The ash dieback pathogen Hymenoscyphus pseudoalbidus is associated with leaf symptoms on ash species (Fraxinus spp.)

Abstract

Ash dieback caused by the ascomycete fungus Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea) is characterized by a wide range of symptoms. Leaf symptoms have previously been related to this emerging infectious disease. In fungal isolations from necrotic lesions on leaf petioles and rachises as well as leaflet veins of Fraxinus excelsior, H. pseudoalbidus was consistently obtained at high frequencies. When inoculated onto leaf rachises of F. excelsior, the ash dieback pathogen induced symptoms identical to those seen after natural infections (necrotic lesions, wilting and leaf dropping). The fungus also caused symptoms on leaves of Fraxinus angustifolia and Fraxinus ornus in artificial inoculations and was re-isolated at considerably high frequencies from all three Fraxinus spp. Kochs postulates were thus fulfilled to conclude that H. pseudoalbidus is associated with leaf symptoms on F. excelsior. On F. angustifolia leaf damage resulting from natural infections still remains to be detected. In contrast, F. ornus may not be a natural host of H. pseudoalbidus, although it proved to be somewhat susceptible in the leaf inoculation experiments.