leading to different virulence levels in pathogenicity tests. It is therefore important to consider both clonal lineage and effector complement as the adaptive potential of each lineage will differ, even if they contain the same race determining effector.
... It infects the host plants through the roots and then colonizes and propagates in xylem vessel (Pegg and Brady, 2002). The list of plant hosts affected by V. dahliae is continually expanding as new hosts are identified (Atallah et al., 2011). In China, apart from the important crops, landscape plants like smoke trees (Cotinus coggygria) are also infested by V. dahliae (Wang et al., 2013). ...
The fungus Verticillium dahliae causes vascular wilt disease on various plant species resulting in devastating yield losses worldwide. The capacity of V. dahliae to colonize in host plant xylem and disseminate by microsclerotia has led to studies to evaluate genes associated with pathogenesis and microsclerotia formation. Here, we identified and characterized a V. dahliae homolog to Skn7, a two-component stress response regulator of Saccharomyces cerevisiae. Results showed that melanized microsclerotia formation and conidiation were significantly inhibited in the VdSkn7 deletion mutants. VdSkn7-deficient mutants displayed severe growth defect under heat shock, cell wall perturbing agents and H2O2, and were significantly less virulent but were not sensitive to osmotic stresses compared to the wild-type strain. Finally, we demonstrated that VdSkn7 is required for the plant penetration. Taken together, our study thus provides new evidence on the functional conservation and divergence of Skn7 orthologs among fungal organisms and indicates that VdSkn7 contributes to microsclerotial development, virulence and stress response of V. dahliae.
... Also, there is a correlation between inoculum density and final disease incidence values (López Escudero and Blanco-López 2007;Bejarano-Alcázar et al. 1995). Moreover, it has been observed that for a given inoculum density, disease incidence varies greatly depending on the crop (Berbegal et al. 2007;Xiao and Subbarao 1998;Grogan et al. 1979;Harris and Yang 1996;Atallah et al. 2011). Consequently, validation and standardization of V. dahliae inoculation methods and inoculum density is needed to provide accurate assessment of wilt resistance in tree hosts. ...
Olive plantations and tree nurseries are economically and ecologically important agricultural sectors. However, Verticillium wilt, caused by Verticillium dahliae Kleb., is a serious problem in olive-growing regions and in tree nurseries worldwide. In this review we describe common and differentiating aspects of Verticillium wilts in some of the main economically woody hosts. The establishment of new planting sites on infested soils, the use of infected plant material and the spread of highly virulent pathogen isolates are the main reasons of increasing problems with Verticillium wilt in tree cultivation. Therefore, protocols for quick and efficient screening of new planting sites as well as planting material for V. dahliae prior to cultivation is an important measure to control Verticillium wilt disease. Furthermore, screening for resistant genotypes that can be included in breeding programs to increase resistance to Verticillium wilt is an important strategy for future disease control. Collectively, these strategies are essential tools in an integrated disease management strategy to control Verticillium wilt in tree plantations and nurseries.
... Verticillium dahliae is a destructive soilborne pathogen that causes Verticillium wilt on many economically important crops, including artichoke, cotton, lettuce, pepper, strawberry, and tomato (Atallah et al. 2011;Pegg and Brady 2002;Subbarao et al. 1997). The fungus internally colonizes the vascular tissues while the host is still alive and, once the host reaches maturity, the fungus forms melanized resting structures called microsclerotia (Vallad and Subbarao 2008). ...
Understanding pathogen evolution over time is vital for plant breeding and deployment of host resistance. In the context of a soilborne pathogen, the potential of host-directed evolution of a Verticillium dahliae race 1 isolate and genotypic variation of V. dahliae associated with two major hosts (lettuce and tomato) were determined. A total of 427 isolates were recovered over six years from a resistance screening nursery infested with a single V. dahliae race 1 isolate. In a separate study, an additional 207 isolates representing 163 and 43 isolates from commercial lettuce and tomato fields were collected, respectively. Analyses of isolates recovered from the screening nursery over six years revealed no changes in the race and mating type composition but did uncover six Simple Sequence Repeat (SSR) variant genotypes. No significant genotypic variation in V. dahliae was observed between or within fields of either lettuce or tomato, but pathogen populations were significantly differentiated between these two hosts. Replicated virulence assays of variant SSR genotypes on lettuce differential cultivars suggested no significant difference in virulence from the wild type race 1 isolate introduced into the field. This suggests that deployed race 1 host resistance will be robust against the widespread race 1 populations in lettuce growing regions at least for six years unless novel pathogen genotypes or races are introduced into the system.
... The genus Verticillium Nees comprises soilborne plant pathogenic fungi that cause vascular wilt diseases on hundreds of plant species, including high-value agricultural crops and landscape plants in many parts of the world (Inderbitzin & Subbarao, 2014;Pegg & Brady, 2002). The disease affects the vascular system of host plants and may cause severe crop losses (Atallah et al., 2011;Jiménez-Díaz et al., 2012). Among the recently revised Verticillium species (Inderbitzin et al., 2011), Verticillium dahliae Kleb. ...
In the Mediterranean basin, Verticillium Wilt of Olive (VWO) is diffused throughout its range of cultivation, causing severe yield losses and tree mortality. The disease was reported in almost all the Mediterranean and Middle East countries, and in Lebanon it is of increasing significance also on many valuable crops. The disease has already been reported on potato, peach and almond in the Bekaa valley; however, to date no information is available about the incidence of VWO and the inoculum density of Verticillium dahliae microsclerotia in soil of the main agricultural areas of Lebanon. Results from the present investigations demonstrate a high V. dahliae frequency in soils (75.3%), coupled with a mean soil inoculum density of 17.0 MS g−1, clearly indicating a great impact on the production of susceptible hosts in Lebanon, mainly in Bekaa region. Molecular method to assess the microsclerotia inoculum density in soil allowed the detection of a higher frequency of infested soils, as compared with the traditional plating, thus confirming its higher sensitivity. The overall Verticillium wilt prevalence in the inspected olive orchards was 46.2%, and the frequency of V. dahliae-infected trees was 25.7%. The widespread presence of V. dahliae in all olive growing areas of Lebanon enforces the adoption of measures aimed at reducing the soil inoculum density before any new olive plantation, and the use of strong phytosanitary regulations to improve the certification schemes of propagating material.
... It causes Verticillium wilt on more than 200 plant species, including many agriculturally important crops (Pegg and Brady, 2002). The list of plant hosts affected by V. dahliae is continually expanding as new hosts are identified (Atallah et al., 2011;Bhat and Subbarao, 1999;Lu et al., 2013). ...
The fungus Verticillium dahliae causes vascular wilt disease on many plant species, including economically important crop and ornamental plants worldwide. It produces darkly pigmented resting structures known as microsclerotia, which are able to survive for up to 14 years in soil, and represent one of the defining characteristics of this species. The pigment produced in V. dahliae is dihydroxynaphthalene (DHN)-melanin, a form of melanin common among fungi and named so for the intermediary of this melanin biosynthetic pathway. In this study, we characterized the function of the V. dahliae Vayg1 gene, whose homologs were involved in melanin biosynthesis in Exophiala dermatitidis (Wayg1) and Aspergillus fumigatus (Aayg1), by deletion and complementation of the gene and co-incubating deletion mutant with wild type strain. Results showed that melanin production and microsclerotial formation in deletion mutants are inhibited. The Vayg1 deletion mutant also exhibited reduced pathogenicity. These results showed that Vayg1 is necessary for melanin and microsclerotium production, and we may thus hypothesize that the Vayg1 product may catalyze two different precursors, one of which is essential for DHN melanin production and the other one is involved in a signal network for microsclerotial formation in V. dahliae.
