Variation in bentgrass susceptibility to Typhula incarnata and in isolate aggressiveness under controlled environment conditions

Typhula incarnata, the causal agent of gray snow mold, is an important winter pathogen of turfgrasses in the northern United States. The relative susceptibility of cultivars of three bentgrass species (creeping, colonial, and velvet bentgrass) to Typhula incarnata and the aggressiveness of 15 T incarnata isolates obtained from infected turfgrasses on golf courses in Michigan, Minnesota, and Wisconsin were evaluated under controlled conditions. A hypersensitive type of resistance response to T incarnata was not observed in any cultivar. Disease severity increased with higher inoculum concentration of T. incarnata. Colonization by gray snow mold gradually decreased with increasing plant age from I I weeks after seeding in most cultivars tested, suggesting that age-related resistance was expressed over time. There were significant differences in disease severity among the three bentgrass species, particularly between tetraploid (creeping and colonial) and diploid (velvet) species, and among cultivars within each species, indicating varying levels of susceptibility to T. incarnata. All 15 isolates were pathogenic on bentgrass and were significantly different in aggressiveness, but aggressiveness was not related to geographic origin. Therefore, turfgrass breeders should be able to use one or a few virulent representative isolates of the pathogen to screen for resistance.