Inheritance pattern of downy mildew resistance in advanced generations of sorghum*

Summary In a project aimed to incorporate downy mildew resistance into sorghum hybrid seed parents, we screened F 4 and F 5 families for resistance to the ICRISAT Centre isolate of the pathogen using a greenhouse seedling screening technique. The families originated from a cross of 296B (susceptible) and IS 18757 [(QL‐3) resistant]. The F 4 s were obtained from agronomic selection in F 2 s and F 3 s, and the F 5 families from advancing plants identified as resistant in segregating F 4 families. The resistant plants were more than double the number of susceptible plants in the F 4 and almost so in the F 5 suggesting that resistance to downy mildew was dominant. Of the four genetic models examined (a single‐locus model and three two‐locus models with complementary, inhibitory, and a combination of complementary and inhibitory interactions), the two‐locus model with independent segregation and a combination of complementary and inhibitory inter‐allelic interaction appeared to be most appropriate in explaining the segregation patterns within and among F 4 and F 5 families. Accordingly, for resistance to P. sorghi , the suggested genotypes for IS 18757 is Pl a Pl a Pl b Pl b and for 296B is Pl a Pl a Pl b Pl b .