CHARACTERIZATION AND PATHOGENICITY OF PYTHIUM SPECIES ISOLATED FROM TURFGRASS WITH SYMPTOMS OF ROOT AND CROWN ROT IN NORTH-CAROLINA

Thirty-three Pythium spp. were obtained from roots and crowns of bentg rass and other turfgrass species with symptoms of Pythium root and cro wn rot. The predominant species recovered were P. arrhenomanes, P. cat enulatum, P. intermedium, P. oligandrum, P. periilum, P. torulosum, an d P. vanterpoolii. Pythium complexes of two or more species from the s ame tissue sample were common. P. catenulatum and P. torulosum, which made up 58% of the total isolates, were the species most frequently fo und in combination with other species of Pythium. Pathogenicity of all Pythium species was analyzed in pre- and postemergence inoculation te sts. Tests were conducted on seedlings of creeping bentgrass grown in tissue-culture well plates and incubated at 28 C and high relative hum idity. fn preemergence tests, 13 species caused moderate or high level s of disease (damping-off) and in postemergence tests, 17 species caus ed moderate or high levels of disease. Symptoms included mild to sever e root and crown rot, blight, and chlorosis. Eight species were highly aggressive (causing 61-100% disease) and included P. arrhenomanes, P. aristosporum, P. aphanidermatum, P. graminicola, P. myriotylum, P. ta rdicrescens, P. vanterpoolii, and P. volutum. Nine species were modera tely aggressive (causing 21-60% disease) and included P. dissotocum, P . irregulare, P. multisporum, P. paroecandrum, P. splendens, P. sylvat icum, P. ultimum sporangiiferum, P. u. ultimum, and P. violae. Twelve species caused low levels of disease (1-20% disease), and four species were not pathogenic under test conditions. In general, the level of d isease caused by a given species was similar in pre- and postemergence tests. Isolates within a species also gave similar results with the e xception of P. vanterpoolii. Among the 14 isolates of P. vanterpoolii tested, two isolates were highly aggressive, nine were moderate, and t hree were nonpathogenic. In tests conducted at 16, 28, and 32 C with s elected species, high temperatures favored disease development by most species. Only P. iwavamai caused more disease at 16 C than at higher temperatures. P. arrhenomanes was the most aggressive root-rotting spe cies tested and along with P. aphanidermatum, P. aristosporum, and sev eral isolates of P. vanterpoolii also caused cottony-blight at 28 and 32 C. All species were easily recovered from roots of symptomatic seed lings 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 r ot of turfgrass. In pre- and postemergence inoculation tests conducted with nine species of Pythium isolated from other hosts, P. tracheiphi lum was highly aggressive, and P. mamillatum and P. spinosum caused mo derate levels of disease. The large number of Pythium species involved in root and crown rot of bentgrass may partially explain the widespre ad distribution of the disease.