DISTRIBUTION OF PHYTOPHTHORA-PARASITICA VAR NICOTIANAE RACES AND THEIR SENSITIVITY TO METALAXYL IN GEORGIA

Tobacco (Nicotiana tabacum L.) samples displaying typical black shank symptoms were collected in 1990-1992 throughout the tobacco-producing area of Georgia. Cultures of Phytophthora parasitica Dastur var. nicotianae (Breda de Haan) Tucker (Ppn) isolated from samples were stored in culture until the fall of each of the years. The tobacco cultivars K 326 (low Ppn resistance), Speight G 70 (moderate Ppn resistance), Coker 371 Gold (high Ppn resistance), and the line 1071 (immune to race 0, susceptible to race 1) were grown in a greenhouse and used to determine virulence and races. Isolates were cultured on toothpicks impregnated with V8 juice agar, and the toothpicks were pushed into the stems of test plants to inoculate them. The cultivars K 326 and Speight G 70 were susceptible to all isolates, with K 326 being the most susceptible. Only 12 isolates of the 75 collected caused disease on line 1071. Coker 371 Gold demonstrated very high stem resistance, even though isolates were collected from decaying roots and stems of the cultivar. Race 1 was isolated from the entire tobacco-growing area, suggesting widespread distribution. Isolates were variable in sensitivity to metalaxyl, with a range of ED50 of 0.96 to <0.01 mug/ml. Typical field rates of metalaxyl may not be sufficient to control isolates that have a low level of sensitivity.