Hymenoscyphus pseudoalbidus , the causal agent of European ash dieback

Abstract

Summary: The ascomycete H ymenoscyphus pseudoalbidus (anamorph C halara fraxinea ) causes a lethal disease known as ash dieback on F raxinus excelsior and F raxinus angustifolia in E urope. The pathogen was probably introduced from E ast A sia and the disease emerged in P oland in the early 1990s; the subsequent epidemic is spreading to the entire native distribution range of the host trees. This pathogen profile represents a comprehensive review of the state of research from the discovery of the pathogen and points out knowledge gaps and research needs. Taxonomy: Members of the genus H ymenoscyphus ( H elotiales, L eotiomycetidae, L eotiomycetes, A scomycota) are small discomycetes which form their ascomata on dead plant material. A phylogeny based on the internal transcribed spacers ( ITS s) of the rDNA indicated the avirulent H ymenoscyphus albidus , a species native to E urope, as the closest relative of H . pseudoalbidus . Symptoms: H ymenoscyphus pseudoalbidus causes necrotic lesions on leaves, twigs and stems, eventually leading to wilting and dieback of girdled shoots. Bark lesions are characterized by a typical dark‐ to cinnamon‐brown discoloration. Life cycle: H ymenoscyphus pseudoalbidus is heterothallic and reproduces sexually on ash petioles in the litter once a year. Ascospores are wind dispersed and infect ash leaves during the summer. The asexual spores only serve as spermatia. Tools and techniques: The most important techniques for fungal handling, such as detection, isolation, culturing, storage, crossing and ascocarp production, are briefly described. Management: 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.