Tracking the invasion: dispersal of Hymenoscyphus fraxineus airborne inoculum at different scales.

Ash dieback is caused by an invasive pathogen Hymenoscyphus fraxineus, which emerged in Europe in the 1990s and jeopardizes the management of ash stands. Although the biological cycle of the pathogen is well understood, its dispersal patterns via airborne spores remain poorly described. We investigated the seasonal and spatial patterns of dispersal in France using both a passive spore-trapping method coupled with a real-time PCR assay and reports of ash dieback based on symptom observations. Spores detection varies from year to year with a detection ability of 30-47% depending on meteorological conditions, which affect both production of inoculum and efficiency of the trapping. Nevertheless, our results are consistent and we showed that the sporulation peak occurred from June to August and that spores were detected up to 50-100 km ahead of the disease front, proving the presence of the pathogen before any observation of symptoms. The spore dispersal gradient was steep, most of inoculum remaining within 50 m of infected ashes. Two dispersal kernels were fitted using Bayesian methods to estimate the mean dispersal distance of H. fraxineus from inoculum sources. The estimated mean distances of dispersal, either local or regional scale, were 1.4 km and 2.6 km, respectively, the best fitting kernel being the inverse power-law. This information may help to design disease management strategies.