Hymenoscyphus species associated with European ash1

The causative agent of dieback on European ash (Fraxinus excelsior) was first described as Chalara fraxinea based on cultural morphology because no sexual stage of the fungus was known. Later, based on culturing of ascospores of a candidate teleomorph, morphological comparison and nuclear ribosomal internal transcribed spacer sequencing, the sexual stage of C. fraxinea was assigned as Hymenoscyphus albidus, a native and widespread species in Europe. Recently, the morphological species concept of H. albidus was shown to cover two species that cannot be separated from each other based on teleomorph characters, but which can be distinguished by several DNA markers. As a result, the strains causing ash dieback were reassigned as H. pseudoalbidus. The closely related H. albidus is presumably a non‐pathogenic endophyte, but pathogenicity tests to confirm this hypothesis have not yet been performed. Genotyping of herbarium specimens has shown that H. pseudoalbidus was present in Switzerland for at least a decade prior to the epidemic outbreak in Europe. The origin of the ash dieback pathogen, and the general importance of correct pathogen identification to development of effective disease control, are discussed.