Family issues: fungal endophyte protects host grass from the closely related pathogen Claviceps purpurea

Certain cool season grasses establish systemic and asymptomatic symbioses with clavicipitaceous fungi of the genus Neotyphodium, which affect multiple biotic interactions within host neighborhood. The presumed symbiont-mediated plant resistance to pathogens is mostly based on studies performed under laboratory and greenhouse conditions. Here we investigated, in two outdoor experiments, the relation between two fungi of the same family with opposite effects on Lolium multiflorum plants: the mutualist endophyte Neotyphodium occultans, and the pathogen Claviceps purpurea. Natural infection and its consequences on symbiotic and non-symbiotic plants were studied under varying conditions of stress by herbicide. In both experiments, N. occultans reduced significantly the infection by C. purpurea at population levels (70 % less). The percentage of spikes infected by C. purpurea was almost three times lower in endophyte-symbiotic plants than in non-symbiotic ones. However, the protective effect was not maintained under stress condition. Our results show that constitutive symbionts such as the systemic fungal endophytes mediate the interaction between host grasses and pathogens, although the effect may depend on the level of stress in the environment.