A New Gene for Resistance toBremia lactucae

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Abstract
The lettuce [Lactuca sativa] cv. Vanguard 75 was free of downy mildew caused by B. lactucae in a lettuce cultivar evaluation trial during 1980 in Oswego County, (New York [USA]), while other cultivars of lettuce in the trial had various amounts of mildew. ''Vanguard'' and ''Vanguard 75'' were resistant when inoculated with a number of isolates of B. lactucae, but they were completely susceptible when inoculated with other isolates, indicating that the resistance in these cultivars is race specific. Analysis of resistance and susceptibility patterns indicates that this is a new resistance factor, different from the 12 previously described. By testing F1, F2, and test cross families from crosses of ''Vanguard 75'' and susceptible cultivars, it was determined that the resistance factor in ''Vanguard 75'' is conditioned by a single dominant gene.