Hymenoscyphus fraxineus is an introduced ascomycete fungus which causes ash dieback and has resulted in widespread mortality of ash throughout Europe. Although H. fraxineus has been present on the continent for at least four decades, it was not identified until 2006. The first record of the pathogen in Britain came in 2012 although it was probably present a decade earlier. The most common host European ash (Fraxinus excelsior L.) is economically and ecologically important to Britain where the cost of ash dieback is estimated at billions of pounds. The impact of ash dieback has stimulated a major research response which we review with the aim of providing up-to-date information relevant to Britain and identifying knowledge gaps where research would contribute to improved disease mitigation. Hymenoscyphus fraxineus is an outcrossing fungus with high genotypic diversity; ascospores produced via sexual reproduction are critical to aerial dispersal and infection. Temperature, moisture, and ground cover influence pathogen fruit body development, the timing of ascospore release, and extent of ascospore germination; they also interact together to affect the likelihood of infection. In addition, stand characteristics, including tree density, tree height, and landscape fragmentation, affect disease dynamics with increased disease severity on moist sites with high ash density. Efforts at finding natural resistance in ash have identified genetic markers associated with disease tolerance, and gene expression analysis is providing insights into the basis of that resistance. Mainland European findings indicate that ash dieback makes trees more vulnerable to other pathogens, whilst endophytes in the ash phyllosphere can suppress infection by H. fraxineus. Possible tools for long-term control of ash dieback include (1) deployment of resistant ash, (2) quantitatively informed management practices based on microclimate models and better understanding of the pathogen life cycle, and (3) manipulation of biocontrol agents from the ash microbiome or pathogen mycoviruses.
