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Microcycle conidiation — A review
Abstract
Microcycle conidiation is defined as the germination of spores by the direct formation of conidia without the intervention of mycelial growth, as occurs in most normal life cycles. It is a method of asexual spore formation in which the normal life cycle of the fungus is bypassed. Spores formed through sexual reproduction and species with unicellular thalli are not included in microcycle conidiation. The term secondary conidium or secondary spore is usually, but not always, synonymous with microcycle conidiation. In the laboratory various factors, but especially temperature, can induce the microcycle condition in such fungi asAspergillus niger, Penicillium andNeurospora crassa, providing a useful tool for research. Microcycle conidiation has also been reported in a broad range of species in nature, and comprises a normal part of the life cycle in several groups, including the Entomophthorales, Taphrinales, Clavicipitales, Uredinales, Ustilaginales, Tremellales and Exobasidiales. The presence of a microcycle in such fungi undoubtedly provides a survival mechanisn for spores that encounter unfavorable conditions.
... Variation in ascospore number is likely determined by the coordination of meiosis, mitosis and spore wall formation, and may be driven by natural selection (Davidow and Goetsch, 1978;Sherwood, 1981;Réblová and Mostert, 2007). For example, an increased ascospore number leads to a higher number of propagules for dispersal and colonization, and four-spored asci may contain heterokaryotic multinucleate and self-fertile ascospores, as in pseudohomothallic species (Hanlin, 1994;Raju and Perkins, 1994;Quijada et al., 2022). Investigating the variation in ascospore size and number in asci helps to understand the cytology, genetics and life cycle of fungi. ...
... The phenomenon of formation of conidia directly from ascospores, without mycelial growth, can be termed microcyclic conidiation (Vinter and Slepecky, 1965;Hawksworth 1987;Hanlin, 1994;Baral, 1999;Quijada, 2015;Quijada et al., 2019). However, we note that Baral (1999) defined the term "ascoconidia" strictly as conidia produced from ascospores contained within living asci (i.e., fresh specimens), in which each ascospore together with its ascoconidia are surrounded by a delicate membrane forming a ball that is violently ejected from asci as a single entity. ...
... In contrast, conidia formed from ejected ascospores or within dead asci (i.e., dried specimens), which are not arranged as balls, are simply referred to as conidia. Such conidia or ascoconidia have been suggested to be beneficially associated with drought-tolerance (Sherwood, 1981), colonization of new habitats (Quijada et al., 2022), infection of new hosts in harsh environments (Juzwik and Hinds, 1984;Hanlin, 1994) and to serve as spermatia (Hanlin, 1994). Observations of ascospore germination and conidial formation provide important information on development in fungi (Karakehian et al., 2021). ...
Spores are important as dispersal and survival propagules in fungi. In this study we investigated the variation in number, shape, size and germination mode of ascospores in Morchella galilaea , the only species of the genus Morchella known to fruit in the autumn. Based on the observation of five samples, we first discovered significant variation in the shape and size of ascospores in Morchella . One to sixteen ascospores were found in the asci. Ascospore size correlated negatively with ascospore number, but positively with ascus size, and ascus size was positively correlated with ascospore number. We noted that ascospores, both from fresh collections and dried specimens, germinated terminally or laterally either by extended germ tubes, or via the production of conidia that were formed directly from ascospores at one, two or multiple sites. The direct formation of conidia from ascospores takes place within asci or after ascospores are discharged. Using laser confocal microscopy, we recorded the number of nuclei in ascospores and in conidia produced from ascospores. In most ascospores of M. galilaea , several nuclei were observed, as is typical of species of Morchella . However, nuclear number varied from zero to around 20 in this species, and larger ascospores harbored more nuclei. One to six nuclei were present in the conidia. Nuclear migration from ascospores to conidia was observed. Conidia forming directly from ascospores has been observed in few species of Pezizomycetes; this is the first report of the phenomenon in Morchella species. Morphological and molecular data show that conidial formation from ascospores is not found in all the specimens of this species and, hence, is not an informative taxonomic character in M. galilaea . Our data suggest that conidia produced from ascospores and successive mitosis within the ascus may contribute to asci with more than eight spores. The absence of mitosis and/or nuclear degeneration, as well as cytokinesis defect, likely results in asci with fewer than eight ascospores. This study provides new insights into the poorly understood life cycle of Morchella species and more broadly improves knowledge of conidia formation and reproductive strategies in Pezizomycetes.
