Inheritance of BRUCHID(Callosobruchus maculatus)resistence in common beans(Phaseolus vulgaris)

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Mwila, Natasha
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Beans (Phaseolus vulgaris) are an important food as a source of protein and fit well in the farming system of smallholders in Zambia. Unfortunately the grain is prone to stor-age losses mainly due to storage insects. Bruchid (Callosobruchus maculatus) attack on dry beans is a serious storage problem causing severe losses, distorting the taste and re-ducing the market value and acceptance to the consumers. In Zambia this is a major problem contributing to food insecurity in smallholder settings. Development of bruchid resistant varieties, therefore, is a key breeding objective. Prelude to the development of bruchid resistance is the need for understanding the genetics of bruchid resistance. This study, therefore, was carried out with specific objectives to evaluate common bean for bruchid resistance, identify phytochemicals related to bruchid resistance in common bean and establish the gene action controlling bruchid resistance in common bean. The study was carried out at the University of Zambia (UNZA) involving crosses from two resistant and six susceptible genotypes in a North Carolina Design II. Beetle emer-gence was evaluated, mean development time derived and phytochemical analysis car-ried out using a thin layer chromatography. Based on the susceptibility index, bean genotypes were categorized into resistant, mod-erately resistant and moderately susceptible. Carioca 38, Rab 608, Carioca x Lukupa and Carioca x Kalungu were identified as resistant, Rab 608 x Lukupa and Rab 608 x Ka-lungu were moderately resistant and Kalungu and Lukupa were moderately susceptible. Host preference for egg laying was exhibited by the bruchids and this was linked to the seed coat colour and seed size. Darker coloured large seeded genotypes showed more number of eggs laid (5 to 13) than lighter coloured small seeded (4 to 5.5). The seed coat thickness seemed to also play an important role in enhancing resistance in the bean vari-eties in this study, suggesting that increased seed coat thickness significantly reduced insect emergence on some genotypes and hence improves their resistance.Reduced bruchid emergence and extended larval development periods in resistant geno-types suggest that antibiosis or anti feedant activity may be the actual resistance mecha-nisms.There was also a distinct presence of methyl esters, of Rf values ranging from 0.14 to 0.47, of fatty acids in the resistant varieties such as the Carioca 38 x Lukupa. This fur-ther confirmed the role phytochemicals can play in enhancing resistance in common beans. The adult emergence and the number of eggs laid significantly influenced, 74% and 18 %, respectively, the susceptibility index and were useful in explaining the re-sistance of the bean genotypes.Carioca 38 and Rab 608 were both categorized as resistant with Carioca 38 showing no methyl esters which manifest in its F2 progeny, while Rab 608 simply did not show any methyl esters in itself nor its progeny. The progeny of Carioca 38 showed resistance while that of Rab 608 was moderately resistant to the bruchids. This suggested different modes of resistance to bruchids for the two resistant bean parents. These results further suggested that Carioca 38 had multi resistance factors such as the seed color, seed size, seed coat thickness and the chemical constituents evident in its progeny.The gene action for most of the bruchid resistance traits considered in this study (suscep-tibility index, number of eggs laid, number of insects emerged, seed coat thickness and protein content) were controlled by additive gene action. The general combining ability effects for Carioca 38 were significant and negative for adult emergence (-19.65), there-fore, Carioca 38 would be good parent for continued use in selection and breeding for bruchid resistance. The heritability of the adult emergence was 79% and ultimately the susceptibility index was 53% indicating selection for these traits should be fairly easy as there is close correspondence between the genotype and the phenotype due to a relative-ly smaller contribution of the environment to the phenotype. It can be concluded that the study demonstrated the presence of adequate genotypic vari-ation among common bean genotypes on their resistance to bruchids (C. maculatus). This suggests that deliberate selection using superior genotypes in bruchid resistance identified targeting low number of eggs laid, low number of insects emerged, high pro-tein content as well as the reduced seed coat thickness which explained variations could lead to development of appropriate varieties which could give lower susceptibility index in common bean varieties. This study also suggested that the resistance of bean geno-types to bruchids (C. maculatus) is a complex one as it is governed by several factors
Plant Pests and Diseases , Bruchid-Resistance in beans , Phaseolus vulgaris , Bean beetle-Bruchid