Mode Of Gene Action And Effects Of Environment On Expression Of Resistance To Diplodia Ear Rot In Tropical Maize
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Diplodia ear rot is one of the most important maize diseases in Kenya. The fungus causes yield losses and also contaminate the grains with mycotoxins, diplodiol and diploidiatoxin, making the maize unhealthy, and can even be lethal, if ingested by humans and livestock. Little work has been conducted on Diplodia ear rot. Two experiments were conducted to evaluate maize germplasm for resistance to Diplodia ear rot disease which could effectively combat the negative effects of maize nutritive quality and production. The objectives of this study were to: (i) assess the performance of hybrids for Diplodia resistance and yield (Experiment 1); (ii) and to estimate the mode of gene action, general and specific combining abilities among hybrids and inbred lines under inoculated, non-inoculated and drought conditions (Experiment 2). In Experiment 1, 80 hybrids and four check varieties were assessed under natural and artificial disease inoculation across six locations. An Alpha lattice design was used with two replications. The plots which were kept free of weeds measured 5 m long and were spaced at 0.75 m between rows and 0.25 m between hills. Each experiment received DAP fertilizer at 80 Kg/ha P2O5 and 31 Kg/ha at planting and top dressed with CAN at 80 Kg N six weeks after planting. Results indicated significant differences among the hybrids (P<0.001) for grain yield and severity of Diplodia disease. Hybrid 31 (incidence and severity of 9 and 14) and H 3 (incidence and severity of 6 and 4) had the lowest disease severity under artificial and natural disease conditions. The best producing hybrids were hybrid 6 (6.6 t/ha-1) and hybrid 66 (6 t/ha-1) on average under both conditions compared with WH505 (4.5 t/ha-1) which was the best check across the six locations. For Experiment 2, 66 F1 diallel hybrids and 12 parental inbred lines were evaluated under natural and artificial disease conditions as well as under managed drought conditions across ten locations. The mode of gene action and combining ability were estimated using Griffing Method 4, Model I and means were separated using least significant difference method of Duncan. Similar to the effects associated with grain yields, general combining ability (GCA) effects were more important than the specific combining ability (SCA) for both disease incidence and severity under stress and non-stress conditions. The non- additive gene effects were more important than additive ones under high Diplodia disease pressure conditions; the contrary was true under low Diplodia disease conditions. Hybrids 5 and H2 were associated with high yields under high and low Diplodia disease respectively while H51 and H2 were the most resistant hybrids. Among the 66 hybrids evaluated it was observed that resistant hybrids produced more yield thus, resistance to Diplodia was closely associated with high grain yields. It was concluded that inbred lines CL-RCW31-# and CKL05003 had the best GCA effects for Diplodia incidence and severity under both conditions. Also inbred line CKL05003, CL-RCW31-# and CKL05019 had best GCA effects for grain yield. It was also concluded that hybrids H51, H2 and parental lines CKL05019 and CKL05003 had the highest potential for commercial exploitation. It was therefore, recommended that these single cross hybrids and their parental inbred lines should be incorporated into breeding programmes for Diplodia disease resistance in East Africa.