Genetic studies of Phytophthora infestans, a late blight pathogen of potato and tomato

Several sexual crosses involving field isolates of Phytophthora infestans which were diverse in metalaxyl sensitivity and in tomato pathogenicity were analyzed. Recombinant progeny were identified from analysis of mating type, allozymes, and (when needed) DNA fingerprinting. Metalaxyl sensitivity of each isolate was determined by comparing growth of an isolate on metalaxyl amended medium (5ppm) with growth on medium containing no metalaxyl. When both parents had the same phenotype for metalaxyl sensitivity (both resistant or both sensitive) there was no segregation in the F1 progeny but all had the parental phenotype. In two crosses (75 and 76) each involving one sensitive and one resistant parent, however, the progeny segregated 1:1, suggesting that the common resistant parent (Bg8) was heterozygous for metalaxyl sensitivity. When an F2 progeny was constructed from resistant F1 isolates in cross 76, the progeny segregated 1:3 (sensitive: resistant), indicating that metalaxyl resistance in Bg8 is conferred by a single dominant gene. Variation in the progeny sensitivity appears to involve minor genes. Meanwhile, a correlation study between metalaxyl resistance and fitness components using one cross did not reveal any association.; The genetic basis of specialization to tomato in P. infestans was examined with the same crosses. Pathogenicity to tomato and potato was measured on detached leaflets. The analyses of crosses involving isolates having the same degree of pathogenicity to either potato (64) or both potato and tomato (74) indicated little segregation. In a cross (76) between one potato/tomato pathogenic parent and one potato pathogenic parent, however, the progeny segregated for pathogenicity to both hosts. In a reciprocal cross (77) between potato pathogenic and potato/tomato pathogenic parents, 22 of 23 progeny were not pathogenic to tomato, indicating recessive genes for tomato pathogenicity in cross 77. Analysis of a sibcross (80) between potato pathogenic F1 progeny of cross 77 identified one progeny with both potato and tomato pathogenicity consistent with a model of recessive genes (more than one) conditioning tomato pathogenicity.