The central question of our study was: Do foliar fungal communities of Norway spruce shift along a tree species diversity gradient in mature European forests? Indeed, we found that the fungal community varied along the tree species diversity gradient in the Hainich forest in Germany. However, we observed no overall trend in other mature forests in this study. Similar variable results to tree species diversity have been previously observed, where mixtures of Norway spruce with other tree species either resulted in increased, decreased or no changes in the susceptibility of Norway spruce to pathogen infection (Kató, 1967, Siepmann, 1984 and Piri et al., 1990). In particular, Nguyen et al. (2016) found that depending on the forest site, different tree species diversity effects could be detected for foliar pathogens. In their study, there were either no correlations with tree species diversity and pathogen incidence, or an increase in disease incidence with diversity. It is thus still unclear to what degree and under what circumstances tree species diversity affects fungal communities of tree foliage.