First report of leaf necrosis on Microstegium vimineum caused by Bipolaris microstegii in Maryland.
Japanese stiltgrass (JSG, Microstegium vimineum) is an invasive weed causing significant ecological changes in the United States. Severely diseased plants in a shaded location 2 × 4 m in size were discovered in August 2012 at a residence on Indian Springs Rd., Frederick, Maryland, USA. JSG in larger monoculture stands at sunny locations within 6 to 10 m of diseased plants had a few, small, necrotic spots. Diseased plants had leaves with brown, often large, elliptical, necrotic spots up to 0.5 × 1.5 cm. Lesions were surrounded by a diffuse chlorotic margin, and larger lesions had tan centres. Diseased plants were smaller in stature than neighbouring, non-symptomatic plants. Based on symptomatology, morphological observations, sequence analysis of the rDNA internal transcribed spacer region and pathogenicity tests, the causal agent was identified as B. microstegii. Bipolaris leaf spot was also found on JSG in Howard and Prince Georges counties in 2009, but the causal agents were not formally characterized. This is thought to be the first confirmed identification of B. microstegii on JSG in Maryland, a plant that occurs as extensive monocultures in natural areas.