Effects of elevated CO2 and ozone on the spillover potential and dilution potential of an insect transmitted plant virus

Background/Question/Methods

Because diseases can have profound effects on community properties such as productivity, diversity, and nutrient cycling, understanding how their dynamics will change with climate is an important emerging facet of climate change research.  For generalist pathogens, if climate change affects the reservoir potential or vector behavior of more competent hosts, we may see changes in spillover to other hosts in the community.  Similarly, if vectors make more use of less competent hosts, we may see a ‘dilution effect’, where competent hosts grown in mixture have reduced disease compared to hosts grown in monoculture.  We used barley yellow dwarf virus (BYDV), an aphid transmitted plant virus, as a model system to investigate the effects of elevated levels of CO₂ and ozone on BYDV transmission between Avena fatua, a highly competent reservoir host, and Setaria lutescens, a much less competent host of the virus.  Using open top chambers in the field, we factorially elevated CO₂ (target 700 ppm) and ozone (target +70 ppb) and examined rates of aphid transmission and viral load in monocultures and 50:50 mixtures of Setaria and Avena. 

Results/Conclusions

Because ozone significantly reduces viral load in both the field and the greenhouse, we expected lower spillover to Setaria under elevated ozone, and recovery of transmission rates of viral loads under CO₂ and ozone combined.   However, at mid-season, transmission rates of BYDV to Setaria were uniformly low across gas treatments, and there were no significant differences in transmission between Setaria plots grown with and without Avena.    This is likely because while aphid colony populations had grown since inoculation in all plots, aphid densities remained low.   Aphids were therefore able to remain restricted to their preferred Avena hosts.     In addition, Setaria grown with Avena were much smaller than Setaria grown in monoculture, and may have been less apparent to aphid species.    In addition, there were no significant differences in Avena viral prevalences in mixtures of Avena and Setaria, suggesting that Setaria is a poor dilution host when in competition with Avena.   Tests for end-of-season viral prevalence are ongoing, and may show a different pattern, as aphid loads were higher at the end of the season.     Overall, these results suggest the importance of understanding the effects of climate on vector biology and behavior as well as within-host processes in order to understand the epidemiology of  this virus.