INHERITANCE OF RESISTANCE TO SOUTHERN CORN RUST IN TROPICAL BY CORN-BELT MAIZE POPULATIONS

The inheritance of resistance to southern rust (caused by Puccinia pol ysora Underw.) was investigated in two F-2:3, populations derived from crossing two temperate-adapted, 100% tropical maize (Zea mays L.) inb red lines (1416-1 and 1497-2) to a susceptible Corn Belt Dent hybrid, B73Ht x Mo17Ht. The inbred lines possess high levels of resistance to southern rust and may be unique sources of resistance genes. Heritabil ity for resistance was estimated as 30% and 50% in the two populations from regression of F-2:3, family mean scores on F-2 parent scores, an d as 65% and 75% from variances among F-2:3 families on a single-plot basis. RFLP loci on three chromosomal regions previously known to poss ess genes for resistance to either southern rust or common rust (P. so rghi Schw.) were used to localize genes affecting resistance to southe rn rust in selected genotypes of both populations, and to estimate the ir genetic effects. A single locus on 10S, bnl3.04, was associated wit h 82-83% of the variation among field resistance scores of selected F- 2:3 families in the two populations. Loci on chromosomes 3 (umc26) and 4 (umc31) were significantly associated with resistance in the 1497-2 population, each accounting for 13-15% of the phenotypic variation fo r F-2:3 field scores. Multiple-marker locus models, including loci fro m chromosomes 3, 4, and 10 and their epistatic interactions, accounted for 96-99% of the variation in F-2:3 field scores. Similar results we re obtained for resistance measured by counting pustules on juvenile p lants in the greenhouse, An attempt was made to determine if the major gene for resistance from 1416-1 was allelic to Rpp9, which is also lo cated on 10S. Testcross families from the cross (1416-1 x B37Rpp9) x B 14AHt were evaluated for resistance to southern rust in Mexico. Neithe r source of resistance was completely effective in this environment, p reventing determination of allelism of the two genes; however, both so urces of resistance had better partial resistance to southern rust tha n did B144Ht.