Pyramiding genes for resistance to major diseases into a market-class common bean variety in Kenya

Common bean (Phaseolus vulgaris L.) production in sub-Saharan Africa (SSA) is persistently threated by three diseases; namely, Anthracnose (Colletotrichum lindemuthianum), bean common mosaic and necrosis viruses (BCMV/BCMNV) and common bacterial blight (Xanthomonas axonopodis pv. Phaseoli). Under favourable conditions, yield losses of between 50 -100 percent have been reported in susceptible cultivars. The objective of this study was to pyramid resistance genes against major common beans pathogens into a susceptible commercial cultivar. SCAR markers, SAS 13 (anthracnose), SU91 (CBB) and SG-6 (BCMV/BCMNV) were used to track respective resistance genes in the breeding populations and phenotypic screening via inoculations to target pathogens was done to confirm resistance among F2 progenies and pryamids. Segregation ratios in F2 populations followed expected Mendelian patterns: 3:1 (anthracnose and CBB) and 1:3 (BCMV/BCMNV). Chi-square tests validated these ratios, and disease severity data were subjected to analysis of variance (SAS software). Marker segregation in F2 populatios for resistance to CBB and anthracnose showed a good fit to the expected ratio for a single dominant gene (3R: 1S); while for BCMV/BCMNV followed that of a reccesive gene (1R:3S). Twelve plants harbouring three-gene combinations were obtained, with nine exhibiting robust resistance upon artificial inoculation to the target pathogens. These results demonstrate successful introgression of anthracnose, BCMV/BCMNV and CBB resistance into a previously susceptible background, confirming the efficacy of marker-assisted selection in breeding common bean varieties with durable resistance.