This thesis is aimed at surveying the influence of tree diversity on richness and infestation of foliar fungal pathogens. The particular experimental design of different European tree diversity experiments offered the potential to consider different aspects of tree diversity. Pathogen species richness and pathogen load of foliar fungal pathogens were determined in a microscopic and macroscopic approach. Statistical analyses showed that tree species diversity negatively affected pathogen load of different tree species and fungus species, whereas with increasing tree clone diversity also pathogen species richness increased. Absent functional tree diversity effects might point on a minor role of functional diversity for foliar fungal pathogens in young experimental stands. Furthermore, host density effects were generally absent, too. However, positive (fungus facilitation) and negative (host facilitation) species identity effects has been numerously determined in all experiments. In conclusion, this thesis has highlighted the role of bottom-up effects for host-pathogen interactions.