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 mutualistic defensive symbionts under most ecological conditions. While many studies have documented the protective effects of these endophytes for their host plants, most of these studies do not quantify intraspecific variation in these potential suppressive effects. We hypothesized that endophyte infection would limit the progression of disease symptoms caused by most fungal pathogens, but that the suppressive effect of the endophyte on growth and disease severity of the pathogen would vary across pathogen strains. To test this, we performed a growth chamber experiment in which we factorially inoculated tall fescue hosts with a fungal endophyte, Epichloë coenophiala, and a fungal pathogen, Rhizoctonia solani. Within the R. solani treatment, we nested three different strains of the pathogen. Following inoculation, we examined pathogen growth and disease severity by measuring lesion size and leaf mortality over time.
Endophyte inoculation had no significant effect on the growth of Rhizoctonia solani on tall fescue. This observation held for all three strains of R. solani used as inoculum. There was, however, a significant difference among strains of R. solani in overall damage caused on tall fescue, suggesting that effects of microbial interactions on overall infection and growth rate of R. solani may also vary among strains. Inoculation with R. solani led to a significant increase in leaf mortality, but there was no significant effect of strain of R. solani on leaf mortality. In conclusion, the presence of a systemic grass endophyte does not necessarily limit growth of a fungal pathogen on tall fescue; however, strains of R. solani did vary in overall damage. Such among-strain variation provides support for future studies that quantify intraspecific variation in effects of within-host microbial interactions.