Verticillium wilt, caused by Verticillium dahliae, is one of the most devastating soilborne diseases of lettuce (Lactuca sativa L.). There are three races of V. dahliae and each race has been characterized by markers representing race-specific effectors. Race 1 is differentiated by the presence of the functional secretory Ave1 effector. Similarly, races 2 and 3 are differentiated by effectors VdR2e and VdR3e, respectively. While the presence of race 1 in coastal California was well-established, the presence of effector-based races 2 and 3 was uncertain. This study therefore focused on characterizing 727 isolates collected from 142 ranches of symptomatic lettuce and other crops from coastal California. Based on this evaluation, 523 isolates were designated as race 1, 20 isolates as race 2, 23 isolates as race 3, and 17 as race undefined. Isolates representing other Verticillium species totaled 110, and 34 were non-Verticillium fungal species. Since the use of resistant cultivars is a key strategy to manage this disease, we evaluated 48 lettuce germplasm lines and one endive (Cichorium endivia L.) line, comprised of commercial cultivars (cv.) and breeding lines, including the race 1-resistant heirloom cv. La Brillante and the susceptible cv. Salinas as controls. Resistance against races 1, 2, and 3 along with VdLs17, a virulent isolate of V. dahliae from lettuce that is currently not assigned to a race was evaluated in replicated greenhouse experiments. Two crisphead lettuce lines, HL28 and HL29, exhibited resistance against race 1 and a partial resistance against race 2 while all other lines were highly susceptible to races 1 and 2 and VdLs17. The majority of lines exhibited higher resistance to race 3 relative to the other two races. This study documents the current distribution of the different races in coastal California. In addition, the sources of resistance currently being developed should be effective or partially effective against these races for targeted deployment as soon as they are available. Keywords: Lactuca sativa, avirulence, effectors, host resistance, disease severity
Verticillium wilt (VW), a devastating vascular wilt disease of strawberry (Fragaria ×$\times$ ananassa), has caused economic losses for nearly a century. This disease is caused by the soil‐borne pathogen Verticillium dahliae, which occurs nearly worldwide and causes disease in numerous agriculturally important plants. The development of VW‐resistant cultivars is critically important for the sustainability of strawberry production. We previously showed that a preponderance of the genetic resources (asexually propagated hybrid individuals) preserved in public germplasm collections were moderately to highly susceptible and that genetic gains for increased resistance to VW have been negligible over the last 60 years. To more fully understand the challenges associated with breeding for increased quantitative resistance to this pathogen, we developed and phenotyped a training population of hybrids (n=564$n = 564$) among elite parents with a wide range of resistance phenotypes. When these data were combined with training data from a population of elite and exotic hybrids (n=386$n = 386$), genomic prediction accuracies of 0.47–0.48 were achieved and were predicted to explain 70%–75% of the additive genetic variance for resistance. We concluded that breeding values for resistance to VW can be predicted with sufficient accuracy for effective genomic selection with routine updating of training populations.
In this study, we examined the effect of crop rotation on the suppression of Verticillium wilt in Chinese cabbage. Compared to leaving cultivation areas fallow (i.e., no rotation), introducing crop rotations with broccoli or cabbage were significantly more effective in controlling this disease. Here we used a generalized linear model to evaluate the effectiveness of crop rotations in all trials. In addition, an optimal model was constructed using Akaike’s Information Criterion to select explanatory variables associated with disease suppression. In the best model, the estimated coefficients for crop rotation with broccoli and cabbage were − 0.56 and − 0.38, respectively, compared to fallow ( p < 0.05). In addition, we found that susceptibility to disease varied by harvest month and year. This study shows that Chinese cabbage wilt can be controlled by crop rotation. Furthermore, the analysis of factors involved in the development of this disease can help cultivators eliminate it from Chinese cabbage production in the future.
Lettuce (Lactuca sativa L.) production is greatly threatened by Verticillium wilt, which is caused by three pathogenic races (races 1, 2, and 3) of the soilborne fungus Verticillium dahliae. Race 1 is predominant, and resistant varieties that provide full protection against it are commercially available. However, heavily relying on race 1-resistant cultivars could shift the population towards resistance-breaking isolates and impact the durability of plant resistance. This study determined the inheritance of partial resistance to isolate VdLs17 of V. dahliae within Lactuca spp. using 258 F 2:3 progeny generated from a cross between two partially resistant accessions, 11G99 (L. serriola) and PI 171674 (L. sativa). Eight experiments were performed under greenhouse and growth room conditions across 3 years using a randomized complete block design, and segregation analysis was conducted to determine the inheritance pattern. The results indicate that partial resistance to isolate VdLs17 of V. dahliae is conditioned by a two-major-gene genetic model with additive-dominance-epistatic effects. Transgressive segregants were infrequent but observed in both directions, indicating that favorable and adverse alleles are dispersed in both parents. Combining favorable alleles of these two partially resistant parents appears to be challenging because of epistatic effects and a significant role of environment in disease severity. The probability of capturing favorable additive genes could be maximized by generating and evaluating a large population and making selections at late generations. This study provides valuable insights into the inheritance pattern of partial resistance to isolate VdLs17 of V. dahliae that will be helpful in designing efficient breeding strategies in lettuce.
[Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .
This datasheet on Verticillium dahliae covers Identity, Overview, Distribution, Dispersal, Hosts/Species Affected, Diagnosis, Biology & Ecology, Seedborne Aspects, Natural Enemies, Impacts, Prevention/Control, Further Information.
Verticillium dahliae is a soil-borne plant pathogenic fungus that causes Verticillium wilt on hundreds of dicotyledonous plant species. V. dahliae is considered an asexually (clonal) reproducing fungus, although both mating type idiomorphs (MAT1-1 and MAT1-2) are present, and is heterothallic. Most of the available information on V. dahliae strains, including their biology, pathology, and genomics comes from studies on isolates with the MAT1-2 idiomorph, and thus little information is available on the MAT1-1 V. dahliae strains in the literature. We therefore evaluated the growth responses of MAT1-1 and MAT1-2 V. dahliae strains to various stimuli. Growth rates and melanin production in response to increased temperature, alkaline pH, light, and H2O2 stress were higher in the MAT1-2 strains than in the MAT1-1 strains. In addition, the MAT1-2 strains showed an enhanced ability to degrade complex polysaccharides, especially starch, pectin, and cellulose. Furthermore, several MAT1-2 strains from both potato and sunflower showed increased virulence on their original hosts, relative to their MAT1-1 counterparts. Thus, compared to MAT1-1 strains, MAT1-2 strains derive their potentially greater fitness from an increased capacity to adapt to their environment and exhibit higher virulence. These competitive advantages might explain the current abundance of MAT1-2 strains relative to MAT1-1 strains in the agricultural and sylvicultural ecosystems, and this study provides the baseline information on the two mating idiomorphs to study sexual reproduction in V. dahliae under natural and laboratory conditions.
Verticillium wilt, a disease caused by the fungus Verticillium dahliae, affects a vast range of crops, including strawberries. A field trial was established to evaluate the effect of broccoli in rotation prior to strawberries, anaerobic soil disinfestation (ASD), and mustard cake (MC) alone and in combination at suppressing V. dahliae. Broccoli, cauliflower and fallow plots were established in year 1, followed by strawberry, winter cover crop, and lettuce across all plots over the next two years. ASD and ASD + MC improved strawberry yields compared to the untreated control (UTC). At the end of the harvest season, V. dahliae infection rates in strawberry crowns were lower in the ASD and ASD + MC treatments compared to the UTC. After harvest of the lettuce crop, the number of V. dahliae microsclerotia in soil treated previously with ASD was lower than in the UTC. Broccoli residue incorporation had no effect and MC alone a limited effect on strawberry yields, V. dahliae in strawberry crowns, and soil microsclerotia levels at lettuce harvest. All rotations treated with ASD showed higher total net returns despite the higher cost of treatments. The ability of ASD to control V. dahliae and its persistent suppressive effect on soil microsclerotia levels have important economic implications for growers.
Booming applications of novel nanomaterials or nanostructures in biomedical and pharmaceutical fields encourage the biogenic synthesis of nanoparticles with reduced toxicity. Green synthesis of nanoparticles generally depends on the phytogenic reduction process and due to its simple mechanism, vast availability, less cost, eco-friendly nature and its one-pot synthesis method, it gains importance in the medical field, agriculture, and bioremediation. The phytochemicals present in the plant extract readily act as reducing agents for the efficient conversion of metal salts or ions into metal/metal oxide nanoparticles. The physico-chemical parameters that influence the successful conversion have been discussed. Varieties of plants, extracts, and compounds that induce the nanoparticle formation have been reviewed in this chapter along with the different techniques used for nanoparticle characterisation. Applications of phytogenic nanoparticles in biotechnology such as antibacterial properties, antiviral activity, cytotoxic activity, antilarval agent, plant tissue culture and crop protection, are discussed. Nanobiotechnological applications in the environmental sector such as bioremediation of heavy metals, dyes, nitrophenol, wastewater reclamation and sensing platforms, are also critically reviewed.
Verticillium wilt (Verticillium dahliae Kleb.) is a fungal disease of sunflower plants causing vascular bundle jamming, leaf wilt, and plant death, all of which affect sunflower production. We devised an artificial inoculation field test to investigate Verticillium wilt incidence during different stages of sunflower growth including seedling, flowering, and maturation stages, to assess the impact of different disease appearance period on plant growth and production. We hypothesized that time of symptom appearance may be a key factor for sunflower development. The results showed that Verticillium wilt dramatically inhibited sunflower growth and production in all three developmental stages. The disease incidence of Verticillium wilt, considered as percentage of symptomatic plants, was higher than 60% at the maturation stage and lower than 20% at the seedling stage. However, the greatest yield suppression was identified at the seedling stage, while yield inhibition was relatively limited during the flowering and maturation stages. Therefore, the earlier the symptom appearance is established in sunflower development, the greater is the suppression of plant growth, indicating that seedling management is crucial in controlling the Verticillium wilt. The outcomes of this study would contribute to the development of disease control strategies to improve sunflower production.
