Genetics of metalaxyl resistance in Phytophthora infestans

Several sexual crosses involving isolates of Phytophthora infestans of dive rse sensitivities to metalaxyl were studied, Metalaxyl sensitivity was dete rmined by comparing the growth of an isolate on metalaxyl-amended agar medi um (5 mu g/ml) with growth on medium containing no metalaxyl. When both par ents had the same phenotype for metalaxyl sensitivity (both resistant or bo th sensitive), all FI progeny had the parental phenotype. In two crosses (7 5 and 76) each involving one sensitive and one resistant parent, however, t he progeny segregated 1:1, suggesting that the common resistant parent (Bg8 ) was heterozygous for metalaxyl sensitivity. When an F2 progeny was constr ucted from resistant Fl isolates in cross 76, the progeny segregated 1:3 (s ensitive: resistant], indicating that metalaxyl resistance in Bg8 is confer red by a single dominant gene. Variation in the progeny sensitivity appears to involve minor genes. A correlation study between metalaxyl resistance a nd fitness components did not reveal any association. (C) 1999 Academic Pre ss.