Snow molds are psychrophilic fungi that grow under snow cover by taking advantage of carbohydrate-depleted, dormant plants. Typhula snow molds caused by Typhula incarnata, T. phacorrhiza, and T. ishikariensis are the most important winter diseases of perennial grasses and winter cereals in the United States. Colonized turfgrass samples with sclerotia were collected from 135 golf courses in Wisconsin, Utah, Michigan, and Minnesota in spring 2001 and 2002. Species and varieties from a total of 2,864 samples were identified using Typhula spp.-specific polymerase chain reaction markers. All three species were found throughout the states sampled, except T. phacorrhiza, which was not found in Minnesota. T. incarnata was distributed in areas of shorter snow cover duration and higher mean temperature than T. ishikariensis. Canonical correspondence analysis indicated that snow cover days and mean temperature were significantly correlated with frequency of Typhula spp. and T. ishikariensis varieties infecting turfgrasses on golf courses in Wisconsin, and that T. incarnata, T. phacorrhiza, and T. ishikariensis were ecologically distinct based on the 2001 data. However, because these two variables accounted for a relatively small proportion of total variation, other environmental variables also may be important in characterizing the distribution of these pathogens and require further study.

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 bent-grass 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 11 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.