... Staurosporous or branched conidia are assumed to enhance the competitive advantage of fungal encounters with the substrata in their ecological niches (Wu and Sutton 1995;Ingold 1975). Microcycle conidiation is presumed to contribute to survival under suboptimal conditions, such as drought, reduced nutrition, and pH (Hanlin 1994). We con rmed microcycles in vivo when cultured in a low-nutrient medium such as 2% water agar, suggesting that the microcycle of Camptophora might also function in a similar manner. ...
... We re-evaluated the ITS sequence comparison among Camptophora species with unidenti ed strains derived from previous environmental investigations and eDNA sequences and found no regional or substrate speci city for each lineage (Fig. 9). This distribution may be explained by the presence of microcycles in their life cycles, as proposed by Hanlin (1994). ...
The generic variety and habitats of Camptophora species, generally known as black yeasts have not been clarified. Here, we re-evaluated Camptophora based on morphological observations and phylogenetic analyses. Because investigations on Camptophora relied only on a few strains/specimens, twenty-four Camptophora -related strains were newly obtained from 13 leaf samples from various plant species to redefine the generic and species concepts of Camptophora . Their molecular phylogenetic relationships were examined based on the small subunit nuclear ribosomal DNA (nSSU, 18S rDNA), internal transcribed spacer rDNA operon (ITS), large subunit nuclear ribosomal DNA (LSU, 28S rDNA), β-tubulin ( tub ), the second largest subunit of RNA polymerase II ( rpb2 ), and mitochondrial small subunit DNA (mtSSU). Single- and multi-locus analyses using SSU-ITS-LSU- rpb2 -mtSSU revealed a robust phylogenetic relationship among Camptophora within the Chaetothyriaceae. Camptophora can be distinguished from other chaetothyriaceous genera by its snake-shaped conidia with microcyclic conidiation and loosely interwoven mycelial masses. Based on the results of the phylogenetic analyses, two undescribed lineages were recognised, and Ca. schimae was considered to be excluded from the genus. ITS sequence comparison with environmental DNA (eDNA) sequences revealed the distribution of the genus limited to the Asia-Pacific region. Camptophora has been isolated or detected from abrupt sources, and the reason for this was inferred to be their microcycle. Mechanisms driving genetic diversity within species are discussed with respect to their phyllosphere habitats.
... Hanlin 1994). This process was described in several fungal species, such as Aspergillus spp. ...
... MC was recognized in some groups of filamentous fungi as a specific process that leads to a rapid formation of new asexual spores following 'regular' conidiogenesis (Hanlin 1994). In fact, the production of basidiospores from teliospores of rust and smut fungi is a similar process, although it leads to sexually formed spores, following meiosis, and thus cannot be classified as MC. ...
Microcyclic conidiogenesis (MC) was recently described in several species of powdery mildew fungi. This process, defined as the production of conidia on a fungal spore without any, or only a minimal, involvement of hyphal growth, was observed on powdery mildew conidia that have already germinated on host plant surfaces and have been attached to the epidermal cells. Most probably, MC contributes to a quick propagation of young powdery mildew colonies because new conidia are sometimes produced in a shorter time on microcyclic conidiophores than on the hyphae of the young mycelium. This article reports MC in Erysiphe necator ex grapevine, Podosphaera leucotricha ex apple, Golovinomyces orontii ex tobacco, and Neoerysiphe galeopsidis ex Lamium purpureum based on light and low-temperature scanning electron microscopic studies.
... Conidia (spores) are important for fungal reproduction and survival (Papagianni 2004;Wang et al. 2016). Usually, filamentous fungi demonstrate the modes of normal and microcycle conidiation (Hanlin 1994;Jung et al. 2014). Normal conidiation is the common mode and is well known under the control of the central developmental pathway (Madi et al. 1994;Busby et al. 1996;Adams et al. 1998;Garzia et al. 2010). ...
... Normal conidiation is the common mode and is well known under the control of the central developmental pathway (Madi et al. 1994;Busby et al. 1996;Adams et al. 1998;Garzia et al. 2010). Microcycle conidiation is an asexual reproduction mode in which the fungus bypasses or greatly reduce mycelial growth under stress conditions (Hanlin 1994;Jung et al. 2014). In our previous study, we found that the acrididspecific fungal pathogen Metarhizium acridum showed microcycle conidiation on microcycle conidiation medium SYA (Zhang et al. 2010). ...
