Landscape epidemiology of ash dieback

Ash dieback is induced by Hymenoscyphus fraxineus , an invasive pathogenic fungus. It is causing severe damage to European ash populations. However, the local environment, such as climate or site conditions, is known to affect ash dieback. We studied the landscape epidemiology of the disease on a 22 km 2 area in north‐eastern France at two stages of the invasion process using Bayesian spatio‐temporal models fitted with integrated nested Laplace approximation (INLA). Several features characterizing disease severity, crown dieback, frequency of collar canker and density of infected leaf debris in the litter were determined on a regular grid over a 3.5 × 6.5 km area. We first analysed the effect of landscape features on the disease establishment stage in 2012, 2 years after the first report of the disease in the area, and then on further disease development, in 2016–2018. Landscape features had little impact on the disease at the establishment stage, but strongly determined its further development. Local fragmentation of tree cover was the most important factor, with trees that are isolated or in hedges far less affected than trees in a forest environment. We showed that they were subjected to different microclimates, with higher crown temperatures unfavourable to pathogen development. Low host density strongly reduced disease development. The presence of large ash populations in the vicinity affected local disease severity up to several hundred metres. Synthesis . We showed that the landscape characteristics strongly affect the development and spread of ash dieback. The disease is far less severe in forest conditions when ash density is low or in open canopies such as hedges and isolated trees. Ash trees are often in these types of landscapes, which should strongly limit the overall impact of ash dieback.