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Characterization and Pathogenicity of Pythium Species Isolated from Turfgrass with Symptoms of Root and Crown Rot in North Carolina. Z. G. Abad, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; H. D. Shew, and L. T. Lucas. Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 84:913-921. Accepted for publication 23 May 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-913.

Thirty-three Pythium spp. were obtained from roots and crowns of bentgrass and other turfgrass species with symptoms of Pythium root and crown rot. The predominant species recovered were P. arrhenomanes, P. catenulatum, P. intermedium, P. oligandrum, P. periilum, P. torulosum, and P. vanterpoolii. Pythium complexes of two or more species from the same tissue sample were common. P. catenulatum and P. torulosum, which made up 58% of the total isolates, were the species most frequently found in combination with other species of Pythium. Pathogenicity of all Pythium species was analyzed in pre- and postemergence inoculation tests. Tests were conducted on seedlings of creeping bentgrass grown in tissue-culture well plates and incubated at 28 C and high relative humidity. In preemergence tests, 13 species caused moderate or high levels of disease (damping-off) and in postemergence tests, 17 species caused moderate or high levels of disease. Symptoms included mild to severe root and crown rot, blight, and chlorosis. Eight species were highly aggressive (causing 61100% disease) and included P. arrhenomanes, P. aristosporum, P. aphanidermatum, P. graminicola, P. myriotylum, P. tardicrescens, P. vanterpoolii, and P. volutum. Nine species were moderately aggressive (causing 2160% disease) and included P. dissotocum, P. irregulare, P. multisporum, P. paroecandrum, P. splendens, P. sylvaticum, P. ultimum sporangiiferum, P. u. ultimum, and P. violae. Twelve species caused low levels of disease (120% disease), and four species were not pathogenic under test conditions. In general, the level of disease caused by a given species was similar in pre- and postemergence tests. Isolates within a species also gave similar results with the exception of P. vanterpoolii. Among the 14 isolates of P. vanterpoolii tested, two isolates were highly aggressive, nine were moderate, and three were nonpathogenic. In tests conducted at 16, 28, and 32 C with selected species, high temperatures favored disease development by most species. Only P. iwayamai caused more disease at 16 C than at higher temperatures. P. arrhenomanes was the most aggressive root-rotting species tested and along with P. aphanidermatum, P. aristosporum, and several isolates of P. vanterpoolii also caused cottony-blight at 28 and 32 C. All species were easily recovered from roots of symptomatic seedlings and sometimes from asymptomatic seedlings. P. tardicrescens, P. volutum, P. dissotocum, P. multisporum, P. paroecandrum, P. sylvaticum, and P. u. sporangiiferum are reported as new pathogens causing root rot of turfgrass. In pre- and postemergence inoculation tests conducted with nine species of Pythium isolated from other hosts, P. tracheiphilum was highly aggressive, and P. mamillatum and P. spinosum caused moderate levels of disease. The large number of Pythium species involved in root and crown rot of bentgrass may partially explain the widespread distribution of the disease.