Brome grasses represent the primary source of Claviceps purpurea inoculum associated with barley fields in the San Luis Valley of Colorado

Abstract The fungal pathogen Claviceps purpurea can infect numerous grass species including important crops like barley, rye, and wheat, but the ecology of ergot disease is rarely studied in non-crop grass species. Recent outbreaks of ergot in barley production systems in the western U.S. have implicated nearby grass hosts as reservoirs for inoculum and spread of C. purpurea into the barley crop. We surveyed the prevalence of ergot infections among native, invasive, and weedy grass species near barley fields in the San Luis Valley of Colorado over three years. We found that barley fields with a history of ergot disease were surrounded by a grass community composed largely of smooth brome, that smooth brome was very susceptible to ergot infection and had the highest infection rates among grass species, and that disease severity among the grass community increased during the growing season. Over the study period there was a low incidence of barley infections and this precluded analysing the effects unmanaged grass composition and disease incidence had on within field disease incidence. Our findings indicate that smooth brome represents a significant source of C. purpurea inoculum and inoculum levels increase as the composition of brome grass in the grass community increases. There is sufficient inoculum present in the brome grass population in the San Luis Valley to cause sporadic disease in barley fields when environmental conditions are optimal for infection. Future disease management strategies should take into account the large pool of inoculum in the grass population near barley production fields.