Diversity analysis and breeding for maize weevil (Sitophilus zeamais Motschulsky) and larger grain borer (Prostephanus truncatus horn) : resistance in productive maize germplasm in Malawi.
Maize (Zea mays L.) is the main staple food crop in Malawi grown by 97% of small holder farmers. However, the potential maize yields are reduced by postharvest losses of grain in storage due to the larger grain borer (Prostephanus truncatus Horn) and maize weevil (Sitophilus zeamais Motschulsky). Limited research is conducted to improve larger grain borer and maize weevil resistance in productive varieties and to exploit their genetic potential for insect resistance breeding programmes. Little is also known about the existing genetic diversity among local maize varieties in Malawi, which is critical for selection of parents for such breeding programmes. In addition, the sustainability of insect resistant materials in farmers’ agro-environments depends on their performance in the field and on farmers’ perceptions on the varieties. Studies were conducted in Malawi between 2012 and 2014 focusing on genetic diversity analysis and breeding for maize weevil and larger grain borer resistance in productive maize germplasm. The objectives of the study were to: i) identify farmers’ perceptions on yield, maize production constraints and storability of local maize varieties; ii) determine genetic diversity of the potential breeding sources for use in introgressing larger grain borer and maize weevil resistance genes in farmer-preferred local varieties; iii) determine levels of larger grain borer and maize weevil resistance in local maize varieties; iv) determine the value for cultivation of larger grain borer and maize weevil resistant hybrids, as reflected by combination of high productivity and stability, under farmer representative conditions in multi-location trials representing the target production environments in Malawi; v) estimate general combining ability (GCA) and specific combining ability (SCA) between maize lines and their hybrids for grain yield and resistance to larger grain borer and maize weevil. Results of participatory rural appraisal showed that both hybrids and local varieties are grown by farmers. Maize hybrids are cultivated mainly because of their high yield potential and early maturity than local varieties, while local maize varieties are grown due to good tolerance to pests and diseases, large cobs, large grain size, good yields under low soil fertility, white color, superior poundability, drought tolerance and high storability than hybrids. Grain hardness, grain size, grain color, poundability and grain texture were the main characteristics used by farmers to select maize varieties tolerant to maize weevil and larger grain borer. The study indicated that farmer requirements should be incorporated in the conventional breeding programmes in Malawi. Storability and other traits should be bred in hybrids that are preferred by farmers. Diversity analysis revealed that phenotypic variation exists among local maize varieties largely due to kernel weight, plant height and ear placement. Phenotypic data produced eight clusters. SSR markers revealed 97.56% polymorphism among the loci. A total of 165 alleles were detected, with a range of 2-9 alleles and an average of four (4) alleles per locus. The mean gene diversity (He) of 0.5115 and mean heterozygosity (Ho) of 0.5233 were observed. The furthest genetic distance of 0.9001 was between local varieties 206 and local 2 and the closest genetic distance of 0.2190 was between local varieties 203 and 811. SSR marker data revealed ten clusters. Both phenotypic and genotypic data support observation of large diversity and variation among open pollinated maize varieties and landraces, which could be exploited by the breeding programme in Malawi. The analysis of resistance for maize weevil (MW) among local maize varieties showed that 14.5% of the varieties were resistant, 21.7% were moderately resistant, 24.6% moderately susceptible, 23.2% susceptible and 16% highly susceptible. Maize varieties, such as, 1772, 1983, 1992, 3243, 3244, 750 and 752 were resistant to maize weevil. For larger grain borer (LGB), all maize varieties were susceptible. However, varieties 1992, 2012, and 1983, representing Five (5) percent of the entire maize population had reasonable levels of resistance against large grain borer. Varieties 1992 and 1983 also showed high levels of resistance against maize weevil, qualifying them as suitable candidates for use in stacking MW and LGB resistance in new hybrids. Designed crosses to combine for maize weevil and larger grain borer resistance in adapted maize lines resulted in the development of 4 to 67% maize weevil resistant hybrids and 4 to 9% larger grain borer resistant hybrids across sets. Stacking of maize weevil and larger grain borer resistance produced 67% maize weevil resistant hybrids, 14% larger grain borer resistant hybrids and 14% maize hybrids with resistance to both larger grain borer and maize weevil. Maize hybrids, MWA06A showed a yield potential of 10 tons/ha, MWMW15106 (9.07 tons/ha) and MWA10A (7.69 tons/ha) and useful resistance to maize weevil. Maize hybrids, lgMW087940 expressed a yield potential of 11.05 tons/ha and MWlg06264 (8.16 tons/ha) and good resistance to both maize weevil and larger grain borer. This demonstrated that the LGB and MW resistance genes can be incorporated into productive cultivars. Analysis for gene action among maize weevil and larger grain borer resistant F1 maize hybrids revealed that both additive and non-additive gene action were responsible for determining weevil resistance. Only additive gene action was responsible for grain yield in maize weevil resistant hybrids. For larger grain borer, additive gene action was responsible for both resistance and grain yield in the F1 maize hybrids. This indicated that both selection and hybridisation strategies would be effective for breeding MW and LGB resistance in productive maize germplasm. The study has demonstrated that maize breeding in Malawi should focus at developing both hybrids and local varieties that meet different needs and traits preferred by farmers. Storability is one of such important traits in local maize varieties. The expressed genetic variation in local maize varieties provides an opportunity to explore for new sources of resistance to maize weevil and larger grain borer. The variation observed in resistance against maize weevil and larger grain borer among local varieties can be exploited to develop new populations or improve resistance in productive maize populations. Breeding for high yielding insect resistant maize hybrids is achievable and would provide a sustainable way of reducing postharvest grain losses in storage and improve net gain in maize yields for smallholder farmers in Malawi. The stacking of maize weevil and larger grain borer resistance in single maize hybrids would offer an effective way of breeding for insect pest resistance to both maize weevil and larger grain borer .The preponderance of additive gene effects over dominance gene effects in the maize hybrids gives a practical option for selection to enhance resistance and grain yield among productive maize germplasm. The exceptional hybrids, which combined high grain yield potential with high levels of resistance to MW and LGB, will be advanced in the breeding programme in Malawi. Overall, findings from the completed research will be useful for devising effective strategies in breeding programmes that emphasize grain resistance to LGB and MW and to those that seek to incorporate selection for these principal postharvest pests in the conventional breeding programmes.
Ph. D. University of KwaZulu-Natal, Durban 2014.
Corn -- Diseases and pests -- Malawi., Corn -- Breeding -- Malawi., Borers (Insects) -- Control -- Malawi., Corn -- Reproduction., Theses -- Plant breeding.