Microcycle conidiation commonly exists in filamentous fungi and has great potential for mass production of mycoinsecticides. L-Arginine metabolism is essential for conidiation and conditional growth and virulence, but its role in microcycle conidiation has not been explored. Here, a unique putative arginase (MaAGA) was characterized in the entomopathogenic fungus Metarhizium acridum. Conidial germination and thermotolerance were facilitated by the disruption of MaAGA. Despite little impact on fungal growth and virulence, the disruption resulted in normal conidiation after a 60-h incubation on microcycle conidiation medium (SYA) under normal culture conditions. In the MaAGA-disruption mutant (ΔMaAGA), intracellular arginine accumulation was sharply increased. Replenishment of the direct metabolites of arginase, namely ornithine and/or urea, was unable to restore the disruption mutant’s microcycle conidiation on SYA. Interestingly, nitric oxide synthase (NOS) activity and nitric oxide (NO) levels of the ΔMaAGA strain were markedly decreased in the 60-h-old SYA cultures. Finally, adding Nω-nitro-L-arginine, an inhibitor of NOS, into the SYA converted the microcycle conidiation of the wild-type strain to normal conidiation. In contrast, adding sodium nitroprusside, an NO donor, into the SYA recovered the mutant’s microcycle conidiation. The results indicate that arginine metabolism controls microcycle conidiation by changing the content of NO.
Key points
• The MaAGA-disruption led to normal conidiation on microcycle conidiation medium SYA.
• Nitric oxide (NO) level of the ΔMaAGA strain was markedly decreased.
• Adding an NO donor into the SYA recovered the microcycle conidiation of ΔMaAGA.
... Most fungi have two types of conidiation: normal conidiation (NC) and microcycle conidiation (MC) [10]. Normally, conidiophores arise from the vegetative mycelia and produce large amounts of conidia [11,12], which is the most common conidiation type for filamentous fungi [13]. ...
... The fungi can bypass the mycelia period in MC and develop secondary conidia from germ tubes or directly from conidial cells [1,4,14]. MC is a special survival mechanism of fungi under adverse conditions, such as high temperature [10,11,15], extreme pH [16], high salt content [17], and nutritional deficiencies [4,[18][19][20]. Among them, nutritional deficiency is the important factor that affects the fungal growth and development. ...
Conidium is the main infection unit and reproductive unit of pathogenic fungi. Exploring the mechanism of conidiation and its regulation contributes to understanding the pathogenicity of pathogenic fungi. Vib-1, a transcription factor, was reported to participate in the conidiation process. However, the regulation mechanism of Vib-1 in conidiation is still unclear. In this study, we analyzed the function of Vib-1 and its regulation mechanism in conidiation through knocking out and overexpression of Vib-1 in entomopathogenic fungus Metarhizium acridum. Results showed that the colonial growth of Mavib-1 disruption mutant (ΔMavib-1) was significantly decreased, and conidiation was earlier compared to wild type (WT), while overexpression of Mavib-1 led to a delayed conidiation especially when carbon or nitrogen sources were insufficient. Overexpression of Mavib-1 resulted in a conidiation pattern shift from microcycle conidiation to normal conidiation on nutrient-limited medium. These results indicated that Mavib-1 acted as a positive regulator in vegetative growth and a negative regulator in conidiation by affecting utilization of carbon and nitrogen sources in M. acridum. Transcription profile analysis demonstrated that many genes related to carbon and nitrogen source metabolisms were differentially expressed in ΔMavib-1 and OE strains compared to WT. Moreover, Mavib-1 affects the conidial germination, tolerance to UV-B and heat stresses, cell wall integrity, conidial surface morphology and conidial hydrophobicity in M. acridum. These findings unravel the regulatory mechanism of Mavib-1 in fungal growth and conidiation, and enrich the knowledge to conidiation pattern shift of filamentous fungi.
... Conidia are more tolerant of adverse environmental conditions, and MC is a crucial survival tactic for P. dubia under Zn 2+ stress. There are many factors that can induce the MC of fungi, such as nutrient deficiency, pH, light, and temperature [53][54][55]. While metal ions induce MC in fungi was found for the first time. ...
