Yeast species interactions and composition modulate leaf rust severity in Populus trichocarpa

Plant microbiomes are recognized for increasing defence against pathogens. Yet, identifying the role of beneficial microbes beyond their individual effects remains poorly explored. In a series of in planta greenhouse experiments, we tested the effects of individual foliar yeasts, yeast interactions and the combination of yeast richness and diversity on disease severity caused by the leaf rust pathogen Melampsora × columbiana in the model tree, Populus trichocarpa. We found that single yeasts can either antagonize or facilitate the rust; nevertheless, most yeasts did not modify disease severity. For some individual disease-modifying yeasts, interactions with additional yeasts changed the outcome of disease severity. In one case, a rust facilitator was neutralized by other interacting yeasts; in another, increasing richness in communities containing a rust antagonist was associated with increasing disease severity. However, we did not find evidence for a positive or negative relationship between yeast richness or diversity and disease severity overall. A follow-up in vitro experiment showed strong interspecific competition between yeasts, which might explain changes in disease severity with varying community membership. Our results highlight the role of microbial interactions and composition for plant disease severity, yet caution against interpreting plant microbiome richness or diversity as inherently disease protective. This article is part of the theme issue 'Wild plant pathosystems'.