Increased winter precipitation benefits the native plant pathogen Ustilago bullata that infects an invasive grass.
Ustilago bullata is a fungal pathogen that infects grasses in western North America. It infects the grass Bromus tectorum, sometimes at epidemic levels, and has been considered as a biocontrol agent in regions where B. tectorum is invasive. During a study of the effects of variation in precipitation on B. tectorum demography in Colorado, USA, we observed infection by U. bullata and recorded data on variation in infection rates among treatments receiving different amounts of winter precipitation. Over 3 years, increased winter precipitation was correlated with a higher prevalence of infection by U. bullata. In one of those years (2012), infection was associated with a 21% decrease in B. tectorum seed yield in plots that received supplemental winter precipitation. While B. tectorum may exhibit higher rates of growth and/or reproduction if winter precipitation increases, results of our pathogen surveys suggest that associated population increases could be moderated by increased rates of fungal infection. Our observations suggest that although invasive species may benefit from climate change, some pathogens may also benefit, and decrease success of invasive plant populations in the future. These results highlight the importance of both direct and interactive effects of climate change on success of invasive species.