Utilizing mycoremediation is an important direction for managing heavy metal pollution. Zn2+ pollution has gradually become apparent, but there are few reports about its pollution remediation. Here, the Zn2+ remediation potential of Paraisaria dubia, an anamorph of the entomopathogenic fungus Ophiocordyceps gracilis, was explored. There was 60% Zn2+ removed by Paraisaria dubia mycelia from a Zn2+-contaminated medium. To reveal the Zn2+ tolerance mechanism of Paraisaria dubia, transcriptomic and metabolomic were executed. Results showed that Zn2+ caused a series of stress responses, such as energy metabolism inhibition, oxidative stress, antioxidant defense system disruption, autophagy obstruction, and DNA damage. Moreover, metabolomic analyses showed that the biosynthesis of some metabolites was affected against Zn2+ stress. In order to improve the tolerance to Zn2+ stress, the metabolic mechanism of metal ion transport, extracellular polysaccharides (EPS) synthesis, and microcycle conidiation were activated in P. dubia. Remarkably, the formation of microcycle conidiation may be triggered by reactive oxygen species (ROS) and mitogen-activated protein kinase (MAPK) signaling pathways. This study supplemented the gap of the Zn2+ resistance mechanism of Paraisaria dubia and provided a reference for the application of Paraisaria dubia in the bioremediation of heavy metals pollution.
... Environmental nutrition can also affect the conidiation mode in filamentous fungi. Most filamentous fungi have two modes of conidiation: "normal" aerial conidiation where conidia are produced from hyphae, typically on more solid substrates, and microcycle conidiation where conidia are directly produced from short hyphae and/or conidia/ blastospore, typically in nutrient-limiting liquid media (Hanlin 1994). Filamentous fungi usually undergo microcycle conidiation when challenged by adverse conditions that can include nutrient deficiency, elevated temperature, and/or other abiotic stresses (Saxena et al. 1992;Ahearm et al. 2007). ...
Carbon sources and their utilization are vital for fungal growth and development. C4-dicarboxylic acids are important carbon and energy sources that function as intermediate products of the tricarboxylic acid cycle. Transport and regulation of C4-dicarboxylic acid uptake are mainly dependent on tetracarboxylic acid transporters (Dcts) in many microbes, although the roles of Dct genes in fungi have only been partially characterized. Here, we report on the functions of two Dct genes (Dct1 and Dct2) in the entomopathogenic fungus Metarhizium acridum. Our data showed that loss of the MaDct1 gene affected utilization of tetracarboxylic acids and other carbon sources. ΔMaDct1 mutants showed larger colony sizes with extensive mycelial growth but were delayed in conidiation with decreased conidia yield as compared to the wild-type parental strain. On the nutrient-deficient medium, SYA, the wild-type strain produced microcycle conidia, whereas the ΔMaDct1 mutant produced (normal) aerial conidia. In addition, ΔMaDct1 had decreased tolerance to cell wall perturbing agents, but increased tolerances to UV-B radiation and osmotic stress. Insect bioassays indicated that loss of MaDct1 did not affect pathogenicity. In contrast, no distinct phenotypic change was observed for the MaDct2 mutant in terms of growth and biocontrol characteristics. Transcriptomic profiling between wild type and ΔMaDct1 showed that differentially expressed genes were enriched in carbohydrate and amino acid metabolism, transport and catabolism, and signal transduction. These results demonstrate that MaDct1 regulates the conidiation pattern shift and mycelial growth by affecting utilization of carbon sources. These findings are helpful for better understanding the effect of intermediates of carbon metabolism on fungal growth and conidiation.
Key points
• MaDct1 influences fungal growth and conidiation by affecting carbon source utilization.
• MaDct1 regulates conidiation pattern shift under nutrient deficiency condition.
• MaDct1 is involved in stress tolerance and has no effect on virulence.
• MaDct2 has no effect on growth and biocontrol characteristic.
... In M. acridum, the microcycle conidiation produces higher yields of conidia with higher quality than those from typical conidiation [13]. In microcycle conidiation, the new conidia are directly separated from the germinated conidia, while in typical conidiation, multicellular mycelia form and extend from the germinated conidia [8,9]. The shift to microcycle conidiation is ultimately regulated by the availability of nutrients and other stress conditions [9]. ...
