Figure 2 - uploaded by Fabio Zottele
Content may be subject to copyright.
Source publication
Chrysomyxa rhododendri de Bary is a well-known pathogen that frequently produces massive and conspicuous attacks on Picea abies Karst. in the Alps. Although the cycle of this rust was described at the end of the 19th century, it is only recently that research has focussed on its real impact on spruce dynamics at the treeline. In Trentino, spruce ne...
Similar publications
Summary: During a three weeks field project in 2013 in the National Park Berchtesgaden
which aimed at evaluating the effects of large scale bark beetle infestations and subsequent
natural succession on species richness and population structure of wood-inhabiting fungi,
the authors were surprised by the high occurrence of Fomitopsis rosea, a species...
Citations
... Continuous growth of the mycelium within the needles causes progressive yellowing of the foliage during summer and enables the fungus to form aeciospore stocks, where after dicaryotization and mitotic splitting aeciospores are released [6]. As fungal growth is restricted to individual current-year needles and infected needles are shed in autumn, each spring a new infection cycle starts and infection intensities as well as subsequent defoliation can strongly vary between individual years [7,8]. ...
Background
Needle rust caused by the fungus Chrysomyxa rhododendri causes significant growth decline and increased mortality of young Norway spruce trees in subalpine forests. Extremely rare trees with enhanced resistance represent promising candidates for practice-oriented reproduction approaches. They also enable the investigation of tree molecular defence and resistance mechanisms against this fungal disease. Here, we combined RNA-Seq, RT-qPCR and secondary metabolite analyses during a period of 38 days following natural infection to investigate differences in constitutive and infection-induced defence between the resistant genotype PRA-R and three susceptible genotypes.
Results
Gene expression and secondary metabolites significantly differed among genotypes from day 7 on and revealed already known, but also novel candidate genes involved in spruce molecular defence against this pathogen. Several key genes related to (here and previously identified) spruce defence pathways to needle rust were differentially expressed in PRA-R compared to susceptible genotypes, both constitutively (in non-symptomatic needles) and infection-induced (in symptomatic needles). These genes encoded both new and well-known antifungal proteins such as endochitinases and chitinases. Specific genetic characteristics concurred with varying phenolic, terpene, and hormone needle contents in the resistant genotype, among them higher accumulation of several flavonoids (mainly kaempferol and taxifolin), stilbenes, geranyl acetone, α-ionone, abscisic acid and salicylic acid.
Conclusions
Combined transcriptional and metabolic profiling of the Norway spruce defence response to infection by C. rhododendri in adult trees under subalpine conditions confirmed the results previously gained on artificially infected young clones in the greenhouse, both regarding timing and development of infection, and providing new insights into genes and metabolic pathways involved. The comparison of genotypes with different degrees of susceptibility proved that several of the identified key genes are differently regulated in PRA-R, and that the resistant genotype combines a strong constitutive defence with an induced response in infected symptomatic needles following fungal invasion. Genetic and metabolic differences between the resistant and susceptible genotypes indicated a more effective hypersensitive response (HR) in needles of PRA-R that prevents penetration and spread of the rust fungus and leads to a lower proportion of symptomatic needles as well as reduced symptom development on the few affected needles.
... The parasite undergoes a host shift between the main host rhododendron (Rhododendron ferrugineum or R. hirsutum) and Norway spruce and its occurrence is therefore restricted to higher elevation areas, where both hosts co-occur. Attacks have been reported since the end of the nineteenth century and continue to be regularly recorded by the forest health monitoring programmes carried out in different parts of the Alps [25,96]. The infection of Norway spruce naturally occurs in spring, when freshly released basidiospores are wind-dispersed and arrive at current-year needles of the trees, where they quickly germinate and enter the young needles [8]. ...
Background: Norway spruce trees in subalpine forests frequently face infections by the needle rust fungus Chrysomyxa rhododendri, which causes significant growth decline and increased mortality of young trees. Yet, it is unknown whether trees actively respond to fungal attack by activating molecular defence responses and/or respective gene expression.
Results: Here, we report results from an infection experiment, in which the transcriptomes
(via RNA-Seq analysis) and phenolic profiles (via UHPLC-MS) of control and infected trees
were compared over a period of 39 days. Gene expression between infected and uninfected ramets significantly differed after 21 days of infection and revealed already known, but also novel candidate genes involved in spruce molecular defence against pathogens.
Conclusions: Combined RNA-Seq and biochemical data suggest that Norway spruce response to infection by C. rhododendri is restricted locally and primarily activated between 9 and 21 days after infestation, involving a potential isolation of the fungus by a hypersensitive response (HR) associated with an activation of phenolic pathways. Identified key regulatory genes represent a solid basis for further specific analyses in spruce varieties with varying susceptibility, to better characterise resistant clones and to elucidate the resistance mechanism.
... In this period, the relationship between tree diversity and defoliation was reversed from positive to negative, suggesting that, in the case of this conifer species, diversity at the plot level plays a stabilising role (Jucker et al., 2014). The worsening conditions of P. abies crowns could be connected to complex causes, including the rainfall decrease in several Alpine areas in the first decade of the XXI century (particularly in the period 2003-2007, data provided by CNR-ISE) and the biotic attacks of Chrysomyxa rododendri de Bary (Zottele et al., 2014) specially in monospecific plots. The correlation between tree diversity and defoliation was always negative in the case of Q. ilex, but it became significant in some specific years depending on the relative fluctuation of defoliation in monospecific and mixed plots. ...