Verticillium dahliae is a widespread fungal pathogen that causes Verticillium wilt on many economically important crops and ornamentals worldwide. Populations of V. dahliae have been divided into two distinct races based upon differential host responses in tomato and lettuce. Recently, the contemporary race 2 isolates were further divided into an additional race in tomato. Herein, we provide a high-quality reference genome for the race 1 strain VdLs.16 isolated from lettuce in California, U.S.A. This resource will contribute to ongoing research that aims to elucidate the genetic basis of V. dahliae pathogenicity and population genomic diversity.
This article is part of the Top 10 Unanswered Questions in MPMI invited review series.
Nonhost resistance is typically considered the ability of a plant species to repel all attempts of a pathogen species to colonize it and reproduce on it. Based on this common definition, nonhost resistance is presumed to be very durable and, thus, of great interest for its potential use in agriculture. Despite considerable research efforts, the molecular basis of this type of plant immunity remains nebulous. We here stress the fact that “nonhost resistance” is a phenomenological rather than a mechanistic concept that comprises more facets than typically considered. We further argue that nonhost resistance essentially relies on the very same genes and pathways as other types of plant immunity, of which some may act as bottlenecks for particular pathogens on a given plant species or under certain conditions. Thus, in our view, the frequently used term “nonhost genes” is misleading and should be avoided. Depending on the plant–pathogen combination, nonhost resistance may involve the recognition of pathogen effectors by host immune sensor proteins, which might give rise to host shifts or host range expansions due to evolutionary-conditioned gains and losses in respective armories. Thus, the extent of nonhost resistance also defines pathogen host ranges. In some instances, immune-related genes can be transferred across plant species to boost defense, resulting in augmented disease resistance. We discuss future routes for deepening our understanding of nonhost resistance and argue that the confusing term “nonhost resistance” should be used more cautiously in the light of a holistic view of plant immunity.
Verticillium dahliae has been recently characterized as a heterothallic fungus having two MAT idiomorphs, MAT1-1 and MAT1-2, which suggested that a putative sexual life cycle of the pathogen might exist. A successful mating in heterothallic fungi requires two individuals of opposite mating types present together in the same site. Co-occurrence of both mating types of V. dahliae has been investigated in many parts of the world but not comprehensively in Turkey. This study aims at determination of the mating types in V. dahliae populations in the major cotton producing districts of Aydın Province. A total of 123 V. dahliae isolates obtained from cotton were subjected to PCR assay with the two pairs of specific primers for mating type determination. All of the isolates produced a single amplicon approximately 600 bp in size, which is known belonging to MAT1-2 idiomorph. The results suggested that MAT1-1 has not been introduced yet to the Aydın Province. Absence of MAT1-1 idiomorph will deprive the pathogen from sexual reproduction and this maintains that the pathogen population remains clonal with presumably having low levels of genetic diversity.
Strawberry production has historically been affected by soilborne diseases such as Verticillium wilt. This disease was a major limiting factor in strawberry production in California in the 1950s, and was the main reason that preplant soil fumigation with methyl bromide (MB) was developed in the late 1950s. MB fumigation was so successful that over 90% of the commercial strawberry fruit production in California utilized this technique. However, MB was subsequently linked to ozone depletion, and its use was phased out in 2005. The California strawberry industry was awarded exemption to the full phaseout until 2016, when all MB use in strawberry fruit production was prohibited. MB use continues in strawberry nurseries under an exemption to prevent spread of nematodes and diseases on planting stock. This review examines the impact of the MB phase-out on the California strawberry industry and evaluates the outlook for the industry in the absence of one of the most effective tools for managing soilborne diseases. New soilborne diseases have emerged, and historically important soilborne diseases have reemerged. Registration of new fumigants has been difficult and replacement of MB with a new and effective alternative is unlikely in the foreseeable future. Thus, crop losses due to soilborne diseases are likely to increase. Host-plant resistance to soilborne diseases has become a top priority for strawberry breeding programs, and cultivars are increasingly selected for their resistance to soilborne diseases. The intelligent integration of a variety of management tactics is necessary to sustain strawberry production in California.
Verticillium wilt is the most important disease threatening the commercial production of mint grown for essential oil. An important long-term goal for mint breeders is the production of cultivars with resistance to Verticillium wilt. Before that can be accomplished, a better understanding of the genetic variation within and among populations of Verticillium dahliae is needed. We characterized the extent of phenotypic and genetic diversity present in contemporary and archival populations of V. dahliae from mint fields in Oregon and other production regions of the United States using genotyping by sequencing, PCR assays for mating type and pathogenic race, vegetative compatibility group (VCG) tests, and aggressiveness assays. We report that the population in the Pacific Northwest can be described as one common genetic group and four relatively rare genetic groups. Eighty-three percent of the isolates belonged to VCG2B, and all isolates possessed the MAT1-2 idiomorph and were characterized as pathogenic race 2. These results indicate low levels of genetic diversity and a negligible risk of sexual recombination in populations of this host-adapted pathogen population. Knowledge of the genetic structure of V. dahliae in the Pacific Northwest will inform breeders about the diversity of pathogenicity factors that may need to be considered in their breeding programs.
Sunflower yellow wilt is a widespread and destructive disease caused by the soil-borne pathogen Verticillium dahliae (V. dahliae). To better understand the pathogenesis mechanism of V. dahliae in sunflower, T-DNA insertion library was generated via Agrobacterium tumefaciens mediated transformation system (ATMT). Eight hundred positive transformants were obtained. Transformants varied in colony morphology, growth rate, conidia production and pathogenicity in sunflower compared to the wild type strain. A mutant, named VdGn3-L2, was chosen for further analysis based on its deprivation on microsclerotia formation. The flanking sequence of T-DNA insertion site of VdGn3-L2 was identified via hiTAIL-PCR, and the interrupted gene encoded an initiation-specific α-1, 6-mannosyltransferase, named as VdOCH1. The deletion mutant ΔVdOCH1 was impaired in certain characteristics such as fungal growth, conidia production, and microsclerotia formation. Also, ΔVdOCH1 mutants were more sensitive to the cell wall perturbing reagents, such as SDS and Congo red, lost their penetration ability through cellophane membrane, and exhibited dramatically decreased pathogenicity to sunflower. The impaired phenotypes could be restored to the wild type level by complementation of the deletion mutant with full-length VdOCH1 gene. In conclusion, VdOCH1, encoded α-1,6-mannosyltransferase, manipulating the biological characteristics, microsclerotia formation and pathogenic ability of V. dahliae in sunflower.
Verticillium dahliae is a highly detrimental pathogen of soil cultivated strawberry ( Fragaria x ananassa ). Breeding of Verticillium wilt resistance into commercially viable strawberry cultivars can help mitigate the impact of the disease. In this study we describe novel sources of resistance identified in biparental strawberry populations, creating a wealth of data for breeders to exploit. Pathogen-informed experiments have allowed the differentiation of subclade-specific resistance responses, through studying V. dahliae subclade II-1 specific resistance in the cultivar ‘Redgauntlet’ and subclade II-2 specific resistance in ‘Fenella’ and ‘Chandler’.
A large-scale low-cost phenotyping platform was developed utilising automated unmanned vehicles and near infrared imaging cameras to assess field-based disease trials. The images were used to calculate disease susceptibility for infected plants through the normalized difference vegetation index score. The automated disease scores showed a strong correlation with the manual scores.
A co-dominant resistant QTL; FaRVd3D , present in both ‘Redgauntlet’ and ‘Hapil’ cultivars exhibited a major effect of 18.3 % when the two resistance alleles were combined. Another allele, FaRVd5D , identified in the ‘Emily’ cultivar was associated with an increase in Verticillium wilt susceptibility of 17.2%, though whether this allele truly represents a susceptibility factor requires further research, due to the nature of the bi-parental cross.
Markers identified in bi-parental populations were validated across a set of 92 accessions to determine whether they remained closely linked to resistance genes in the wider germplasm. The resistant markers FaRVd2B from ‘Redgauntlet’ and FaRVd6D from ‘Chandler’ were associated with resistance across the wider germplasm. Furthermore, comparison of imaging versus manual phenotyping revealed the automated platform could identify three out of four disease resistance markers. As such, this automated wilt disease phenotyping platform is considered to be a good, time saving, substitute for manual assessment.
Biological significance:
Mounting of defense responses requires a substantial flux of carbon and nitrogen from primary to secondary metabolites. In-depth understanding of these key metabolic pathways required for growth and defense responses, especially at proteome level, will allow the development of breeding strategies for crops where Verticillium tolerance is absent. Our data show early and late responses of tomato root proteins towards pathogen infection and identify primary metabolism enzymes affected by V. dahliae. Those proteins represent candidates for plant improvement.
