Pythium root rot associated with cool-season dieback of turfgrass in Ontario and Quebec

Pythium species were isolated from diseased samples of creeping bentgrass and annual bluegrass. Among fourteen isolates from decayed roots and crowns, seven were P. graminicola, five were P. torulosum and two were P. ultimum. These species were identified by comparison of cultural and morphological characteristics to literature and reference isolates. In a tissue culture plate assay for quantifying virulence of Pythium spp., significant differences in seedling emergence were found among isolates. In general, isolates produced more severe root rot on perennial ryegrass than on creeping bentgrass, and isolates of P. ultimum were more virulent than those of P. graminicola or P. torulosum. Furthermore, greater virulence was generally found at 15C than at 30C. Virulence differences were confirmed in greenhouse pot tests. Oospores and sporangia were recovered from inoculated symptomatic samples. We conclude that Pythium species are a cause of cool-season dieback of golf course turfgrass in Ontario and Quebec.