GENETIC VARIABILITY FOR STEM BORER RESISTANCE IN TWO ADAPTED EARLY-MATURING MAIZE (Zea mays L.) POPULATIONS

Stem borers are among the major biotic stresses limiting the grain yield of maize, an important cereal crop in Africa. Breeding for multiple resistance to maize stem borers has been reported to be a promising method of control. Understanding the genetic variability of crop populations will help in improving them for pest resistance. Thus, genetic variability for dual resistance to two stem borer species were determined in a white (DMR ESR-W) and a yellow (DMR ESR-Y) maize populations to provide information required for improving their levels of resistance to the borers. Two experiments were conducted. In Experiment 1, a set of 100 S1 plants of DMR ESR-W were selected to produce 250 full-sib and half-sib progenies using the North Carolina Design II (NCD II) mating scheme. The progenies with six checks were evaluated under artificial infestation with two borer species (Sesamia calamistis and (Eldana saccharina) at Ibadan, and non-infested conditions at Ibadan and Ikenne in 2008 and 2009 using Randomized Incomplete Block Design with two replications. Experiment 2 was with DMR ESR-Y and the same methodology was used. In both experiments, days to 50% anthesis and silking, plant and ear height, plant and ear aspect, ear length and grain yield were measured. Resistance was measured according to levels of leaf feeding damage, dead heart, stalk breakage, cob damage and stem tunneling. Analyses of variance for NCD II were conducted to estimate genetic variances and Narrow-Sense Heritability (NSH). Correlation coefficients were determined and partitioned into direct and indirect effects. Predicted responses to selection were estimated to measure expected genetic gains. Correlated response was used to determine traits that could hasten selection progress. Tests of significance were conducted at p < 0.05. 

Infestation significantly reduced plant height (6.0 -11.1%), ear length (20.9 - 25.6%) and grain yield (23.9 - 30.4%) in both maize populations. Additive variance was significant for grain yield and stalk breakage in DMR ESR-W, and for stalk breakage, cob damage and stem tunneling in DMR ESR-Y. Narrow-sense heritability was low to moderate, but low for damage parameters except stalk breakage (40.6%) in DMR ESR-W, and cob damage (40.1%) in DMR ESR-Y. Negative correlations exist between grain yield and increasing levels of stem borer damage, with genotypic correlation between grain yield and stem tunneling being the highest (- 0.52*) in DMR ESR-Y. Stem tunneling and cob damage had high positive direct effects on grain yield reduction. Estimated genetic gain per generation was 4.0 - 6.1% (= 210kg/ha) for grain yield under infestation in DMR ESR-W, but low for damage parameters in both maize populations. Direct selection for individual trait gave better response than indirect selection through other traits.

The significant additive variances and moderate heritability estimates obtained for stalk breakage and cob damage indicate that the traits are heritable, therefore, improvement of the maize populations for stem borer resistance using these traits is feasible. Direct selection for grain yield is recommended in improving the maize populations. Stem tunneling and cob damage by the borers are major causes of grain yield reduction.