Pathogenicity of fungi associated with ash dieback towardsFraxinus excelsior

Abstract

A large part of the area in Europe in whichFraxinus excelsioris native is currently affected by ash dieback, a threatening disease caused by the ascomycetous fungusHymenoscyphus fraxineus. Fungi other thanH. fraxineusalso occur in large numbers on stems of the dying ash trees. To clarify their possible role in the dieback process, six fungal species common on dying stems and twigs of ash in Poland, i.e.Cytospora pruinosa,Diaporthe eres,Diplodia mutila,Fusarium avenaceum,F. lateritiumandF. solani, were tested for pathogenicity using a test based on artificial wound inoculations of 6‐year‐oldF. excelsiorplants under field conditions, withH. fraxineusincluded for comparison. There were significant differences in index of pathogenicity among the fungi tested.Hymenoscyphus fraxineus(mean index 5.78) was the most pathogenic.Diplodia mutila(4.23) andC. pruinosa(4.02) were significantly less pathogenic thanH. fraxineus, but significantly more than the other fungi.Diaporthe eres(2.43),F. avenaceum(1.92),F. solani(1.86) andF. lateritium(1.08) were the least pathogenic (P < 0.0001). The extent of disease symptoms caused byF. solaniandF. lateritiumwas statistically similar to the control (P = 0.05). All tested fungi were successfully reisolated from inoculated stems. The contribution of the results to understanding the possible role of these fungi in the ash dieback process inF. excelsior, particularly in trees weakened after primary infection byH. fraxineus, is discussed.