Coexistence is stabilized by conspecific negative density dependence via fungal pathogens more than oomycete pathogens.

Plant pathogens are often hypothesized to promote species coexistence by generating conspecific negative density dependence (CNDD). However, the relative importance of fungal versus oomycete pathogens in maintaining plant species coexistence and community composition remains unresolved, despite their recognized effects on plant performance. Here, we use fungicide application to investigate how fungal versus oomycete pathogens affect plant species coexistence in an alpine meadow. We found that the severity of foliar fungal disease was density-dependent at both intra- and interspecific levels. Fungal pathogen-exclusion treatment successfully decreased the severity of foliar fungal diseases, with no detectable effects on root colonization by arbuscular mycorrhizal fungi or on soil chemical properties. Fungal pathogens were important factors shaping CNDD across 25 coexisting plant species. Exclusion of fungal pathogens significantly reduced plant species richness and Shannon's evenness. Treatments that excluded fungal pathogens also led to significant shifts in plant community composition toward more Poaceae and Cyperaceae. These results indicate that fungal pathogens, especially those affecting aboveground plant parts, may play a larger role in maintaining species coexistence and shaping community composition than has been previously recognized.