Isolates of Magnaporthe poae from turfgrass hosts were analyzed for mating type, genetic relatedness according to ITS sequences, reaction to a previously developed species-specific poly-merase chain reaction (PCR) assay, and virulence on two creeping bentgrass cultivars in growth chamber experiments. Analysis of internal transcribed spacer (ITS) sequences revealed three clades, designated A, B, and C. Clade A contained isolates of both mating types from creeping bentgrass, annual bluegrass, and Kentucky bluegrass. Clade B contained only mating type ‘A’ isolates from annual bluegrass, whereas Clade C contained only mating type ‘a’ isolates from creeping bentgrass. The M. poae PCR assay failed to positively identify several North Carolina isolates from annual bluegrass and creeping bentgrass. M. poae isolates from Kentucky blue-grass were most virulent toward creeping bentgrass in growth chamber experiments. Although isolates of M. poae are not host specific, certain ITS clades may have a limited host or geographical range. The improved creeping bentgrass cv. Penn A-4 was more susceptible to summer patch than cv. Penncross. Additional research is needed to develop methods for accurate diagnosis of summer patch and other patch diseases in creeping bentgrass and to determine how creeping bentgrass cultivars vary in their susceptibility to these root pathogens.