Ash dieback

Ash dieback in Europe is caused by an invasive alien pathogen originating from East Asia, the helotialean ascomycete fungus Hymenoscyphus fraxineus. This disease first emerged in the early 1990s in North-Eastern Poland, and the pathogen successively invaded most of Europe, in total 32 countries, in the next decades, causing substantial damage and mortality of the highly susceptible Fraxinus excelsior and Fraxinus angustifolia, while the third European ash species, Fraxinus ornus, is not affected. In this chapter, we review the general characteristics of the disease, in particular its disease cycle, and the perspectives that tolerance to H. fraxineus in the populations of ash may lead to remission of ash dieback over time through natural selection and accelerated by breeding. Likewise, an updated view of the possible long-term ecological impact of ash dieback is presented with a review of the role of stand, site, and environmental factors in the epidemiology of the disease. While disease intensity is generally favored by high moisture levels (soil humidity and precipitation), the sensitivity of H. fraxineus to high summer temperatures constrains the impact of ash dieback in Southern Europe. Likewise, the impact of the disease is often lower on ash trees that occur isolated or at low frequency (solitary trees in the open landscape or in hedges and mixed forest stands with low proportion of ash) compared to trees in ash-dominated forest stands. Finally, we discuss the role of the microbiota associated with ash and of H. fraxineus mycoviruses in relation to ash dieback.