Entomopathogenic fungi are promising biocontrol agents of insect-mediated crop damage. Microcycle conidiation has shown great potential in enhancing the conidial yield and quality of entomopathogenic fungi. Homologs of Cts1, an endochitinase of Saccharomyces cerevisiae, participate in cell separation in several fungal spp. and may contribute to the morphological differences that occur during the shift to microcycle conidiation. However, the precise functions of Cts1 in entomopathogenic fungi remain unclear. Herein, the endochitinase gene, MaCts1, was characterized in the model entomopathogen, Metarhizium acridum. A loss of function line for MaCts1 led to a delay of 1 h in the median germination time, a 28% reduction in conidial yield and significant defects in fungal resistances to UV-irradiation (18%) and heat-shock (15%), while fungal tolerances to cell wall stressors, oxidative and hyperosmotic stresses and virulence remained unchanged. The MaCts1-disruption strain displayed typical conidiation on the microcycle conidiation induction medium, SYA. In contrast, deletion of key genes in the morphogenesis-related NDR kinase network (MOR pathway)/regulation of Ace2 and morphogenesis (RAM pathway) did not affect the SYA-induction of microcycle conidiation. This indicates that MaCts1 makes contributions to the microcycle conidiation, which may not be dependent on the MOR/RAM pathway in M. acridum.
... Two conidiation patterns, the normal conidiation pattern and the microcycle conidaition pattern, are found in most filamentous fungi [9,10]. Normal conidiation must go through a period of hyphae elongation and then form conidiophores at the tip of the long hyphae; however, microcycle conidiation can bypass the long hyphae growth and produce conidia directly [10]. ...
Opy2 is an important membrane-anchored protein upstream of the HOG-MAPK signaling pathway and plays important roles in both the HOG-MAPK and Fus3/Kss1 MAPK. In this study, the roles of MaOpy2 in Metarhizium acridum were systematically elucidated. The results showed that the MaOpy2 disruption significantly reduced fungal tolerances to UV, heat shock and cell-wall-disrupting agents. Bioassays showed that the decreased fungal pathogenicity by topical inoculation mainly resulted from the impaired penetration ability. However, the growth ability of ΔMaOpy2 was enhanced in insect hemolymph. Importantly, MaOpy2 deletion could significantly increase the conidial yield of M. acridum by shifting the conidiation pattern from normal conidiation to microcycle conidiation on the 1/4SDAY medium. Sixty-two differentially expressed genes (DEGs) during the conidiation pattern shift, including 37 up-regulated genes and 25 down-regulated genes in ΔMaOpy2, were identified by RNA-seq. Further analysis revealed that some DEGs were related to conidiation and hyphal development. This study will provide not only the theoretical basis for elucidating the regulation mechanism for improving the conidial yield and quality in M. acridum but also theoretical guidance for the molecular improvement of entomopathogenic fungi.
In this study, the micromorphology and surface details of Pseudocercospora pseudostigmina‐platani conidia were investigated on American sycamore (Platanus occidentalis) leaves. Sooty films were collected from diseased leaves for fungal DNA extraction and observed using light and field‐emission scanning electron microscopy. Two types of conidia were present on the leaves: (i) hyaline and curved Cercostigmina‐like conidia and (ii) brown and straight Stigmina‐like conidia. The fungal pathogen was identified as P. pseudostigmina‐platani based on morphological characteristics and internal transcribed spacer region sequences. Some non‐glandular trichomes were found near the centre of the conidial masses on the abaxial surface of the naturally infected leaves. Leaf ultrasonication to dislodge conidia revealed sporodochia erupting through the stomata. Hyphae from sporodochia grew on the leaf surface and entered host tissues through the stomata. Conidiogenous cells had terminal rings and tapered towards the flat region where a conidium was attached. The conidia produced secondary conidia directly without hyphae. After dislodging the conidia by ultrasonication, scars were observed. These results indicate that the fungus has dimorphic conidia and microcycle conidiation for simple and rapid asexual reproduction.
