Evidence of a component Allee effect for an invasive pathogen: Hymenoscyphus fraxineus , the ash dieback agent

Invasive pathogens are a major threat to forest health especially in managed forest with a low diversity of tree species. The dieback of Fraxinus spp. caused by the fungus Hymenoscyphus fraxineus that occurs in Europe is the latest example of pathogen invasion causing widespread damage in forests. Ash dieback severity has been shown to be strongly affected by environment, in particular by stands features such as overall tree density or proportion of ashes. The fact that H. fraxineus reproduce mostly through heterothallic sexual reproduction suggest that an Allee effect could limit the mating success at low host densities, thus limiting inoculum production and disease development. Populations of H. fraxineus were monitored during the vegetation period in a network of stands across a host density gradient in forest and non-forest environment (hedges and small woods). Ash dieback, basal area of ash, density of infected ash leaf debris (rachis) and apothecia in the litter and ascospores load in the air were determined in the different environments during two years. We showed significant differences between forest and non-forest environment with ash dieback, infection rate and inoculum production higher in forest settings. Host density significantly affected disease development, with crown dieback, density of infected rachis in the litter and inoculum production increasing with host density. We also demonstrated that fruiting rate, i.e. the number of apothecia per infected rachis dry weight, is strongly dependent on infected rachis density. Inoculum production is therefore limited at low host densities. Such a component Allee effect could be important in H. fraxineus epidemiology and invasion dynamic.