Race 1 resistance against Verticillium dahliae in lettuce was originally shown in the cultivar La Brillante to be conditioned by a single dominant gene (Verticillium resistance 1, Vr1). Multiple, morphologically diverse sources of germplasm have been identified as resistant to race 1. In this study, allelism tests indicated that resistance in these different lettuce cultivars is closely linked or allelic to the Vr1 gene. The Vr1 gene is defeated by race 2 isolates of V. dahliae. Only partial resistance to race 2 isolates is available in a few plant introductions (PIs). Greenhouse and field experiments conducted with these PIs demonstrated partial resistance to V. dahliae race 1 as well as race 2 isolates from lettuce. Cultivars resistant to race 1 and PIs with partial resistance to race 2 were challenged with several race 1 and 2 isolates originating from hosts other than lettuce. This indicated that cultivars resistant to race 1 and the breeding lines derived from them would also be resistant to race 1 isolates from other hosts; similarly, the partial resistance would be effective against race 1 and 2 isolates from hosts other than lettuce. Nevertheless, there were specific interactions that warrant further study. Although race 1 currently predominates in the major lettuce production area of the Salinas Valley, CA, breeding lettuce for resistance to V. dahliae should take both races into account.
In growth chamber experiments with five concentrations of NH 4 NO 3 and inoculation of lettuce (Lactuca sativa L.) cv . Salinas with Rhizomonas suberifaciens, the causal agent of corky root (CR), symptoms of noninfectious corky root induced by high rates of N were distinct from those of infectious corky root (ICR). Nitrogen toxicity was observed at 350 kg·ha ⁻¹ and above, and was not affected by inoculation with R. suberifaciens. There was a curvilinear relationship between concentration of NH 4 NO 3 applied and ICR severity with a maximum at 525 kg·ha ⁻¹ . In a similar growth chamber experiment with NH 4 NO 3 plus urea, ICR severity decreased and N toxicity increased at increasing N levels (N at 160 to 650 kg·ha ⁻¹ ). In microplots at Davis, Calif., sidedressing with NH 4 NO 3 (N at 170 kg·ha ⁻¹ ) increased ICR severity on `Salinas' lettuce over the nonfertilized control. There was a significant interaction between N fertilization and soil-infestation with R. suberifaciens with respect to head fresh weight: sidedressing with NH 4 NO 3 increased head weight in noninfested plots, but decreased head weight in infested plots. In four field experiments at Salinas, Calif., sidedressing with N at 78 to 213 kg·ha ⁻¹ , with N as (NH 4 ) 1 SO 4 , NH 4 NO 3 , urea, or Ca(NO 3 ) 2 , increased ICR over the control, but there were no significant differences between the forms of N. Head fresh and dry weights were either increased or unaffected by sidedressing with N fertilizers, depending on the residual concentrations of N in the soil. The increase in ICR was likely related to concentrations of soil NO 3 rather than NH 4 .
du Toit, L. J., Derie, M. L., and Hernandez-Perez, P. 2005. Verticillium wilt in spinach seed production. Plant Dis. 89:4-11. There are no previous reports of Verticillium wilt in fresh and processing spinach (Spinacia oleracea) crops in the United States. In 2002, a hybrid spinach seed crop in the Pacific North- west developed late-season wilt symptoms. Assays of the harvested seed and stock seed of the male and female parents revealed 59.5, 44.0, and 1.5%, respectively, were infected with Verticil- lium dahliae. Assays of 13 stock or commercial seed lots grown in 2002 and 62 commercial lots harvested in 2003 in Denmark, Holland, New Zealand, and the United States revealed the preva- lence of Verticillium spp. in commercial spinach seed. Sixty-eight lots (89%) were infected with Verticillium spp. at incidences ranging from 0.3 to 84.8%. Five spinach seed isolates of V. dahl- iae were pathogenic on each of three spinach cultivars by root-dip inoculation. V. dahliae was detected on 26.4% of the seed from 7 of 11 inoculated plants but on none of the seed from 6 control plants, demonstrating systemic movement of V. dahliae. Seed-to-seed transmission was also demonstrated by planting naturally infected seed lots. This is the first report of Verticillium wilt of spinach in the primary region of spinach seed production in the United States.
The recent phase-out of the soil fumigant methyl bromide (MB) due to its impact on
stratospheric ozone presents a huge challenge to strawberry nursery producers. We
evaluated the effectiveness of alternative fumigants on soil pests and plant productivity,
as well as production costs in California strawberry nurseries. Our trials followed
nursery stock through low- and high-elevation phases of runnerplant propagation and
a complete cycle of fruit production in coastal fields. Plant yields from the nurseries
and fruit yields from Oxnard and Watsonville indicated that nursery plots treated
with iodomethane plus chloropicrin, with 1,3-dichloropropene followed by dazomet,
and with chloropicrin followed by dazomet produced runner-plant yields that were similar
to methyl bromide plus chloropicrin. However, our economic analysis suggests that
nursery profitability may nonetheless suffer from the loss of methyl bromide.
Gibellulopsis, a suitable genus for Verticillium nigrescens, and Musicillium, a new genus for V. theobromae. -Nova Hedwigia 85: 463-489. Abstract: Verticillium nigrescens is not congeneric with Verticillium s. str., as shown in cladograms based on LSU and ITS sequences. Both genera and a few other taxa form a separate, new family, the Plectosphaerellaceae, in which Plectosphaerella cucumerina is the only known teleomorph taxon. The species is conspecific with Gibellulopsis piscis, which provides a suitable generic name. The available isolates are not completely homogeneous as shown in tef1 analysis. This frequent saprotrophic species is neotypified with an isolate from a Dutch soil. It has hyaline, rather short conidiophores arising from vegetative hyphae generally in terminal position, with one or two scant whorls of aculeate phialides. Brown, intercalary or terminal chlamydospores give the colony a more or less dark grey aspect. In the same family, the causal agent of cigar-end rot of bananas, Verticillium theobromae, is also generically distinct and described in the new genus Musicillium. The species is also neotypified with an Egyptian isolate. It has long, brown conidiophores, bearing several nodes, each with whorls of 3-6 aculeate phialides, no chlamydospores, but chains of monilioid cells that gradually darken.
For the 1980–2003 period, we analyzed the relationship between crop yield and three climatic variables (minimum temperature,
maximum temperature, and precipitation) for 12 major Californian crops: wine grapes, lettuce, almonds, strawberries, table
grapes, hay, oranges, cotton, tomatoes, walnuts, avocados, and pistachios. The months and climatic variables of greatest importance
to each crop were used to develop regressions relating yield to climatic conditions. For most crops, fairly simple equations
using only 2–3 variables explained more than two-thirds of observed yield variance. The types of variables and months identified
suggest that relatively poorly understood processes such as crop infection, pollination, and dormancy may be important mechanisms
by which climate influences crop yield. Recent climatic trends have had mixed effects on crop yields, with orange and walnut
yields aided, avocado yields hurt, and most crops little affected by recent climatic trends. Yield-climate relationships can
provide a foundation for forecasting crop production within a year and for projecting the impact of future climate changes.
Wild plant species are often adapted to more stressful environments than their cultivated relatives. Roots are critical in
exploiting soil resources that enable plants to withstand environmental stresses, but they are difficult to study. Cultivated
lettuce (Lactuca sativa L.) and wild L. serriola L. differ greatly in both shoot and root characteristics. Approximately 100 F2:3 families derived from an interspecific cross were evaluated in greenhouse and field experiments. In the greenhouse, root
traits (taproot length, number of laterals emerging from the taproot, and biomass) and shoot biomass were measured 4 weeks
after planting. In the field, plants were grown for 9 weeks (close to harvest maturity of the cultivated parent); mild drought
stress was induced by withholding water for 1 week, and gravimetric moisture of soil was then determined for five depth increments
between 0–100 cm. The families were genotyped using codominantly scored AFLP markers distributed throughout the genome. Composite
interval mapping was used to analyze marker-trait associations. Quantitative trait loci were identified for differences between
wild and cultivated lettuce for root architectural traits and water acquisition. Thirteen QTL were detected that each accounted
for 28–83% of the phenotypic variation. The loci for taproot length (i.e., cm taproot length g–1 plant biomass) and the ability to extract water from deep in the soil profile co-localized in the genome. These coincident
loci were identified in separate experiments. The wild L. serriola is therefore a potential source of agriculturally important alleles to optimize resource acquisition by cultivated lettuce,
thereby minimizing water and fertilizer inputs and ultimately enhancing water quality.
