Competition among plants has a strong impact on the structure of plant communities, but the outcome of competition can be modified by interactions with parasites, herbivores, and mutualists.  In field experiments, we examined the impact of a generalist pathogen, the barley yellow dwarf virus, on competitive interactions between annual grasses.  We planted two-species mixtures of eight common annual grass species in mesocosms that were inoculated with virus or mock-inoculated with non-infectious vectors and protected from virus infection.  All grass species were hosts of the virus, but experiments showed that they varied significantly in epidemiological traits such as resistance to infection, tolerance to disease, and attractiveness to vectors. They also differed in host competence, the probability of an infected host individual transmitting the pathogen to a vector individual feeding on it.  These simple grass mixtures were maintained over the growing season, then harvested to measure reproductive and vegetative biomass, and assayed for virus prevalence.

Results/Conclusions

We found that the presence of wild oats (Avena fatua), a highly susceptible reservoir host of the virus, increased pathogen prevalence in most other grass species through the process of pathogen spillover.  For several host species, pathogen spillover from wild oats decreased the abundance of other host species through pathogen-mediated apparent competition.  This indirect interaction with Avena often outweighed the direct effects of competition.  In contrast, other hosts exhibited tolerance to disease that resulted in few negative impacts of pathogen spillover.  These results provide experimental support for theoretical predictions of strong feedbacks between generalist disease dynamics and host community structure, mediated by specific epidemiological traits of host species.