Examining the ecology of Puccinia coronata: An emerging infectious disease on the invasive grass, Phalaris aquatica, in northern California

Background/Question/Methods

The study of infectious diseases can help avoid ecological and agricultural damage, and be used to better understand disease ecology. The emergence of the rust Puccinia coronata presents a unique opportunity to investigate aspects of disease ecology, including variables influencing pathogen spread, effects of the pathogen on host fitness, and management options.

In 2011, staff at Pepperwood Preserve, Santa Rosa California, noticed infections of the leaf crown rust, P. coronata on the exotic grass, Phalaris aquatica; this was the first documented occurrence of the pathogen on P. aquatica outside its native range. We observed varying levels of infection severity between 2011 and 2012, which led us to establish a joint research program between the University of North Carolina and Santa Rosa Junior College. We expected that pathogen infections would respond to three variables: (1) host exposure to spores in the air, (2) host density, and (3) local microclimate. We hypothesized that more infections would occur in plants taller than the surrounding vegetation, stands of higher density, and microclimates with wetter soil. In 2013, we initiated a long-term monitoring project and a series of lab inoculations to measure transmission, infection severity, and effects of the pathogen on grassland community dynamics.

Results/Conclusions

In 2013, we found limited support for our hypotheses. We measured rust infections in 45, one-meter-squared quadrats distributed across six distinct populations of P. aquatica. Although there was significant variation in infection severity between populations (P<0.05), we found no effect of host density on pathogen infections. There was a small but significant increase in infection as plants increased in height relative to the surrounding vegetation (R²=0.05, P<0.05). However, we expect that damage is primarily driven by climate, and that higher levels of damage during the 2014 growing season may offer greater support for our hypotheses.

Moving forward, we plan to extend our original hypotheses to include variation in inter-annual climate as a potential contributor to variation in infection severity. Doing so may advance a greater understanding of rust infections in wild plants, emerging infectious diseases, and disease ecology in general.