The Agricultural Research Service, U.S. Department of Agriculture and the University of California, Davis, announce the release of two breeding lines of lettuce (Lactuca saliva L.). Lines RH08-0472 and RH08-0475 are F9 iceberg-type lettuce breeding lines with resistance to verticillium wilt caused by V. dahliae. They were selected from the cross ‘Tiber’ x (‘La Brillante’ x ‘Pacific’). Resistance is derived from ‘La Brillante’, a Batavia-type lettuce cultivar with resistance to Race 1 isolates of V. dahliae. These breeding lines are suitable for commercial production in both V. dahliae infested and non-infested fields. They are being released for commercialization of verticillium wilt-resistant iceberg cultivars and for use as parents in further breeding of resistant cultivars
Verticillium wilt of lettuce caused by Verticillium dahliae can cause severe
economic damage to lettuce producers. The pathogen exists as two races (Races 1 and 2)
in lettuce, and complete resistance to Race 1 is known. Resistance to Race 2 isolates has
not been reported, and production of Race 1-resistant cultivars will likely increase the
frequency of Race 2 strains. The objective of this research was to select lettuce accessions
for resistance to Race 2 isolates of V. dahliae. Two independent populations totaling 314
randomly sampled PIs were evaluated for Verticillium wilt disease incidence (DI) caused
by V. dahliae isolate VdLs17 in one unreplicated and two replicated greenhouse experiments.
Selection for PIs with reduced DI was conducted between each experiment
and plant stems were plated on semiselective media to identify colonized plants that
remained non-symptomatic. No accession with complete resistance was identified,
although accessions with partial resistance were selected. Genetic variation for the
frequency of V. dahliae-colonized plants that remain symptomless was detected. Four PIs
(169511, 171674, 204707, and 226641) were selected for further testing in three replicated
greenhouse experiments and demonstrated significantly lower disease incidence than the
susceptible control cultivars. The results indicate that lettuce has genetic variation for
partial resistance to a Race 2 isolate of V. dahliae. The resistant PIs selected in this
research are morphologically diverse, and no dependence between rate of bolting and
resistance was found. PIs with partial resistance may be useful for breeding lettuce
cultivars with resistance to Race 2 isolates of V. dahliae.
Pezizomycotina is the largest subphylum of Ascomycota and includes the vast majority of filamentous, ascoma-producing species. Here we report the results from weighted parsimony, maximum likelihood and Bayesian phylogenetic analyses of five nuclear loci (SSU rDNA, LSU rDNA, RPB1, RPB2 and EF-1α) from 191 taxa. Nine of the 10 Pezizomycotina classes currently recognized were represented in the sampling. These data strongly supported the monophyly of Pezizomycotina, Arthoniomycetes, Eurotiomycetes, Orbiliomycetes and Sordariomycetes. Pezizomycetes and Dothideomycetes also were resolved as monophyletic but not strongly supported by the data. Lecanoromycetes was resolved as paraphyletic in parsimony analyses but monophyletic in maximum likelihood and Bayesian analyses. Leotiomycetes was polyphyletic due to exclusion of Geoglossaceae. The two most basal classes of Pezizomycotina were Orbiliomycetes and Pezizomycetes, both of which comprise species that produce apothecial ascomata. The seven remaining classes formed a monophyletic group that corresponds to Leotiomyceta. Within Leotiomyceta, the supraclass clades of Leotiomycetes s.s. plus Sordariomycetes and Arthoniomycetes plus Dothideomycetes were resolved with moderate support.
The Sordariomycetes is one of the largest classes in the Ascomycota, and the majority of its species are characterized by perithecial ascomata and inoperculate unitunicate asci. It includes more than 600 genera with over 3000 species and represents a wide range of ecologies including pathogens and endophytes of plants, animal pathogens and mycoparasites. To test and refine the classification of the Sordariomycetes sensu Eriksson (2006) Eriksson OE, ed. 2006. Outline of Ascomycota—2006. Myconet 12:1–82. [Google Scholar], the phylogenetic relationship among 106 taxa from 12 orders out of 16 in the Sordariomycetes was investigated based on four nuclear loci (nSSU and nLSU rDNA, TEF and RPB2), using three species of the Leotiomycetes as outgroups. Three subclasses (i.e. Hypocreomycetidae, Sordariomycetidae and Xylariomycetidae) currently recognized in the classification are well supported with the placement of the Lulworthiales in either a basal group of the Sordariomycetes or a sister group of the Hypocreomycetidae. Except for the Microascales, our results recognize most of the orders as monophyletic groups. Melanospora species form a clade outside of the Hypocreales and are recognized as a distinct order in the Hypocreomycetidae. Glomerellaceae is excluded from the Phyllachorales and placed in Hypocreomycetidae incertae sedis. In the Sordariomycetidae, the Sordariales is a strongly supported clade and occurs within a well supported clade containing the Boliniales and Chaetosphaeriales. Aspects of morphology, ecology and evolution are discussed.
Eight tomato (Lycopersicon esculentum Mill.) genotypes were evaluated based on shoot dry weight for resistance to four isolates of Verticillium dahliae Kleb. race 2 in two greenhouse seedling experiments. The race 2 isolates, obtained from North Carolina, Brazil, and Spain, demonstrated no differences in pathogenicity on the eight lines tested, thus precluding the identification of a third V. dahliae race in this collection. However, highly significant differences in virulence were observed among the isolates. The Brazilian isolate was the most virulent. No tomato genotype showed resistance comparable to that conferred by the single dominant Ve gene to V. dahliae race 1. While all tomato lines were susceptible to all race 2 isolates tested, there were significant differences in susceptibility equal to differences in levels of resistance. IRAT L3, Morden Lac, Okitsu Sozai, and `UC82' significantly outperformed the lowest ranking line XXIV-a. `Earlypak 7', Morden Mel, and Philippine 2 performance was statistically indistinguishable from that of either the highest- or lowest-ranked lines. Genetic diversity in the host and pathogen and environmental conditions favoring the pathogen likely contributed to the genotype × isolate interactions observed in Expt. 1. These results suggest using diverse isolates when screening for improved race 2 resistance.
Lettuce (Lactuca sativa L.) is an important crop used for fresh and processing markets in Italy and is grown on more than 21,000 ha. During October and November of 2006, wilt symptoms were observed on field-grown lettuce, cv. Estelle, in Forlì, Emila Romagna (northeastern Italy) and on cv. Ballerina grown under plastichouses in Piedmont (northwestern Italy). Both lettuce cultivars were of a butterhead type. Affected plants were stunted and developed yellow leaves with brown or black streaks in the vascular tissue. Yellowing started from the external leaves. Discoloration was observed in the vascular tissue of roots, crown, and leaves. A fungus was consistently and readily isolated from symptomatic vascular tissue, previously disinfested in 1% sodium hypochlorite, when cultured on potato dextrose agar (PDA). Microscopic observations revealed hyaline hyphae with many ovoid, dark microsclerotia measuring 32 to 43 × 16 to 26 μm developing after 15 days of growth at 18°C in the dark. Conidiophores showed two verticils of three elements. Conidia were hyaline, elliptical, single celled, and measured 3.5 to 8.5 × 1.8 to 4.3 μm (average 5.5 × 2.5 μm). According to its morphological characteristics, the fungus was identified as Verticillium dahliae (2). Healthy, 20-day-old lettuce plants, cvs. Principessa and Maxima, both belonging to the butterhead type, were separately inoculated by root dip with a conidial suspension (10 ⁶ /ml) of two isolates of V. dahliae isolated, respectively, at Forlì and Torino. Noninoculated lettuce plants served as control treatments. Plants (10 per treatment) were grown in pots (10-liter vol.) in a steam-disinfested peat/perlite/sand (3:1:1 vol/vol) substrate and were maintained in a glasshouse at temperatures ranging between 17 and 22°C and relative humidity ranging between 60 and 70%. First wilt symptoms and vascular discoloration in the roots, crown, and veins developed 40 days after the artificial inoculation. Forty percent of the plants were affected in the case of cv. Maxima and 30% for cv. Principessa. Noninoculated plants remained healthy. The pathogenicity tests were repeated twice. To our knowledge, this is the first report in Italy of Verticillium wilt on lettuce. The disease has been previously reported in Greece (1) and the United States (3). Currently, Verticillium wilt of lettuce seems restricted in Italy to very few farms in the two locations; moreover, its incidence is very low (0.05%).
References: (1) E. K. Ligoxigakis et al. Phytoparasitica 30:141, 2002. (2) G. F. Pegg and B. L. Brady. Verticillium Wilts. CABI Publishing, Wallingford, UK, 2002. (3) G. E. Vallad et al. Plant Dis. 89:317, 2005.
