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Identification and Comparative Pathogenicity of Pythium spp. from Roots and Crowns of Turfgrasses Exhibiting Symptoms of Root Rot. Eric B. Nelson, Assistant professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; Cheryl M. Craft, Research support specialist, Department of Plant Pathology, New York State College of Agriculture and Life Sciences. Phytopathology 81:1529-1536. Accepted for publication 7 August 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1529.

Pathogenicity of Pythium species recovered from roots and crowns of creeping bentgrass and annual bluegrass was determined under laboratory, growth chamber, and field conditions. Of the 121 isolates of Pythium recovered from diseased roots and crowns, 46 were pathogenic (disease rating ? 2.0) in laboratory experiments. Of the pathogenic species, P. graminicola was isolated most frequently (18.2% of all isolates), and nearly all isolates tested were highly virulent to creeping bentgrass and perennial ryegrass. Additional pathogenic species recovered were isolates of P. aphanidermatum, P. aristosporum, P. torulosum, and P. vanterpoolii. These species were recovered at frequencies of 6.6, 2.5, 4.1, and 5.0% of all isolates, respectively. At least one isolate within each species was highly virulent on creeping bentgrass. P. torulosum was the most frequently recovered species from turfgrass roots and crowns, but nearly all isolates were nonpathogenic. Five pathogenic isolates of P. torulosum were recovered and, with the exception of one isolate, all were only weakly virulent to creeping bentgrass at 13 or 28 C. The majority of the P. graminicola isolates and all of the P. aristosporum isolates tested were highly virulent at both 13 and 28 C. The virulence of specific isolates of P. graminicola and P. vanterpoolii on creeping bentgrass was favored by either cool or warm temperatures. Although isolates of P. aphanidermatum were virulent at both temperatures, in general they were more virulent at 28 C than at 13 C. At 28 C, some isolates of P. graminicola, P. aphanidermatum, and P. aristosporum were pathogenic to perennial ryegrass in growth chamber experiments, whereas none of the isolates of P. torulosum and P. vanterpoolii was pathogenic. On perennial ryegrass, isolates of P. graminicola ranged from nonpathogenic to highly virulent. In field plantings of creeping bentgrass and perennial ryegrass, symptoms of P. graminicola-induced root rot were evident by 5 days after inoculation. By 15 days after inoculation, disease ratings among all cultivars ranged from 2.7 to 6.7. Results suggest that P. graminicola is the principal root-rotting species affecting creeping bentgrass and perennial ryegrass under both cool and warm temperatures in New York State.