Rusts are economically important diseases of switchgrass (Panicum virgatum) and other Paniceae grasses. Phylogenetic analyses based on sequences of the nuc rDNA 5.8S internal transcribed spacer 2 region (ITS2), partial 28S region, and intergenic spacer region (IGS) of nuc rDNA showed that species of rust fungi infecting switchgrass are closely related within Puccinia. Variation among rbcLa sequences for the associated hosts sampled concurred with the original identifications. Five species infecting switchgrass were recognized: Puccinia graminicola (≡ Uromyces graminicola), P. pammelii (= P. panici), and the proposed new species P. amari, P. novopanici, and P. pascua. These species were distinct from P. emaculata, the species previously considered the principal rust pathogen infecting switchgrass but that was found exclusively on witchgrass (Panicum capillare) in this study. Rust fungi on switchgrass previously identified as P. emaculata were identified as the morphologically similar species P. amari, P. novopanici, and P. pammelii. The morphological species Puccinia graminicola was found to comprise three species, P. graminicola and the proposed new species P. pascua on switchgrass and P. cumminsii on Panicum sp.
Keywords: Pucciniales; herbarium specimens; molecular phylogeny; switchgrass; warm-season grasses.