Dieback of European Ash: What Can We Learn from the Microbial Community and Species-Specific Traits of Endophytic Fungi Associated with Ash?

European ash( Fraxinusexcelsior), a keystone specieswith wide distribution and habitat range in Europe, is threatened at a continental scale by an invasive alien ascomycete, Hymenoscyphus fraxineus. In its native range of Asia, this fungus is a leaf endophytewith weak parasitic capacity and robust saprobic competence in local ashspecies that are closely related to European ash. In European ash, H. fraxineus has a similar functional role as in Asia, but the fungus also aggressively kills shoots, resulting in crown dieback and tree death. H. fraxineus is a typical invasive species, as its spread relies on high propagule pressure. While crown dieback of European ashis the most obvious symptom of ashdieback, the annual colonization of ashleaves is a crucial key dependency for the invasiveness of H. fraxineus, since its fruiting bodies are formed on overwintered leaf vein tissues in soil debris. Leaves of European ashhost a wide range of indigenous epiphytes, endophytes, facultative parasitesand biotrophic fungi, including Hymenoscyphus albidus, a relative of H. fraxineus that competes for the same sporulation niche as the invader. At face value, leaves of European ashare colonized by a large and diverse indigenous mycobiome. In order to understand why this invader became successful in Europe, we discuss and summarize the current knowledge of diversity, seasonal dynamics and traits of H. fraxineus and indigenous fungi associated with leaves of European ash.