More closely related organisms are expected to function more similarly than distantly related organisms due to shared ancestry and functional trait heritability. However, there have been few tests of this hypothesis for fungal leaf endophytes, which can modify host plant disease severity by a variety of mechanisms. We tested whether phylogenetic relatedness within Cladosporium, a genus including many common fungal leaf endophyte species, predicts endophyte effects on cottonwood leaf rust disease severity caused by Melampsora × columbiana. First, we used multilocus sequence typing to infer phylogenetic relationships among 96 Cladosporium isolates collected from wild cottonwood trees growing in Pacific Northwest of North America. Next, we conducted a double-inoculation leaf-disk assay (endophyte inoculated first, then rust pathogen) for a subset of 50 Cladosporium isolates to characterize disease modification for the endophyte isolates; data on endophytes parasitizing rust was collected simultaneously for each isolate. We used generalized linear models to link disease modification and mycoparisitic ability to endophyte phylogeny, while accounting for endophyte geographic origin. We recognized 17 distinct species of Cladosporium; all fifty isolates of Cladosporium reduced rust disease severity in our leaf disk assay (by as much as 79% and as little as 45%). Cladosporium phylogeny was a significant predictor of rust disease severity and was also correlated with mycoparasitism.  The geographic origin of the isolates explained only a small amount of the overall variation in disease reduction. Our results demonstrate that fungal endophyte phylogenetic relatedness can help predict differences in endophyte function.