Mapping and validation of quantitative trait loci for resistance to Cercospora zeae-maydis infection in tropical maize (Zea mays L.)

Breeding for resistance to gray leaf spot, caused by Cercospora zeae-maydis (Cz) is paramount for many maize environments, in particular under warm and humid growing conditions. In this study, we mapped and characterized quantitative trait loci (QTL) involved in the resistance of maize against Cz. We confirmed the impact of the QTL on disease severity using near-isogenic lines (NILs), and estimated their effects on three major agronomic traits using their respective near isogenic hybrids (NIHs), which we obtained by crossing the NILs with an inbred from a complementary heterotic pool. We further validated three of the four QTL that were mapped using the Multiple Interval Mapping approach and showed LOD values >2.5. NILs genotype included all combinations between favorable alleles of the two QTL located in chromosome 1 (Q ₁ in bin 1.05 and Q ₂ in bin 1.07), and the allele in chromosome 3 (Q ₃ in bin 3.07). Each of the three QTL separately significantly reduced the severity of Cz. However, we found an unfavorable epistatic interaction between Q ₁ and Q ₂: presence of the favorable allele at one of the QTL allele effectively nullified the effect of the favorable allele at the other. In contrast, the interaction between Q ₂ and Q ₃ was additive, promoting the reduction of the severity to a greater extent than the sum of their individual effects. When evaluating the NIH we found significant individual effects for Q ₁ and Q ₃ on gray leaf spot severity, for Q ₂ on stalk lodging and grain yield, and for Q ₃ on grain moisture and stalk lodging. We detected significant epitasis between Q ₁ and Q ₂ for grain moisture and between Q ₁ and Q ₃ for stalk lodging. These results suggest that the combination of QTL impacts the effectiveness of marker-assisted selection procedures in commercial product development programs.