The genetics of race specific resistance in lettuce (Lactuca sativa) to downy mildew (Bremia lactucae)

Data are presented on the segregation of resistance to five British races and two Dutch races of Bremia lactucae in the F₂ progenies of crosses involving seven resistant and several susceptible lettuce cultivars. These data and also those previously published by other workers are considered in relation to the systematic model proposed by Crute & Johnson (1976) to explain the genetics of race specific resistance to B. lactucae in lettuce. It is shown that, with minor modifications, the model accommodates almost all of the previously published data and correctly predicts the new data, except for one set which cannot at present be interpreted. It is concluded that genetic evidence exists for the presence, among various cultivars of lettuce, of at least four and possibly five different dominant resistance genes of major effect designated Dm₂, Dm₃, Dm₄, Dm₆ and Dm₈; and of a pair of dominant genes with complementary effect designated Dm7/1 and Dm7/2. The resistance conferred by these genes is specified in relation to five British races, five Dutch, three Israeli and one United States race of the fungus. Resistance genotypes are proposed for cultivars Avoncrisp, Avondefiance, Calmar, Great Lakes 659, Kares, Meikoningen, Mildura, Proeftuins Blackpool, Solito, Valverde, Ventura and the USDA line PI 164937.