Fungal disease incidence along tree diversity gradients depends on latitude in European forests

European forests host a diverse cohort of tree species that are increasingly threatened by fungal pathogens. These pathogens may have devastating consequences for forest ecosystems and their functioning. Previous studies suggest that foliar and root pathogen abundance and disease severity decrease with increasing tree species diversity, but evidence from natural forests are rare. Here we tested whether foliar fungal disease incidence was negatively affected by tree species diversity in different forest types across Europe. We measured the foliar fungal disease incidence on 16 different tree species in 209 plots in six European countries, representing a forest type gradient from the Mediterranean to boreal forests. Forest plots of single species (monoculture plots) and those with different combinations of two to five tree species (mixed species plots) were compared. Specifically, we analyzed the influence of tree species richness, functional type and phylogenetic diversity on overall fungal disease incidence. The relationship between tree species richness and overall fungal disease incidence a) increased along a latitudinal gradient, and b) decreased with functional type of the tree. However, for specific damage symptoms, no tree species richness effects were observed. The advantage of tree species mixtures, as observed in this study, depended on the forest site and tree type.