Genetic population structure of the invasive ash dieback pathogen Hymenoscyphus fraxineus in its expanding range

Introduced plant pathogens are increasingly recognized as a major threat to biodiversity and ecosystem functioning. One such pathogen, the causal agent of the devastating ash dieback in Europe, Hymenoscyphus fraxineus, was most likely introduced into Europe from eastern Asia in the 1990s. To investigate the genetic population structure of this invasive fungus at the epidemic disease front (Switzerland) and in the post-epidemic phase (Lithuania), a total of 847 H. fraxineus isolates were genotyped at 11 microsatellite loci. Among these isolates, 244 multilocus genotypes were found in five post-epidemic subpopulations (367 isolates) of the fungus and 263 in five epidemic subpopulations (480 isolates). No genetic differentiation was found between isolates recovered from bark lesions and fallen leaf petioles, which suggests that all H. fraxineus genotypes have the potential to induce bark infections on living trees and to survive saprophytically. Moreover, no genetic differentiation and no difference in genetic diversity were detected between the epidemic and post-epidemic populations. The entire genetic diversity present in the original founding populations in north-eastern Europe seems to have been transmitted to the epidemic disease front. Nonetheless, gene flow among post-epidemic subpopulations occurs slightly more random than among epidemic subpopulations. Furthermore, the probability of correct assignment of a particular H. fraxineus genotype to its subpopulation of origin was greater in Switzerland than in Lithuania. These two analyses point to weak founder effects at the disease front.