Once the disease is established, management is hardly possible. The occurrence of a small fraction of partially tolerant trees constitutes hope for resistance breeding in the future. Healthy-looking trees should be preserved.
Understanding the genetic diversity and structure of invasive pathogens in source and in introduced areas is crucial to the revelation of hidden biological features of an organism, to the reconstruction of the course of invasions and to the establishment of effective control measures. Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) is an invasive and highly destructive fungal pathogen found on common ash Fraxinus excelsior in Europe and is native to East Asia. To gain insights into its dispersal mechanisms and history of invasion, we used microsatellite markers and characterized the genetic structure and diversity of H. pseudoalbidus populations at three spatial levels: (i) between Europe and Japan, (ii) in Europe and (iii) at the epidemic's front in Switzerland. Phylogenetic and network analysis demonstrated that individuals from both regions are conspecific. However, populations from Japan harboured a higher genetic diversity and were genetically differentiated from European ones. No evident population structure was found among the 1208 European strains using Bayesian and multivariate clustering analysis. Only the distribution of genetic diversity in space, pairwise population differentiation (GST) and the spatial analysis of principal components revealed a faint geographical pattern around Europe. A significant allele deficiency in most European populations pointed to a recent genetic bottleneck, whereas no pattern of isolation by distance was found. Our data suggest that H. pseudoalbidus was introduced just once by at least two individuals. The potential source region of H. pseudoalbidus is vast, and further investigations are required for a more accurate localization of the source population.
Pseudodidymella fagi is a leaf blotch pathogen of Fagus crenata in Japan. This pathogen is now reported for the first time on F. sylvatica in Switzerland and Germany. Species identity was verified by morphological assessment of the asexual and sexual morphs and by comparing the internal transcribed spacer (ITS) sequence of the type material from Japan and of freshly collected samples from Europe. ITS sequences proofed to be completely identical. The asexual morph Pycnopleiospora of Ps. fagi is formed on necrotic leaf spots during summer and autumn. In early spring, its sexual morph is formed in the litter of F. sylvatica. The connection between sexual and asexual morphs was verified by sequencing the ITS region of single conidium and ascospore isolates. The pathogenicity of Ps. fagi on F. sylvatica was tested by inoculations on detached leaves in vitro and Koch's postulates were fulfilled. The second pathogen we report in central Europe for the first time is Petrakia deviata, which causes a leaf blotch disease of field maple (Acer campestre). This species was collected only once in the central Caucasus region in 1929 and was never found again after its first discovery. Now it was rediscovered in two different locations in Switzerland on field maple (A. campestre) and on the new host Norway maple (A. platanoides). Species identity was verified by morphology and by comparison with the ITS sequence of the holotype specimen and freshly collected samples. Phylogenetic analysis based on ITS sequences suggested that Ps. fagi and P. deviata are closely related to each other. Whether these species were simply overlooked so far, profit from climate change, or represent newly introduced invasive species remains to be studied. Moreover, deeper phylogenetic analysis using multiple sequence markers should be conducted to verify species identities.
The recently described ascomycete fungus Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) causes the current dieback of ash (Fraxinus excelsior) in large parts of Europe. The origin of this species and its relation to the native cryptic species Hymenoscyphus albidus are still enigmatic. The spatiotemporal pattern of the epidemic is typical for an introduced invasive species. However, the presence of two cryptic species indicates that hybridization or mutation might have been involved in driving speciation in this case. In this study, we present a set of 18 polymorphic microsatellite markers to study these processes in more detail on a population genetic level. Markers were designed such that they can be amplified in three individual multiplex PCRs and analysed in two fragment analysis runs. We thoroughly tested the marker set for pairwise linkage among loci, selective neutrality and Mendelian inheritance. Additionally, the markers were applied on two large collections of isolates derived from study sites in Germany. Population genetic calculations suggested a low yet significant level of differentiation, a large genotypic diversity and a limited genetic diversity within populations. Furthermore, we present additional data concerning the phylogenetic relation between H. albidus and H. pseudoalbidus, which seems to be more distantly related to each other than expected previously.
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