Pseudodidymella fagi and Petrakia deviata: Two closely related tree pathogens new to central Europe

Abstract

SummaryPseudodidymella 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.