Pythium species associated with root dysfunction of creeping bentgrass in Maryland.
Putting green samples (n=109) were inspected for the presence of Pythium oospores in roots of plants from golf courses (n=39) in Maryland and adjacent states in the USA. Twenty-eight Pythium isolates were recovered from creeping bentgrass (Agrostis palustris)[A. stolonifera var. palustris] (n=25) and annual bluegrass (Poa annua) (n=3) plants. Most isolates associated with Pythium-induced root dysfunction were from greens less than 3 years of age and were obtained primarily between March and June, 1995-97. Eight Pythium species (P. aristosporum, P. aphanidermatum, P. catenulatum, P. graminicola, P. torulosum, P. vanterpoolii, P. volutum and P. ultimum var. ultimum) were isolated from creeping bentgrass and 2 species (P. graminicola and P. torulosum) were from annual bluegrass. All species, except P. catenulatum, were pathogenic to 'Crenshaw' creeping bentgrass seedlings in postemergence pathogenicity tests. P. aristosporum (n=3) and P. aphanidermatum (n=1) were highly aggressive at a low (18°C) and a high temperature (28°). P. graminicola (n=1) was low to moderately aggressive. P. torulosum (n=12) was the most frequently isolated species, but most isolates were either nonpathogenic or caused very little disease. P. aristosporum (n=3) and P. aphanidermatum (n=1) were highly aggressive and were associated with rapid growth at 18 and 28° on cornmeal agar. P. volutum (n=1) was highly aggressive at 18°, but was one of slowest growing isolates. Infected roots were generally symptomless, and the number of oospores observed in roots was not always a good indicator of disease or of the aggressiveness of an isolate. Large numbers of oospores of low or even nonpathogenic species may cause dysfunction of creeping bentgrass roots.