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Genetics of Virulence in Californian Populations of Bremia lactucae (Lettuce Downy Mildew). T. W. Ilott, Department of Vegetable Crops, University of California, Davis 95616, Present address: The Plant Breeding Institute, Cambridge, England; M. E. Durgan(2), and R. W. Michelmore(3). (2)(3)Department of Vegetable Crops, University of California, Davis 95616; (2)Present address: Bud of California Ag Research, Salinas 93902. Phytopathology 77:1381-1386. Accepted for publication 16 March 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1381.

Virulence phenotypes and sexual compatibility types were determined for 116 Californian isolates of Bremia lactucae collected between 1982 and 1986. All but one isolate could be grouped into one of three distinct pathotypes on the basis of their virulence phenotypes. Sexual compatibility type (SCT) was absolutely correlated with pathotype. All pathotype I isolates had the B1 SCT, and all pathotype II and III isolates had the B2 SCT. The genetic basis of avirulence was also studied for representative isolates of each pathotype. The avirulence genotypes and the lack of diversity in virulence phenotype suggested that the sexual cycle has not been important in the generation of variation in the Californian population of B. lactucae. This may have been because of the infertility of matings between the predominant pathotypes.

Additional keywords: Lactuca sativa, resistance.