Genetic structure of Magnaporthe grisea populations associated with turfgrass hosts in Georgia

Abstract

Magnaporthe grisea, causal agent of gray leaf spot, is a pathogen of increasing importance in the turfgrass industry, yet little is known about the genetic structure of populations from turfgrasses. Populations of M. grisea from St. Augustinegrass and tall fescue in Georgia were sampled in 1999 and 2000. Populations were selected to allow separation of factors that may influence population structure, such as host species, host cultivar, and geographic region. A PCR-based assay was developed for mating type determination in M. grisea. Populations from St. Augustinegrass and tall fescue were dominated by Mat1-1 and Mat1-2, respectively, but the opposite mating type was also obtained from each host. No female fertile isolates were identified in crosses with fertile tester strains in vitro. Although both mating types exist in populations of M. grisea from turfgrass, the potential for sexual reproduction appears to be low due to the absence of female fertile isolates. According to AFLP markers, each M. grisea population was dominated by a single lineage, with other lineages present in low frequencies. Host species significantly influenced population structure, but differences were observed among populations which could not be explained by host species, host cultivar, or geographic region. M. grisea populations from tall fescue and perennial ryegrass appear to have been established from a common source, but no alternative host for the dominant lineage from these hosts was identified. Weeping lovegrass was an alternative host for lineage E from tall fescue, and isolates from wheat were also closely related, but distinct. In growth chamber experiments, isolates from lineage G4 induced a hypersensitive response on wheat cultivar Roberts, but other M. grisea lineages from tall fescue induced typical blast symptoms. The clonal lineages associated with tall fescue in Georgia may correspond to distinct pathogenic races of M. grisea. Several components were responsible for the M. grisea resistance of ‘Coyote’ and ‘Coronado’ tall fescue, and the resistance was equally effective against all lineages associated with tall fescue in Georgia. These cultivars represent sources of partial resistance to M. grisea and will be a valuable component of integrated programs for management of gray leaf spot.