Winter Survival of the Perennial Ryegrass Pathogen Magnaporthe oryzae in North Central Indiana

Winter survival of Magnaporthe oryzae in north central Indiana was investigated in response to reports and observations of sporadic disease incidence. Survival of the fungus in perennial ryegrass residue was assessed. Time course studies were designed to assess the conidia production potential of infested perennial ryegrass residue exposed to ambient and predetermined treatments. Approximately 50,000 conidia per gram dry weight were produced initially on infested residue. In all years of the study, ambient winter conditions in Lafayette, IN, reduced conidia production on residue to fewer than 60 conidia per gram by spring. Unless residue was dried prior to treatment, storage of residue at all temperatures tested reduced conidia production potential. Airborne M. oryzae conidia, over the plot of perennial ryegrass where the winter survival studies were conducted, were estimated from particles collected with a volumetric air sampler. The stand of perennial ryegrass was inoculated with residue infested with M. oryzae in the summer of 2000 and late fall of 2000 and 2001. The stand of ryegrass was not inoculated in the summers of 200l or 2002. Conidia were captured with the air sampler each year in early July, before outbreaks had been observed in north central Indiana, but capture peaked in September when outbreaks most often occur. Cumulative conidia capture followed disease severity in 2000; however, no disease was observed in 2001 or 2002. Results of this study suggest poor survival of M. oryzae, and inadequate populations of viable pathogen limit disease development in north central Indiana. Disease risk assessment in north central Indiana should include an estimate of viable inoculum.