Overwintering Hosts, Compatibility Types, and Races of Phytophthora infestans on Tomato in Southern California. Varoujan G. Vartanian, Graduate Research Assistant, Department of Plant Pathology, University of California, Riverside 92521. Robert M. Endo, Professor, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 69:516-519. Accepted for publication 14 December 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-516.

Although 11 solanaceous species of plants were susceptible to local isolates of Phytophthora infestans in the greenhouse, only tomato, potato, and Solanum sarrachoides were infected in the field. Tomato was the most common host. Although potato is not grown commercially in coastal southern California, potato occasionally was found infected in home gardens. Blighted plants of S. sarrachoides were encountered commonly in field surveys throughout the year. In cross-inoculation tests, isolates of P. infestans recovered from tomato, potato, and S. sarrachoides were pathogenic on all three hosts. Thirty-eight isolates of P. infestans from tomato, potato, and S. sarrachoides collected from southern California coastal counties, the lower San Joaquin Valley, and Baja California, Mexico, were evaluated for their race composition on tomato differential hosts. Ninety-five percent were tomato race 1 (T1) and 5% were tomato race 0 (T0). When 57 isolates were evaluated for their compatibility type, 54 were A1, one was neuter, and two presumably were homothallic.