Genetic analysis and response to selection for resistance to two stem borers, Busseola fusca and Chilo partellus, in tropical maize germplasm.
Mwimali, Murenga Geoffrey.
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Maize is the principal staple food in sub-Saharan Africa (SSA), but production lags behind population growth. The African stem borer, Busseola fusca, Fuller (Lepidoptera, Noctuidae), and the spotted stem borer, Chilo partellus, Swinhoe (Lepidoptera, Crambidae) are serious insect pests of maize in tropical environments. The damage can be managed by breeding stem borer resistant maize varieties but there is limited information that can be used to devise appropriate breeding programs. Therefore breeding investigations were conducted to appraise germplasm screening methods, and to determine combining ability, heterosis and response of maize populations to S1 progeny recurrent selection. The study was conducted in Kenya during 2010 to 2013. The results showed that most of the test genotypes were susceptible to B. fusca and less so to C. partellus, indicating that breeding for B. fusca would be more challenging. Therefore more resources would be required to improve maize germplasm for resistance to B. fusca to broaden the base from which breeders will select suitable lines for breeding. There was a highly significant (r=0.947, p≤ 0.01) correlation between rank selection index in the greenhouse and laboratory. The detached leaf disk bioassay method was effective for screening maize genotypes for resistance to both stem borers. Therefore it will be recommended for use in screening maize genotypes in future studies. The line x tester studies indicated a preponderance of the additive gene effects for borer resistance traits. Specific combining ability effects were significant for resistance traits and grain yield indicating that non-additive effects were also influential. Findings from the breeding investigations will impact positively on both food security and plant breeding capacity. The completed study was successful in identifying new maize inbred lines with resistance to both stem borers. These lines have high utility to maize breeding programmes that emphasise stem borer resistance in tropical environments. For the hybrid-oriented programmes, combining ability and heterotic orientation data for the 66 maize inbred lines will be crucial. In this regard the study was very successful in classifying the lines into three heterotic groups according to single cross testers (CML395/CML444, and CML312/CML442) that are widely used at CIMMYT, and by public breeding programs throughout SSA. Importantly, this was done based on grain yield potential of hybrids under B. fusca and C. partellus infestations in three mega environments. The study demonstrates that S1 progeny recurrent selection is effective for improving stem borer resistance, without compromising yield. There was significant reduction (69%) in maize plant damage by both pests, and yield gains of 25% to 70% were realised in two populations. This represents significant contribution to plant breeding capacity, especially to maize breeding programmes that emphasise stem borer resistance in hybrids.