Melampodium divaricatum (Rich. ex Pers.) DC. (=M. paludosum H.B.K.), a member of the family Asteraceae and native to South America, is a recent introduction for use as a summer bedding ornamental. In September 1999, melampodium plants in multiple Baton Rouge landscapes were observed with signs of powdery mildew and symptoms of a virus-like disease. Powdery mildew spread throughout one of the plantings by late November and infected flowers and leaves. An Oidium species sporulated on both leaf surfaces but was more common on the adaxial surface. Ellipsoid conidia were produced in chains, lacked fibrosin bodies, and averaged 31 × 19 µm. No sexual stage was observed. Eight of 63 plants (cv. Derby) in one of the plantings showed virus disease symptoms that included severe leaf mosaic, leaf malformation, and stunting. Leaves from infected plants were used to sap inoculate seedling plants of melampodium and Nicotiana benthamiana. Melampodium seedlings developed typical mosaic symptoms after 48 to 56 days. N. benthamiana developed severe chlorosis and mosaic, then wilted and died after 14 days. Noninoculated plants of both species remained healthy. The virus in both plant species was identified as Tomato spotted wilt virus (TSWV) by enzyme-linked immunosorbent assay (ELISA) (Agdia, Elkhart, IN). ELISA tests for presence of Impatiens necrotic spot virus were negative. This is the first report of powdery mildew and TSWV on M. divaricatum.
