The effects of generalist pathogens on hosts in a community context will be mediated both by epidemiological traits (e.g. vector preference) and variation among host species in traits such as resistance and tolerance to infection. The dilution hypothesis suggests that host community composition can alter the prevalence of a generalist pathogen, and therefore its importance on host fitness. Specifically, when hosts that are highly competent pathogen reservoirs are grown with less competent hosts, the presence of the less competent host(s) could slow the spread of the pathogen through the more competent host as well. To test the dilution hypothesis, we grew bi-cultures of eight annual grass species that are hosts to the vector transmitted generalist barley yellow dwarf virus (BYDV) in mesocosms exposed to or protected from viral inoculation. All of these simple communities consisted of the most competent host, Avena fatua, and other hosts that vary in their competence.

Results/Conclusions

We found that virus prevalence in Avena was reduced in mixtures with less competent hosts, but the degree of reduction was not always predicted by virus prevalence in host monocultures. Avena grown in monocultures inoculated with BYDV showed a reduction in biomass compared to uninoculated Avena. In some cases, Avena grown in mixtures showed a similar reduction in biomass when inoculated with BYDV. However, in other cases, inoculated Avena grown in mixtures had higher biomass relative to inoculated Avena grown in monoculture, providing evidence in support of the dilution effect for BYDV.