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 F4 and F5 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 F4s were obtained from agronomic selection in F2s and F3s, and the F5 families from advancing plants identified as resistant in segregating F4 families. The resistant plants were more than double the number of susceptible plants in the F4 and almost so in the F5 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 F4 and F5 families. Accordingly, for resistance to P. sorghi, the suggested genotypes for IS 18757 is PlaPlaPlbPlb and for 296B is PlaPlaPlbPlb.