Background: Systemic Epichloë endophytes are common fungal symbionts of many cool-season grasses. They are known for their capability of increasing host plant tolerance against biotic and abiotic stressors, including grass pathogens. However, results on endophyte-mediated disease resistance have been ambiguous, and the underlying mechanisms of disease resistance remain unknown.

Aims: We studied how Epichloë endophytes affect naturally occurring pathogen, leaf blotch (Rhynchosporium sp.), infections of wild and cultivated tall fescues (Schedonorus phoenix).

Methods: Endophytic and endophyte-free tall fescues were grown in a common garden experiment in southern Finland for eight growing seasons. The experimental plants were subjected to nutrient and water treatments.

Results: Our results show that the effects of endophytes on leaf blotch infection incidences varied with plant origins and environmental conditions. Overall, the American cultivar ‘Kentucky-31ʹ appeared to have less disease symptoms compared to the Finnish cultivar ‘Retu’ or the wild grasses. Endophytic wild tall fescues from Gotland Island had lower Rhynchosporium incidences than grasses from Åland Islands or coastal Sweden.

Conclusions: These results show that Epichloë endophytes can suppress pathogen Rhynchosporium infections in tall fescues, but pathogen resistance depends on grass origin and environmental conditions.