Phylogenetic relatedness among Cladosporium leaf endophytes predicts their ability to reduce the severity of a poplar leaf rust disease

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.