Association of slugs with the fungal pathogen Epichloë typhina (Ascomycotina: Clavicipitaceae): potential role in stroma fertilisation and disease spread

Epichloë spp. are endophytes of grasses, and form epiphytic external stromata on flowering tillers. E. typhina was first noticed infecting Dactylis glomerata (= orchardgrass, cocksfoot) stands in the Willamette Valley in 1996, and soon became the primary factor limiting the longevity of seed production fields. Several species of slugs are present in these fields, and we investigated their role in E. typhina biology. Pre-dawn surveys of D. glomerata fields in 2009 and 2010 found Prophysaon andersoni and Arion subfuscus slugs feeding on the fungal stromata. When unfertilised and fertilised immature stromata predominated, approximately 80% of the individuals of these two species that were seen on plants were found on the stromata. As the majority of stromata reached maturity the presence of these species on stromata declined to between 20-40%. The common agricultural slug pest, Deroceras reticulatum, was on stromata only 20% of the time early in the season, and declined to <5% at stromata maturity. Observations of frass from slugs determined that the most common constituent was the food sources upon which the slug species was usually found during these surveys. Typically 100% of the frass from P. andersoni and A. subfuscus contained stroma material, compared to 25% for D. reticulatum. Spermatia, and ascospores later in the season, were commonly seen in the frass of slugs that consumed stromata. Some slugs that had no stroma material in their frass appeared to have consumed spermatia and ascospores from the leaf surface. A multiple-choice laboratory test confirmed the different proportional preferences of P. andersoni and D. reticulatum for stroma (0.72 vs 0.20) and leaf (0.07 vs 0.38), respectively. Two laboratory multiple-choice tests, and a field survey, found that P. andersoni preferred unfertilised and immature stroma over mature stroma. D. reticulatum is the most common and abundant slug in Willamette Valley grass seed fields, yet it is the least likely to move spermatia between unfertilised stromata, or ascospores to uninfected plants. P. andersoni and A. subfuscus are mycophagous, frequently transport viable spermatia and ascospores in their frass; yet they are generally confined to field edges. Data and observations suggest the role of slugs in the epidemiology of E. typhina is small compared to other factors.