Rowe and Powelson (21) identify five key areas that need attention for advancing successful management of Verticillium wilt in changing times. (i) Suppression of soilborne inoculum: some very effective management practices are already available and practiced by a number of growers; namely, green manures. (ii) Clean planting stock: with the possible exception of a few special situations, we doubt that availability of clean planting stock will be a significant management tool for this disease. (iii) New plant-protection chemicals: although desirable, the current toughening environment restrictions on pesticide use, with little relaxation in sight, make this an unlikely avenue in the future. (iv) Enhanced host resistance: a selection of resistant and highly tolerant, high-quality cultivars are available already. Tolerant cultivars such as Ranger Russet can be grown successfully with current management practices. Pathogens are not static and eventually adapt to most resistant cultivars; therefore, continuous efforts at enhancing host resistance always will be necessary. (v) Improved pathogen monitoring tools: current soil and plant assay methods are both reliable and reproducible in competent hands. Improvements in any or all of these areas are both desirable and welcome. However, the available current management practices can manage this important disease effectively. A number of growers are using them with success. In coming years, continued work with green manures and rotations is strongly recommended and should be encouraged wherever possible. Historically, green manures have shown long-term benefits to soil and have been found equal to and even superior to conventional pesticide controls. Investigations into this key area of agriculture offer many potential rewards that can extend all the way from a basic understanding of the interactions of plants and soil microflora to plant nutrition. Although plant nutrition commonly is ignored by plant pathologists, an understanding of nutrition is paramount to an understanding of Verticillium spp. control-particularly in potato.
Epidemics of Verticillium wilt in pepper fields of the central coast of California and isolates of Verticillium dahliae associated with these epidemics were characterized. The mean incidence of wilted plants per field ranged from 6.3 to 97.8% in fields with Anaheim, jalapeno, paprika, or bell peppers. In general, incidence of wilt in jalapeno and bell pepper crops was lower than in crops of other types of pepper. Inoculum density of V. dahliae in the surveyed pepper fields ranged from 2.7 to 66.6 microsclerotia g(-1) dry soil, and the correlation between disease incidence and density of microsclerotia was high (r = 0.81, P < 0.01). Distribution of Verticillium wilt was aggregated in a majority of the pepper fields surveyed, but the degree of aggregation varied. Vegetative compatibility group (VCG) characterization of 67 isolates of V dahliae indicated that 67% belonged to VCG 2, 22% to VCG 4, and 11% to a new group, designated VCG 6. The pathogenicity of isolates of V dahliae from bell pepper and tomato plants was tested by inoculating 1-month-old bell pepper (cv. Cal Wonder) and tomato (cv. EP 7) seedlings and incubating the inoculated plants in the greenhouse. Seedlings of bell pepper were susceptible only to the isolates of V dahliae from pepper, whereas seedlings of tomato were susceptible to both pepper and tomato isolates. Pepper isolates belonging to VCG 2, VCG 4, and VCG 6 were highly pathogenic to bell pepper and chili pepper. Temperatures between 15 and 25degreesC were optimal for mycelial growth of a majority of isolates of V dahliae. Molecular characterization of pepper isolates of V dahliae using a polymerase chain reaction (PCR)-based random amplified polymorphic DNA (RAPD) technique revealed minor variation among these isolates, but unique polymorphic banding patterns were observed for isolates belonging to VCG 6. Verticillium wilt of pepper is a major production constraint in the central coast of California. More aggressive isolates of V dahliae may have been selected in this region as a result of intensive cropping practices.
The effects of dry, fresh, and no broccoli amendments on Verticillium dahliae microsclerotia in soil were evaluated at 10, 15, 20, 25, 30, and 35°C. Aliquots of 25 g of field soil naturally infested with V. dahliae microsclerotia (80 to 100 per gram of soil) were placed in plastic bottles to which dry (1% wt/wt), fresh (8.7% wt/wt), or no broccoli treatments were randomly assigned. One randomly chosen set of each treatment was assigned to the above temperatures and incubated for 45 days. The soil was subsequently assayed for microsclerotia using the Anderson sampler technique. The experiment was repeated nine times. At all temperatures, both dry and fresh broccoli significantly (P < 0.05) reduced the number of microsclerotia in soil compared with the unamended soil. At ≤30°C, fresh broccoli had a significantly (P < 0.05) greater remissive effect than dry broccoli. At 35°C, however, the number of V. dahliae microsclerotia were significantly (P < 0.05) reduced after 45 days in unamended soil; both fresh and dry broccoli almost completely eliminated the pathogen from soil. Temporal dynamics of the V. dahliae microsclerotia as affected by the treatments at the above temperatures were determined in nine experiments identical to the above by sampling and assaying soil three times at 15-day intervals. Regardless of the temperature, maximum reductions in the number of microsclerotia in treatments involving broccoli occurred within 15 days, with fresh broccoli providing significantly (P < 0.05) greater reductions. Further reductions in the number of microsclerotia were not significant in broccoli treatments at the subsequent sampling dates. In unamended soil, the number of microsclerotia at all temperatures except 35°C changed little throughout the experiment. The optimum temperatures for the broccoli-mediated reductions in V. dahliae microsclerotia in both experiments were 25 and 30°C. Four greenhouse experiments were conducted to test the effectiveness of broccoli treatments in reducing wilt incidence in cauliflower. Consistently, cauliflower plants in the fresh broccoli treatment were taller, bad greater root and shoot weights, were significantly more robust, and had the least number of infected plants (number of microsclerotia was lower) than in other treatments. The number of infected plants in the dry broccoli treatment was intermediate, similar to the number of microsclerotia. For maximal reductions in soilborne V. dahliae microsclerotia and the subsequent lower wilt incidence in cauliflower, the broccoli residue incorporation should occur when the temperatures are at least 20°C.
Improved understanding of the genetic diversity within fungi in the genusVerticillium has resulted from recent studies based on vegetative compatibility analysis and several techniques of molecular biology.
Although the method used to identify vegetative compatibility groups (VCGs) does affect the results, vegetative compatibility
appears to be a stable characteristic among isolates. Fairly low VCG diversity has been detected withinV. dahliae andV. albo-atrum using nitrate non-utilizing mutants. VCGs do not appear to be related to pathogenicity to particular host species, with the
exception ofV. albo-atrum on alfalfa. However, there is some correlation with virulence on certain hosts and with the ability ofV. dahliae to interact with root-lesion nematodes. Studies based on DNA analysis indicate thatV. dahliae andV. albo-atrum are closely related but separate species. Restriction fragment length polymorphism (RFLP) studies have identified several
subspecific groups withinV. dahliae, including two non-host-adapted groups and two that are host-adapted. They also have confirmed that alfalfa strains ofV. albo-atrum are a distinct subgroup that is probably a separate population of clonal origin. Using polymerase chain reaction (PCR), a
second non-host-adapted subgroup withinV. albo-atrum was identified that was previously unknown.
A survey of common and uncommon weed species usually showing Verticillium wilt symptoms was carried out during 1992–2000 in
Crete, Greece.Verticillium dahliae was isolated in 48 out of 182 sampled fields, in which several weed species were grown, from several locations in Oropedio,
Lasithi. Altogether, 124 isolates ofV. dahliae were recovered from the vascular stem-tissue of 19 weed species, belonging to ten botanical families. Pathogenicity trials
with 13 out of 19 weed species that have never been reported as hosts of the fungus, using for inoculation isolates which
originated from the same weed species, resulted in infection of all of them, showing various disease symptoms. Seven weed
species (Anthemis melanolepis, Cardaria draba, Convolvulus arvensis, Erodium sp.,Euphorbia helioscopia, Helminthotheca echioides andSinapis alba) are new hosts worldwide, and six additional species (Euphorbia sp.,Lactuca serriola, Raphanus raphanistrum, Sinapis arvensis, Sonchus oleraceus andTrifolium sp.) are new hosts for Greece. The most susceptible (isolation frequency: 27.9–52.8%, moderate disease severity) species
were:Capsella bursa-pastoris, C. draba, Chenopodium album, Senecio vulgaris andSolanum nigrum. Less susceptible (isolation frequency: 4.8–17.8%, slight disease severity) were:Amaranthus sp.,A. melanolepis, C. arvensis, Erodium sp.,Euphorbia sp.,E. helioscopia, H. echioides, L. serriola, Malva sylvestris, R. raphanistrum, S. alba, S. arvensis, S. oleraceus andTrifolium sp. Some species —C. draba, C. album, L. serriola andS. nigrum L. — that usually showed external and vascular wilt symptoms, occasionally exhibited only reduced growth. Visible symptoms
under natural field conditions in all 13 weed species that had never been reported as hosts ofV. dahliae were similar to those observed after their artificial inoculation. The fungus was not isolated fromFoeniculum vulgare ssp.piperitum, Oxalis corniculata andStellaria media, among other species.
The incidence of wilt was recorded in runner and fruiting crops of 13 strawberry cultivars at 72 locations in southern England in 1989 and 1990, and soil samples from the sites were analysed for Verticillium dahliae. Linear regressions of wilt incidence on inoculum concentration in soil for runner crops of the susceptible cv. Elsanta in both years were significant whilst that for runner crops of the susceptible cv. Hapil in 1989 approached significance; the regression for cv. Elsanta fruiting crops in 1990 was not significant. The inclusion of sand content of soil in the regression model improved the fit for the cv. Hapil data but not for the cv. Elsanta data; neither clay nor silt content of soil significantly improved the fit of the models for any data set. There were insufficient data in either year for regression analysis for other cultivars, but the levels of wilt generally corresponded with the degree of soil infestation and broadly reflected known field resistance. The data were used to estimate an inoculum concentration which corresponds to 5% wilt incidence (IC5) for cv. Elsanta. It is suggested that this could be used as a yardstick for determining the risk of unacceptable levels of wilt in susceptible cultivars on the basis of pre-planting soil analysis. For the 44 sites where the cropping history over the 15 years prior to soil analysis was available there was no clear association between any crop and soil infestation levels at or above the IC5. However, V. dahliae was more common at sites with a history of vegetatively propagated crops than at sites which had only supported crops grown from true seed.