Tree diversity is found to enhance ecosystem functions in forests and increase the resistance and resilience of trees subjected to environmental stress, including climate change. This effect, however, can be different depending on tree species assemblages and ecological contexts. The pan-European programme for monitoring forest health (ICP Forests) considers crown defoliation as an indicator of tree vitality. Only a few studies have analysed the role of tree diversity in crown defoliation, with contrasting results. This paper analyses the relationships between defoliation and forest diversity in a country (Italy) characterised by heterogeneous biocli-matic and edaphic characteristics (from Mediterranean to Alpine) and with a wide range of forest tree species and functional groups. National-scale results show that defoliation increased in more diverse forest stands. This result can be explained by the fact that the tree mixtures reflect varying ecological conditions in heterogeneous environments. The most relevant factors that explain tree defoliation at the national level, investigated through random forest analysis, were the geographic position, tree species identity and forest stand age and structure. Cluster analysis was applied to identify homogeneous groups of plots based on their ecological and vegetation features. Four clusters were selected. Within each homogeneous cluster, the crown defoliation of the tree species showed different and contrasting patterns, changing their relationships with diversity over time. A beneficial role of diversity on defoliation (i.e. decrease of defoliation) has been observed in Quercus ilex, growing in Mediterranean conditions, and Picea abies in Alpine areas after dry periods. In the latter species, drought led to a greater increase in defoliation in monospecific than in mixed stands, suggesting the stabilising role of tree diversity.
... In most regions of the Alps, district foresters estimate and report the extent of forest area affected by C. rhododendri to the responsible forest head office. The values are based on a visual estimate and do not include indications of the extent of symptom development, which can vary considerably between individual years, sites and specimens (Ganthaler et al., 2014;Zottele et al., 2014). In several physiological and biochemical studies, disease severity of individual trees was estimated via a semiquantitative method Ganthaler et al., 2017a); the percentage of current-year needles with disease symptoms was evaluated by visual assessment and using a discrete disease scale with five classes between 0% and 100%. ...
High elevation spruce forests of the European Alps are frequently infected by the needle rust Chrysomyxa rhododendri, a pathogen causing remarkable defoliation, reduced tree growth, and limited rejuvenation. Exact quantification of the disease severity on different spatial scales is crucial for monitoring, management and resistance breeding activities. Based on the distinct yellow discoloration of attacked needles, we investigated whether image analysis of digital photographs can be used to quantify the disease severity and to improve phenotyping compared to conventional assessment in terms of time effort and application range. The developed protocol for pre-processing and analysis of digital RGB images enabled to identify disease symptoms and healthy needle areas on images gained in ground surveys (total number of analysed images n = 62) and by the use of a semi-professional quadcopter (n = 13). Obtained disease severities correlated linearly with results obtained by manual counting of healthy and diseased needles for all approaches, including images of individual branches with natural background (R² = 0.87) and with black background (R² = 0.95), juvenile plants (R² = 0.94), and top views and side views of entire tree crowns of adult trees (R² = 0.98 and 0.88, respectively). Results underline that a well-defined signal related to needle rust symptoms of Norway spruce can be extracted from images recorded by standard digital cameras and using drones. The presented protocol enables precise and time-efficient quantification of disease symptoms caused by C. rhododendri and provides several advantages compared to conventional assessment by manual counting or visual estimations.
The fungal pathogen Chrysomyxa rhododendri undergoes a host shift between Rhododendron spp. and Picea abies and has a considerable negative impact on the latter by infecting the new sprouting needles and reducing photosynthesis, growth and seedling survival. Repeated high infection rates in the Central European Alps were reported in recent years, and although the life cycle of the pathogen is well understood, knowledge on temporal patterns of host infection, spore dispersal and influence of weather conditions is limited. We analysed the period of needle infection by a staggered application of fungicide on twigs of Norway spruce. Seasonal and diurnal patterns of spore dispersal were investigated using a Burkard volumetric spore trap, and airborne spore concentrations were correlated with several weather parameters. Needle infection occurred within 3 weeks (19 June–9 July). Basidiospores (infecting Norway spruce) were trapped from May to July, and airborne spore concentration was positively correlated with air temperature, global radiation and wind speed, and negatively with air humidity and precipitation. Aeciospores (infecting rhododendrons) appeared from August to October, and high concentrations were significantly associated with rain events. Needle infection started with bud sprouting and was limited either by decreasing spore concentrations or morphological and chemical differentiation of the needles, which prevented infection. Both spore types showed distinct periods of dispersal based on the life cycle of the pathogen, and the variability in concentration within these periods was explained by local weather conditions. The correlation analysis indicated different mechanisms of spore dispersal for both spore types and, with respect to global warming, more favourable conditions for the pathogen in future. The results can explain the high year- and site-specific variation in Chrysomyxa infection intensities of P. abies and may be helpful for developing successful strategies to control the pathogen and protect alpine spruce forests.
