Curvularia muehlenbeckiae causing leaf spot on Johnson grass in Mexico

Johnson grass, a common weed in agricultural fields, is widely distributed in warm regions of the world. In Mexico, this grass grows mainly during the rainy season, along railways and roadsides, and in abandoned fields and urban communities. Significantly, it competes with a wide range of crops during the fall-winter season. Recently, a high incidence of severe leaf spot was observed on Johnson grass in northern Sinaloa, Mexico. The aims of this study were to identify the causal agent of this leaf spot based on morphology and molecular techniques and to determine the pathogenicity of the fungal isolates associated with the disease. The combination of morphological and phylogenetic analyses of the ITS and GAPDH allowed the identification of ten fungal isolates from Johnson grass as Curvularia muehlenbeckiae. Inoculation tests under greenhouse conditions demonstrated the pathogenicity of C. muehlenbeckiae on Johnson grass. Leaf spots displayed on the inoculated plants were similar to those observed under field conditions and in two independent inoculation tests. This is the first report implicating C. muehlenbeckiae as a causal agent of leaf spot on Johnson grass in México.