Evaluation of resistance to bruchids within the common dry bean germplasm in Uganda.
Ebinu, Joseph Aston
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Dry bean (Phaseolus vulgaris L.) is economically and nutritionally an important legume in many parts of Africa and Latin America. Unfortunately, two bruchid species, Acanthoscelides obtectus Say, and Zabrotes subfasciatus Boheman are known to cause extensive damage in storage, reducing the economic importance, food value and planting value of the crop. This particular work was aimed at determining the distribution pattern of the two bruchid species in bean cropping systems within Uganda; explore Ugandan dry bean germplasms for effective and adaptable sources of resistance to the two bruchid species; and identify the mode of resistance deployed by dry bean genotypes to bruchid species. Qualitative method of data collection was used. A semi structured questionnaire was used individually to get the information from farmers who were physically visited within the four major bean cropping systems of Uganda. Southwestern highlands (Kabale) and Eastern highlands (Sironko, Kween & Kapchorwa) represented bean production areas in the high altitude areas. Mpigi represented bean production areas in the Central humid lowland while Lira and Oyam represented the mid North warm lowland. To determine the bruchid distribution, bean samples were collected from farmers in the cropping systems and transported to the laboratory at NaCRRI and incubated. Results confirmed that the two bruchid species are well established storage pests, causing serious storage losses among smallholder farmers. In the absence of any control measures, farmers reported more than 90% loss of their stored beans to bruchids. To explore the problem of bruchid damage experienced by smallholder farmers, a total of 45 dry bean genotypes, comprising 20 landraces, 22 improved varieties (obtained from NaCRRI) and 3 advanced lines were tested under laboratory infestation (no choice test method). The objective of this study was to identify effective sources of resistance to the two bruchid species. Results of the study showed that there was a wide variation among the genotypes for resistance to the two bruchid species. Overall results showed that 97.7% of the genotypes were moderately resistant to Z. subfasciatus and only 2.3% of the genotypes were susceptible. Genotype screening for resistance to A. obtectus showed that only 6.7% were moderately resistant, whereas 57.7% were susceptible and 35.6% were very susceptible. All the improved genotypes had a susceptibility index ranging from susceptible to very susceptible in reaction to A. obtectus in storage. One landrace, U15-2, one improved variety, NABE 7C and one advanced line, NABE 22 consistently showed moderate resistance to both bruchid species under laboratory infestation. However, performance of most genotypes was not consistent in laboratory screenings, suggesting that mechanisms of bruchid resistance within genotypes are different. The seed coat played a significant role in conferring resistance to both bruchid species in the laboratory, whereas arcelin-like protein did not play any significant role in conferring resistance in the genotypes. Through this study, important sources of moderately resistant genotypes to bruchids in common dry bean have been identified in Ugandan bean genotypes. These moderately resistant sources can be used in a breeding programme to develop bruchid resistant bean cultivars, as well as improve resistance in susceptible commercial bean cultivars currently grown by farmers in Uganda.