Comparison of Genetic Gain from Phenotypic, Genotypic, and Marker-assisted Selection for Improved Resistance of Maize to Multiple Foliar Pathogens.

Northern corn leaf blight (NCLB) incited by Exserohilum turcicum, gray leaf spot (GLS) incited by Cercospora zeae-maydis and maize streak incited by Maize streak mastrevirus (MSV), are among the most destructive diseases limiting maize production in sub-Saharan Africa. Most foliar diseases of maize are managed using quantitative (partial) resistance and previous studies have reported quantitative trait loci (QTL) associated with host-resistance. A population of 410 F_2:3 progenies, derived from hybridization between inbred line CML202 (E. turcicum and MSV resistant), and VP31 (C. zeae-maydis resistant breeding line), were planted in field nurseries inoculated separately with each pathogen.  Broad and narrow-sense heritability estimates were obtained using F_2:3 and selected and non-selected F_2:4 progenies.  Gene action of the candidate QTL was determined using orthogonal contrasts. We used simple sequence repeat (SSR) markers flanking six target QTL associated with partial resistance and compared three different selection strategies (phenotypic, genotypic, and marker-assisted selection (MAS) index; for improvement of resistance to each pathogen during early generation pedigree selection. Narrow-sense heritability estimates were 0.22, 0.25 and 0.39 for MSV, NCLB and GLS, respectively and mostly dominant gene action was displayed by all resistance QTL. The genetic gains from genotypic selection were highest for MSV followed by GLS and then NCLB. Cumulative genetic gains for improved resistance were practically the same for both phenotypic and genotypic selection and MAS index produced the highest gains for all diseases. Values of predicted genetic gains were higher than realized gain, but the relative values for the different selection procedures were consistent with the trend for actual genetic gains. Estimates of costs based on lower boundary values indicated the cost of marker-based selection was lower than that of phenotypic selection. Our results indicate that markers linked to target QTL can facilitate pyramiding resistance to multiple diseases during early generation pedigree selection.