Pythium root rot associated with cool-season dieback of turfgrass in Ontario and Quebec.
Pythium species were isolated from diseased samples of Agrostis stolonifera var. palustris and Poa annua. Of 14 isolates from decayed roots and crowns, 7 were P. graminicola, 5 were P. torulosum and 2 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 Lolium perenne than on A. palustris and isolates of P. ultimum were more virulent than those of P. graminicola or P. torulosum. Furthermore, greater virulence was generally found at 15 than at 30°C. Virulence differences were confirmed in greenhouse pot tests. Oospores and sporangia were recovered from inoculated symptomatic samples. It is concluded that Pythium species are a cause of cool-season dieback of golf course turfgrass in Ontario and Quebec, Canada.