The role of biotic and abiotic factors in altering pathogen dynamics for tall fescue in a managed grassland

Background/Question/Methods

Under climate change, shifts in precipitation and temperature regimes are expected to impact ecosystem structure and function. These impacts may be determined by feedbacks between plants and associated microbes, including both endophytic fungal symbionts and viral pathogens. To begin to test for such feedbacks, we manipulated temperature, precipitation, and endophyte infection in a managed grassland in central Kentucky. In a split plot design, we factorially increased precipitation and temperature across 20 plots planted with a mixture of forage species including tall fescue (Schedonorus phoenix; 50% endophyte-infected and 50% endophyte-free).  Plants were tested for naturally transmitted infections by three species of barley and cereal yellow dwarf viruses (B/CYDVs) for three growing seasons (2009-2011). B/CYDVs are widespread aphid-vectored viruses that infect hundreds of grass species. The impacts of climate change on interactions among these microbes and plants are not straight-forward to predict.  For example, endophytes produce alkaloids that can deter aphids from feeding, which should decrease B/CYDV prevalence in endophyte-infected plants. On the other hand, endophyte infection can also confer drought resistance to the host plant, which may increase aphid feeding and virus prevalence under drought conditions.

Results/Conclusions

Bayesian statistical analysis indicated that both abiotic and biotic factors had important impacts on virus dynamics. There was a year by precipitation by fescue interaction such that when precipitation was added, endophyte infection increased virus prevalence, but only in 2011. Additionally, increased temperature decreased overall B/CYDV prevalence regardless of year. This affect does not appear to be driven by vector abundance because increased temperature increased aphid load on plants. Viral species also had differential responses to the manipulations of temperature and precipitation, as some species increased in prevalence while others decreased. These changes in viral species prevalence may be responsible for the changes in overall virus prevalence in this system. Our experiment suggests disease dynamics in managed grasslands are complex with both biotic and abiotic factors having important roles. Changes in climate are likely to have strong impacts on disease dynamics by not only altering vector loads, but by changing the prevalence of individual viral species which scales up to changes in overall disease dynamics.