A fungus isolated from leaves of barley was determined to be Verticillium dahliae. Its pathogenicity was compared with that of known isolates of this fungus from tomatoes, cotton, and potatoes, and with members of various vegetative compatibility groups. Among small grain cereals, oats and barley were susceptible. Wheat was infected but did not develop severe symptoms. Wounding of roots facilitated infection, but was not required. (…)
Symptoms of Verticillium wilt on inoculated plants of the sunflower cultivar Sunrise appeared sooner and were more severe under long days (16 h light, 8 h dark), or short days with interrupted dark period (10 h light, dark, light, dark), than under short days (10 h light, 14 h dark). Uninoculated control plants flowered in 45 to 47 days under short-day conditions, but reached only the yellow bud stage in 50 to 55 days under long days or short days with interrupted dark period. Development of Verticillium wilt of sunflower is not dependent on the initiation of flowering as it is in some other hosts.
Sunflower seed harvested from diseased plants in a Verticillium nursery in Manitoba, surface-sterilized and plated on potato sucrose agar, gave rise to colonies of Verticillium albo-atrum Reinke and Berth. Microsclerotia of the organism were confined to the hull and testa of the infested seeds. The fungus was not found in any portion of the seed inside the testa. Approximately 50 per cent of the infested seeds germinated on the agar plates; the rest apparently were dead. Symptoms typical of Verticillium infection developed in sunflower plants in a plot on clean soil sown with seed harvested from diseased plants in a Verticillium nursery. No diseased plants were found in adjacent plots sown with seed from healthy plants.Failure to isolate the pathogen from seed taken from typically diseased plants in a farm field near the Verticillium nursery may signify that seed infestation occurs only under favorable circumstances. The factors predisposing to such infestation are not yet known. Lack of consistency in isolation of the pathogen from seed taken from diseased plants from various sources parallels the contradictory results obtained by workers studying seed dissemination of V. albo-atrum in other crops.
Root architecture is described for intact root systems of cultivated (Lactuca sativa L.) and wild (L. serriola L.) lettuce, grown for 5 to 6 weeks in greenhouse pot and cylinder experiments in coarse-textured soil. L. sativa cv. Salinas and a sclinas line of L. serriola attained the same biomass at 4 to 6 weeks after planting. Root biomass allocation was also similar, but root architecture differed. In the top zone along the tap root (0 to 5 cm), Salinas tended to produce more laterals, a greater total root length, and more external links (segments that originate at a branch point and end in a meristem) than wild lettuce. In the 5 to 55cm zone of the tap root, these measures were greater in the wild species. These patterns of root structure were generally corroborated by a second cylinder study with a different pair of L. sativa and L. serriola. Regressions of root structural traits were made against total root dry weight as a means to compare root architecture independently of the size of the root system. Regressions demonstrated that production of root segments differed between the two species; for example the slope for the regression of summed external link length in the top 0 to 5 cm with total root dry weight was significantly higher for Salinas, indicating that the rate of construction in the top 0 to 5cm was greater for cultivated than for wild lettuce. Yet, from 5 cm depth to the tap root tip, the rate of construction of external link length was greater in L. serriola. For many of these types of regression, r2 and mean slope ± SE suggested that more variation occurred in cultivated than wild lettuce, yet genetic heterogeneity was probably low within the studied taxa. Inadvertent selection may have occurred in the breeding of cultivated lettuce varieties for increased root growth in the surface zone where water and fertilizer are applied, and for greater plasticity in construction of root segments, which might maximize the efficiency of exploitation of soil moisture and nutrients.
Seven wild Lactuca species: L. serriola L., L. aculeata Boiss. & Ky., L. scarioloides Boiss., L. azerbaijanica Rech., L. georgica Grossh., L. dregeana DC. and L. altaica Fisch. & C.A. Mey., are taxonomically closely related to the cultivated lettuce. Together with L. sativa they form a distinct natural group. Only scanty information is available on genetic affinities among the various members of this group. However, the combined taxonomic and genetic evidence indicates that all these 7 wild lettuces are very likely interfertile (or largely interfertile) with the crop, and constitute its primary wild gene pool. Also the taxonomically more distant L. saligna L. is partly cross-fertile with the crop.
Lettuce (Lactuca sativa L.) is a major fresh vegetable and its leaves are commonly found in salad mixtures and sandwiches. In some eastern countries
like China and Egypt, stems instead of leaves of lettuce are consumed, either cooked, raw, pickled, dried, or as a sauce.
Some less common uses for lettuce include a cigarette without nicotine made from lettuce leaves, edible oil extracted from
seeds of a primitive lettuce, and a sedative made of dried latex contained in stems and other tissues. Lactucarium, the dried
latex produced from a wild lettuce relative, Lactuca virosa L., is used to make a sleep-inducing medicine (Ryder, 1986).
An acidic endochitinase gene (pcht28) isolated from Lycopersicon chilense was introduced into tomato (L. esculentum) through Agrobacterium-mediated transformation, using the CAMV 35S promoter. Transgenic plants demonstrated a high level of constitutive expression
of pcht28 and chitinase enzyme activity. Kanamycin-resistant R1 plants (resulting from self-pollination of transgenic plants)
as well as R2 plants were evaluated for their tolerance to Verticillium dahliae (race 1 and 2 for R1 plants and race 2 for R2 plants) in the greenhouse. They demonstrated a significantly (P<0.05) higher level of tolerance to the fungi compared to the nontransgenic plants, as measured by foliar disease symptoms,
vascular discoloration, and vascular discoloration index. The transgenic plants produced in this study represent a source
of genetic resistance to Verticillium dahliae.
A broad survey of available world literature showed that at least 98 wildLactuca spp. (Asteraceae) have been described taxonomically. The distribution of the genusLactuca worldwide includes 17 species in Europe, 51 in Asia, 43 in Africa, and 12 in the Americas (mostly the North American subcontinent).
Species originating in Asia, Africa, and the Americas form ca. 83% of knownLactuca spp. richness; however, they are very poorly documented from the viewpoint of taxonomic relationships, ecogeography, and
variability. The phytogeography ofLactuca spp. regarding their distribution on different continents and in relation to the structure of the lettuce gene pool is discussed.
A more detailed analysis of geographical distribution and habitats is given for some species (L. serriola, L. saligna, L. virosa, L. perennis, L. quercina, L. tatarica), which represent the primary, secondary, and tertiary gene pools of cultivated lettuce (L. sativa). Original and synanthropic distributions ofLactuca spp. and their occurrence in natural and secondary habitats are discussed, along with the representation of wildLactuca spp. in world gene-bank collections. Global biodiversity ofLactuca spp. and their representation in germplasm collections are poorly documented. Future studies of taxonomy, phytogeography,
ecology, phylogenetic relationships, and genetic diversity are needed for a more complete understanding of this genus and
taxonomically related genera.
Discusses self/nonself recognition in the sexual cycle, with emphasis on the ascomycetes, Neurospora crassa, Podospora anserina and Cochliobolus heterostrophus. The recent cloning and sequencing of the mating-type genes from these organisms has allowed the clarification of molecular aspects regarding the regulation of the sexual cycle. The sex cycle can generate genetic diversity that can adversely affect resistance breeding or chemical control strategies. Sexual reproduction results in ascospores that can function as overwintering structures or infective propagules and can be an important component of the disease cycle. The second section of this review covers vegetative incompatibility and its role in self/nonself recognition during asexual reproduction. The genetics of N. crassa, Aspergillus nidulans and P. anserina, and the pouplation dynamics of Cryphonectria parasitica, Ophiostoma ulmi and several species of Fusarium are emphasized. -from Authors
A wilt disease of sainfoin caused by Verticillium Dahliae Kleb. is described, and it is shown that the fungus can penetrate sainfoin seedlings through unwounded roots as well as through ruptures caused by the emergence of lateral rootlets. V. Dahliae was isolated from naturally infected soil only in June, July and August, although another species, V. nigrescens , was obtained throughout the year.
Comparative studies of the longevity of cultures of V. Dahliae, V: albo‐atrum and V. nigrescens proved that all are viable for at least 3 years on agar media. On sterilized wheat grains V. Dahliae dies within 8 weeks after inoculation, V. albo‐atrum and V. nigrescens within 12 weeks, while the hyaline variants of the first two remain viable for 6 months.
