Genetic resistance of selected maize inbred lines to Striga hermonthica
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Maize (Zea mays L.) is one of the most important food security crops in Uganda. It is annually cultivated on an area of 1,063,000 hectares representing 57% of the total area allocated to crop production. However, maize yields are very low in the country due to several biotic and abiotic stresses, institutional and socio-economic constraints. Among the biotic factors, Striga hermonthica inflicts significant yield losses reaching up to 100% in highly infested fields if not managed. Development of high yielding Striga resistant maize genotypes is the most sustainable strategy to boost maize productivity in Striga infested areas of Uganda. Therefore, the main objective of this study was to contribute to the development of high yielding Striga resistant maize varieties through generating knowledge on the mechanisms of resistance of the maize inbred lines to Striga hermonthica. Fifty-six maize inbred lines were evaluated in farmers abandoned naturally Striga infested fields in Namutumba district, Eastern Uganda. Field experiments were established during growing season 2016B using a 7×8 alpha lattice design with two replications. Analyses of variance on quantitative traits revealed highly significant (P ≤ 0.01) variations among inbred lines. Inbred lines TZISTR1174, TZISTR1199, 1368STR, TZISTR1181, TZISTR1198 and TZISTR1162 were identified as the most resistant to Striga infestation. They showed low Striga count, low Striga vigor, low Striga severity, low Area under Striga number progressive curve (AUSNPC) and low Area under Striga severity progressive curve (AUSVPC) suggesting their suitability for hybrid breeding to achieve high yielding Striga resistant genotypes. In bred lines CML 312 and CML 442 were identified as the least resistant genotypes. Ten inbred lines of varying resistance to Striga hermonthica were selected from the preceding study, crossed and 45 F1 hybrids developed from a 10x10 half diallel mating design. These were evaluated using a 9×5 lattice design with two replications at Nakyeere (Namutumba district), Ngerekyomu (Tororo district), and Kinyamaseka (Kasese district) during 2017A growing season. General combining ability (GCA) of the parents and Specific combining ability (SCA) of hybrids for resistance to Striga, grain yield and related maize agronomic traits were estimated. The mean squares of GCA and SCA effects showed significant differences for almost all Striga and maize agronomic traits. The GCA effects were most important for Striga traits while SCA effects were important for almost all Striga and maize agronomic traits. Inbred lines TZISTR1199, TZISTR1192, TZISTR1174 and TZISTR1162 were good general combiners for resistance to Striga (basing on their respective AUSNPC) showing highly significant negative GCA effects of -646.99, -428.21, -338.00, and -76.51 respectively while lines TZISTR1174, TZISTR1162, TZISTR1192, and CML442 were good general combiners for grain yield showing highly significant positive GCA effects of 0.40, 0.2, 0.17, and 0.22, respectively. These inbred lines could be exploited in hybrid breeding to develop high yielding Striga resistant maize varieties. Hybrids such as TZISTR1162×TZISTR1198, TZISTR1199×TZISTR1181, TZISTR1192×1368STR had highest negative significant SCA effects of -1453.19, -1058.28, and-808.252 for AUSNPC whereas crosses like TZISTR1174 x CML312, TZISTR1192 x CML442and TZISTR1174 x 1368STR had significant positive SCA effects for grain yield, which can be used for direct production as single cross hybrids or developed further as three-way cross hybrids. Genotype by environmental interactions (GXE) for resistance to Striga and grain yield were also investigated among newly developed maize hybrids from the same trial. Genotype main effect and genotype by environmental interaction (GGE) biplot and Additive Main Effect and Multiplicative Interaction (AMMI) models were used to assess the magnitude of GXE interactions for resistance to Striga and grain yield among test genotypes. Results from the analysis of variance revealed high contribution of the environmental effect for AUSNPC and grain yield compared to genotypes and GXE interaction. Experimental hybrids TZISTR1199×TZISTR1181, TZISTR1192×TZISTR1174, TZISTR1162×TZISTR1198, TZISTR1174×CML442, TZISTR1181×CML312 and TZISTR1199×TZISTR1162 were the most resistant and stable whereas, TZISTR1132×CML312, 1368STR×TZISTR1198, TZISTR1174×TZISTR1198, TZISTR1199×TZISTR1174 and TZISTR1199×CML442 had the highest grain yield means and were the most stable genotypes. Kasese was the most discriminating environment for both resistance to Striga and grain yield whereas, Tororo and Namutumba were the most representative for resistance to Striga and grain yield, respectively. TZISTR1174, TZISTR1199, TZISTR1181, TZISTR1192, TZISTR1132, 1368STR, TZISTR1162 and TZISTR1198 were resistant to Striga hermonthica. TZISTR1199, TZISTR1192, TZISTR1174 and TZISTR1162 displayed negative GCA effects for resistance to Striga hence could be used as sources of resistance genes to Striga and could be used to introgess resistance to popular susceptible maize varieties. TZISTR1174, TZISTR1162, TZISTR1192, TZISTR1132 and CML442 could be used as sources of genes for grain yield increment in a breeding programme. TZISTR1199×TZISTR1181, TZISTR1192×TZISTR1174, TZISTR1162×TZISTR1198, TZISTR1174×CML442, TZISTR1181×CML312 and TZISTR1199×TZISTR1162 were the most resistant and stable hybrids. TZISTR1132×CML312, 1368STR×TZISTR1198, TZISTR1174×TZISTR1198, TZISTR1199×TZISTR1174 and TZISTR1199×CML442 had the highest grain yield means and were the most stable. Kasese was the most discriminative environment for resistance to Striga and grain yield. Namutumba and Tororo were the most representative for resistance to Striga and yield respectively. The study recommends that TZISTR1174, TZISTR1199, TZISTR1181, TZISTR1192, TZISTR1132, 1368STR, TZISTR1162 and TZISTR1198 should be used in breeding new high yielding resistant to Striga maize varieties and the identified possible adaptation areas of test genotypes should be used for large scale production or further evaluation to bring a good return on investment to maize growers who use production inputs such as fertilizers and agro-chemicals in Striga infested areas.