VIEW ARTICLE

Disease Control and Pest Management

Insensitivity to Metalaxyl in California Populations of Bremia lactucae and Resistance of California Lettuce Cultivars to Downy Mildew. T. M. Schettini, Department of Vegetable Crops, University of California, Davis, 95616; E. J. Legg, and R. W. Michelmore. Department of Vegetable Crops, University of California, Davis, 95616 Phytopathology 81:64-70. Accepted for publication 27 July 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-64.

Ninety-seven California isolates of Bremia lactucae were characterized for virulence phenotype, metalaxyl insensitivity, and sexual compatibility 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. Two isolates derived from pathotype IV isolates, however, had virulence phenotypes that deviated from virulence phenotypes previously reported for California. When assayed for fungicide insensitivity, the two pathotype IA isolates were sensitive to metalaxyl, while the four pathotype II isolates, 73% of the pathotype III isolates, and 82% of the pathotype IV isolates were insensitive to metalaxyl. When pathotype IV isolates were separated into pathotype II and pathotype III components, both derivatives expressed the same sensitivity to metalaxyl as their pathotype IV progenitor. Of the 175 lettuce accessions tested, 114 were resistant to at least one California pathotype, with 51, 12, and 82 being resistant to at least pathotype 1A, II, or III, respectively. In addition, 11 accessions were resistant to all three pathotypes. Of the 21 crisphead cultivars that were resistant to at least pathotype III, five may be adapted for the coastal valleys where downy mildew can be severe. Combining genetic and chemical strategies may provide effective control of downy mildew in California if metalaxyl-insensitive pathotype II derivatives do not cause epidemics in the field.