Common foliar fungi of Populus trichocarpa modify Melampsora rust disease severity

Summary Nonpathogenic foliar fungi (i.e. endophytes and epiphytes) can modify plant disease severity in controlled experiments. However, experiments have not been combined with ecological studies in wild plant pathosystems to determine whether disease‐modifying fungi are common enough to be ecologically important. We used culture‐based methods and DNA sequencing to characterize the abundance and distribution of foliar fungi of Populus trichocarpa in wild populations across its native range (Pacific Northwest, USA ). We conducted complementary, manipulative experiments to test how foliar fungi commonly isolated from those populations influence the severity of Melampsora leaf rust disease. Finally, we examined correlative relationships between the abundance of disease‐modifying foliar fungi and disease severity in wild trees. A taxonomically and geographically diverse group of common foliar fungi significantly modified disease severity in experiments, either increasing or decreasing disease severity. Spatial patterns in the abundance of some of these foliar fungi were significantly correlated (in predicted directions) with disease severity in wild trees. Our study reveals that disease modification is an ecological function shared by common foliar fungal symbionts of P. trichocarpa . This finding raises new questions about plant disease ecology and plant biodiversity, and has applied potential for disease management.