QTL for resistance to Setosphaeria turcica in tropical African maize

Setosphaeria turcica (Luttrell) Leonard Sr Suggs causes northern corn leaf blight (NCLB), an important disease of maize (Zea mays L,) in sub-Saharan Africa and other tropical and temperate regions. Quantitative trait loci (QTL) for resistance were mapped and characterized in a population of F(2:3) lines derived from a cross between the susceptible line Lo951 and the highly resistant CIMMYT line CML202, and tested in three inoculated trials in Kenya (n = 194-256 lines). Incubation period (IP) and area under the disease progress curve (AUDPC, based on three readings) were assessed. Both traits showed a continuous distribution in the population with no transgressive segregation. Variance components for genotypes (G) and environments (E) were highly significant for both traits. G x E interaction was also significant but less important. Heritability was 0.86 for IP and 0.81 for AUDPC, Genotypic and phenotypic correlations between IP and AUDPC were tight (r(g) = -0.84, r(p) = -0.71). The genetic map encompassed 110 RFLP marker loci with an average interval length of 17 centimorgans (cM). Composite interval mapping revealed different but overlapping sets of QTL in the individual environments, In the combined analysis, 10 QTL were detected for IP and AUDPC. These included two QTL with resistance alleles coming from the susceptible parent, Lo951, Six QTL (on Chromosomes 2, 3, 5, 8, 9) were associated with both traits, two affected only IF, another two only AUDPC. The QTL explained 61% of the genotypic and 52% of the phenotypic variance of IP (AUDPC: 69 and 56%). The magnitude of the individual QTL effects was homogeneous and their gene action additive. Marker-assisted improvement of NCLB resistance is considered feasible, though not necessarily economically viable.