Insensitivity to Metalaxyl in California Populations ofBremia lactucaeand Resistance of California Lettuce Cultivars to Downy Mildew

Ninety-seven California isolates of Bremia lactucae were characterized for virulence phenotype, metalaxyl insensitivity, and sexual compability type from 1987 to 1989. Seventy-eight lettuce cultivars and 85 lettuce accessions were screened for resistance to lettuce downy mildew caused by the three major California pathotypes of B. lactucae. Of the 97 B. lactucae isolates, two were California pathotype IA, four were pathotype II, 30 were pathotype III, and 61 seemed to be pathotype IV. Several pathotype IV isolates were separated easily into derivatives having virulence phenotypes identical to either pathotype II or pathotype III (...)