The history and diagnostic characteristics of the Calosphaeriales, family Calosphaeriaceae (Class Ascomycetes) are reviewed. A combination of features of ascomata and asci permits recognition of eight genera: Calosphaeria, Scoptria, Enchnoa, Jattaea, Romellia, Graphostroma, Togninia, and Pleurostoma. Some of the species are described and illustrated. One new species, Romellia tympanoides, is described; the new combinations Scoptria discreta, Jattaea curvicolla, J. ceanothina, Romellia cornina, and Pleurostoma ootheca are proposed.
A method was perfected for inducing single-spore formation in uninucleate lag-phase germlings of Blastocladiella emersonii. Whether one or more spores was produced was related to the age of the germlings, the number of nuclei present prior to induction, and their apparent position in the mitotic cycle. Amino acids and peptone-yeast extract reversed induction of sporogenesis more effectively than glucose. Changes in cell morphology, polysaccharide, protein, nucleic acids, and dry weight during induction in uninucleate lag-phase germlings were similar to those known to occur in multinucleate log-phase ordinary colorless plants. The viability and general morphology of zoospores produced by lag- and logphase plants resembled one another closely.
Ascospores of Phillipsia domingensis and P. lutea germinated readily when plated on 2% Difco Noble agar. The two species differ in the manner of germination. The ascospores of P. domingensis become one septate during germination, and produce one or two germ tubes, which frequently bear conidialike structures. The conidialike structures are holoblastic, and are borne on sympodially proliferating conidiogenous cells. The ascospores of P. lutea germinate by a single germ tube which does not bear conidialike structures and they do not become septate. In both species the ascospore wall becomes double during germination, with the actual spore separating from an outer wall layer.
Ascospore germination in P. domingensis is compared to germination in Sarcoscypha coccinea. The genera Phillipsia and Sarcoscypha are considered to be closely related.
Secondary sporidial formation and discharge in Tilletia foetida were investigated using light- and electron-microscopy. Allantoid secondary sporidia developed asymmetrically at the tips of enteroblastic sterigmata. The hilar appendix consisted of a slight protrusion ca. 1.7 μm diam. The cell wall was thinner at the hilar appendix than elsewhere in the cell. A septum consisting of electron-dense material sandwiched between electron-transparent material, developed between the sterigma and sporidium. During sporidial discharge, the cell wall and septum abscised at the proximal margin of the dense material. On ejected sporidia, a patch of cell wall was absent at the hilar appendix. Cell wall generation occurred at the hilar end of sporidia after ejection but not at the tips of sterigmata. Just prior to sporidial discharge, a drop of liquid formed at the hilar appendix. These drops were removed with a microneedle and induced to evaporate and reform by shifting them between dry and humid atmospheres. The mechanism of sporidial discharge apparently involved the rapid movement of the drop from the hilar appendix to the sporidium.
Nine genera of minute nonmycelial fungi characterized by a thallus of one to 15 cells in linear arrangement, a usually darkened holdfast, and often endospores cut off in succession are known from arthropod hosts. The taxonomic relationship of these minute fungi, including Thaxteriola, has been the subject of speculation, and morphological comparisons have been made to the Laboulbeniales. One genus has been synonymized with Pyxidiophora in the Hypocreales. The discovery of a new species of Pyxidiophora with a Thaxteriola anamorph on mites associated with the southern pine beetle gives new evidence of relationships and points out the importance of a mycological curiosity as a dispersal stage. This anamorph is nonmycelial, highly specialized, and unlike any known for an ascomycete.
The status of the generic name Syzygospora is reviewed and the holomorph name is applied to a group of Basidiomycetes. Nine species are accepted in the genus. Three are proposed as new species (S. marasmoidea, S. norvegica and S. subsolida) and five others are transferred to Syzygospora. Carcinomyces, Christiansenia and Heterocephalacria are treated as synonyms of Syzygospora.
Valsaria insitiva was studied in culture. Ascospores germinated on agar and on host bark by forming yeast-like cells that reproduced by budding or from percurrently proliferating sporogenous loci. Yeast-like cells gave rise to mycelial colonies that formed oval to elongate conidia from lateral, percurrently proliferating loci. Hyphal cells sometimes disarticulated to form arthrospore-like cells. Eventually, colonies produced multiloculate pycnidial conidiomata which contained phialidic conidiogenous cells that proliferated to form periclinally thickened collarettes, or annellations, or sometimes underwent large proliferations which increased greatly the cell lengths. Conidia from conidiomata did not germinate.