Ash dieback, induced by an invasive ascomycete, Hymenoscyphus fraxineus, has emerged in the late 1990s as a severe disease threatening ash populations in Europe. Future prospects for ash are improved by the existence of individuals with natural genetic resistance or tolerance to the disease and by limited disease impact in many environmental conditions where ash is common. Nevertheless, it was suggested that, even in those conditions, ash trees are infected and enable pathogen transmission. We studied the influence of climate and local environment on the ability of H. fraxineus to infect, be transmitted and cause damage on its host. We showed that healthy carriers, i.e. individuals showing no dieback but carrying H. fraxineus, exist and may play a significant role in ash dieback epidemiology. The environment strongly influenced H. fraxineus with different parameters being important depending on the life cycle stage. The ability of H. fraxineus to establish on ash leaves and to reproduce on the leaf debris in the litter (rachises) mainly depended on total precipitation in July-August and was not influenced by local tree cover. By contrast, damage to the host, and in particular shoot mortality was significantly reduced by high summer temperature in July-August and by high autumn average temperature. As a consequence, in many situations, ash trees are infected and enable H. fraxineus transmission while showing limited or even no damage. We also observed a decreasing trend of severity (leaf necrosis and shoot mortality probability) with the time of disease presence in a plot that could be significant for the future of ash dieback.

Data collected by visual observation in the field (dieback score, leaf necrosis, shoot mortality)

Data from DNA extraction / qPCR with specific primers for detection of Hymenoscyphus fraxineus in infected tissues

Data on the ability of H. fraxineus to produce apothecia on rachises collected at leaf fall. Rachises that were just shed were collected in autumn and placed outdoors for overwintering in common gardens on the soil in a forest situation. They were retrieved in spring and placed in humid chamber to assess the ability of Hymenoscyphus fraxineus to produce apothecia on them. The part producing / not producing apothecia were separated and the proportion of rachises-producing apothecia were computed