Plants can host a diversity of mutualistic, commensal, and parasitic microbes, and the interactions among these microbes may be antagonistic or beneficial. Epichloë endophytes, symbiotic fungi of many grass species, are thought to be mutualistic defensive symbionts under most ecological conditions. We hypothesized that endophyte infection would decrease the progression of disease symptoms caused by most fungal pathogens, as well as limit the spread between hosts. To determine the effect of this endophyte on within- and between-host spread of a fungal pathogen, Rhizoctonia solani, we set up populations of the host grass species, tall fescue, in field mesocosms within the Duke Forest Research and Teaching Laboratory. At this site, tall fescue experiences epidemics of R. solani, therefore, each population was contained to limit disease caused by R. solani from the surrounding soil. Each population was comprised of entirely endophyte-inoculated plants or entirely endophyte-free plants. A focal plant was inoculated with R. solani. Following inoculation, we examined pathogen spread within that focal plant and from that focal plant to surrounding plants by observing all leaves within the population for symptoms of the pathogen over time.
Results/Conclusions
Endophyte inoculation had no significant effect on the within-host spread of R. solani on tall fescue. Specifically, there was no significant difference in the spread of R. solani symptoms to other leaves of the focal plant between the endophyte-inoculated and the endophyte-free treatments. There was, however, a significant effect of endophyte inoculation on between-host spread of R. solani. We quantified disease intensity over time by using the proportion of total leaves observed with R. solani symptoms to calculate the area under the disease progress curve stairs (AUDPS) for each treatment. Populations comprised of endophyte-inoculated plants experienced less disease caused by the fungal pathogen over time than populations comprised of endophyte-free plants. In conclusion, a fungal defensive symbiont decreased the spread of fungal plant pathogen, R. solani, through a host population but not within hosts. As populations of tall fescue can vary in endophyte prevalence, future work aims to quantify how endophyte prevalence affects pathogen epidemics.