Evidence was obtained that in artificially inoculated soil V. Dahliae persists mainly as microsclerotia. The fungus may also exist in the soil as hyaline mycelium or conidia, but only for a relatively short time.
The incidence of this disease in sainfoin is reduced by an increase in soil‐water content, but is unaffected by the application of lime to the soil.
Summary Ethylene evolved during compatible or susceptible disease interactions may hasten and/or worsen disease symptom development; if so, the prevention of disease-response ethylene should reduce disease symptoms. We have examined the effects of reduced ethylene synthesis on Verticillium wilt (causal organism, Verticillium dahliae) of tomato by transforming tomato with ACC deaminase, which cleaves ACC, the immediate biosynthetic precursor of ethylene in plants. Three promoters were used to express ACC deaminase in the plant: (i) CaMV 35S (constitutive expression); (ii) rolD (limits expression specifically to the site of Verticillium infection, i.e. the roots); and (iii) prb-1b (limits expression to certain environmental cues, e.g. disease infection). Significant reductions in the symptoms of Verticillium wilt were obtained for rolD- and prb-1b-, but not for 35S-transformants. The pathogen was detected in stem sections of plants with reduced symptoms, suggesting that reduced ethylene synthesis results in increased disease tolerance. The effective control of formerly recalcitrant diseases such as Verticillium wilt may thus be obtained by preventing disease-related ethylene production via the tissue-specific expression of ACC deaminase.
Metam sodium (sodium N-methyl dithiocarbamate, metam-Na) is widely used in agricultural and floricultural production for controlling soilborne plant pathogens, parasitic nematodes, and weeds. It undergoes rapid decomposition to the biocide methyl isothiocyanate (MITC) in moist soils. In this study, the efficacy of 12 concentrations of metam-Na (10 to 2,650 μmol kg-1 soil) to control seeds or tubers of five major weed species, three soilborne pathogens, and one parasitic nematode was evaluated in a sandy loam soil under controlled conditions. Soils were exposed to the fumigant in microcosms for 24 h at 10 and 20°C. Generation and dissipation curves of MITC in soil under controlled conditions showed that MITC concentrations in soils were highest 2 h after metam-Na application and decreased steadily over the 24-h incubation period. After 24 h, remaining MITC concentrations in soil microcosms at 10 and 20°C were 53 and 38% of the original amount applied, respectively, indicating a 20% reduction in MITC dissipation at the lower soil temperature. Logistic dose-response models were used to estimate the effective concentration necessary to reduce soil pest viability by 50 (LC₅₀) or 90 (LC₉₀) percent under both temperatures. Seed of Portulaca oleracea, with LC₉₀ values of < or =1,242 μmol kg-1 soil, was the most sensitive to soil fumigation with metam-Na, followed by Polygonum arenastrum with LC₉₀ values of < or =1,922 μmol kg-1 soil. At 10°C fumigation temperature, metam-Na at the highest dose tested in this study, 2,650 μmol kg-1 soil, was not sufficient to achieve adequate control of Stellaria media and Malva parviflora seed and Cyperus esculentus tubers. Weed control efficacy (average reduction in LC₉₀ values) of metam-Na was between 25 and 60% higher if soils were fumigated at 20°C compared with 10°C, with the exception of M. parviflora. Phytophthora cactorum and Pythium ultimum were more sensitive to soil fumigation with metam-Na (LC₉₀ < or = 165 μmol kg-1 soil) than Verticillium dahliae (LC₉₀ < or = 737 μmol kg-1 soil). The nematode Tylenchulus semipenetrans was highly sensitive to soil fumigation with metam-Na (LC₉₀ < or = 98 μmol kg-1 soil), and the efficacy of control increased by 30% if soil was fumigated at 20°C compared with 10°C. In this sandy loam soil, metam-Na at a concentration of 850 μmol kg-1 reduced the viability of Portulaca oleracea and Polygonum arenastrum seeds, C. esculentus tubers, and all soilborne pathogens and parasitic nematodes tested by 90% at 20°C after 24 h exposure. These results indicate that metam-Na can provide effective pest and disease control at maximum label rate for the commercial formulation, but there was a reduction in efficacy at low temperature.
The influence of crop rotation on soilborne diseases and yield of strawberry (Fragaria x ananassa) was determined at a site infested with Verticillium dahliae microsclerotia and at another with no known history of V. dahliae infestation during 1997 to 2000. The rotations studied at the V. dahliae-infested site were (i) broccoli-broccoli-strawberry, (ii) Brussels sprouts-strawberry, and (iii) lettuce-lettuce-strawberry; the treatments at the site with no history of V. dahliae were (i) broccoli-broccoli-strawberry, (ii) cauliflower-cauliflower-strawberry, and (iii) lettuce-lettuce-strawberry. The effects of rotation on V. dahliae and Pythium populations, strawberry vigor, Verticillium wilt severity, and strawberry fruit yield were compared with a standard methyl bromide + chloropicrin fumigated control treatment at both sites. Rotations did not alter total population levels of Pythium spp. at either study site. However, V. dahliae microsclerotia were significantly reduced with br occoli and Brussels sprouts rotations compared with lettuce rotations at the V. dahliae-infested site. Reduced propagules led to lower Verticillium wilt severity on strawberry plants in the broccoli and Brussels sprouts rotations than in lettuce-rotated plots. Strawberry vigor and fruit yield were significantly lower in lettuce-rotated plots than in broccoli- and Brussels sprouts-rotated plots. Despite no detectable microsclerotia at the other site, strawberry vigor and fruit yield were greatest in plots rotated with broccoli, intermediate with cauliflower, and lowest with lettuce. None of the rotation treatments were better than the fumigated control for all variables measured. In the absence of fumigation, rotation with broccoli and Brussels sprouts is an effective cultural practice for managing Verticillium wilt in strawberry production; whereas, in fields with no detectable V. dahliae, broccoli is also a feasible rotational crop that enhances strawberry growth and yield. According to a cost-benefit analysis, the broccoli-strawberry rotation system could be an economically viable option provided growers are able to alternate years for strawberry cultivation.
Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (Lactuca sativa). In 2004 and 2005, 107 and 22 lettuce cultivars, respectively, were screened for resistance in a field infested with Verticillium dahliae, and disease progress on resistant and susceptible cultivars was determined. Greenhouse experiments were conducted to evaluate 16 cultivars for resistance to a race 1 and a race 2 isolate. Significant differences for resistance were observed within cultivated lettuce. In susceptible cultivars, disease levels increased through the season, whereas disease in resistant cultivars remained constant. Resistance in greenhouse tests was dependant upon the race used. Seven cultivars were resistant to race 1, whereas all were susceptible to race 2. Cultivar reactions to race 1 in greenhouse and field experiments were correlated, indicating the utility of greenhouse evaluations. The identification of resistance in diverse lettuce types is beneficial to the breeding process. However, because of the existence of resistance-breaking race 2 isolates, this resistance may not be durable. Alternatively, targeted releases of race-1-resistant cultivars to fields with only race 1 pathogen genotypes may extend the life of these cultivars.
Big vein is an economically damaging disease of lettuce (Lactuca sativa L.) incited by Mirafiori lettuce big vein virus, which is vectored by the soil-borne fungus Olpidium brassicae (Woronin) P.A. Dang. Resistance to this disease is needed because no feasible cultural control methods have been identified. Partial resistance is available within cultivated lettuce and is expressed as delayed appearance of symptoms in combination with a reduced percentage of symptomatic plants. Complete resistance has been identified only in accessions of L. virosa L., an incongruent wild relative of lettuce. Resistance from L. virosa has not been introgressed into lettuce. The objective of this research was to determine whether big vein resistance from L. virosa can be introgressed into lettuce. Progenies of backcross (BC) hybrids between L. virosa and L. sativa cultivars were greenhouse tested for big vein resistance over four generations of self-pollination. Selected plants from resistant BC families were used as parents to create BC2 progeny from crosses with high partial-resistant cultivars, intermediate partial-resistant cultivars, and susceptible cultivars to test for the presence of transgressive segregants. Experiments were conducted in the greenhouse by infecting seedlings with O. brassicae zoospores collected from big vein symptomatic plants. Plots were evaluated for area under the disease progress curve and the percentage of symptomatic plants; asymptomatic plants from resistant families were retained in every generation. Complete resistance to big vein was not recovered, and may be the result of insufficient sampling of BCF2 progeny or linkage between resistance alleles and alleles causing incongruity. Variation for partial resistance was observed in all BC generations, and transgressive segregants were identified among BC2 families from crosses using partially resistant and susceptible parents. This research suggests that L. virosa contains alleles that confer partial resistance to big vein when introgressed into an L. sativa background, and these alleles are distinct from those present in partially resistant lettuce cultivars. Alternative breeding strategies should be pursued to introgress complete resistance from L. virosa into cultivated lettuce.