Resistance to Common Bacterial Blight <i>amongPhaseolus</i> Species and Common Bean Improvement

Common bean ( Phaseolus vulgaris L.) is highly susceptible to common bacterial blight (CBB), caused by Xanthomonas campestris pv. phaseoli (Smith) Dye. High levels of cultivar resistance would minimize yield losses, reduce bactericide use and production costs, and facilitate integrated disease‐and‐crop management and the production and distribution of pathogen‐free seed. We aimed to (i) assess the levels of CBB resistance of different Phaseolus species in the tropics, (ii) determine the CBB reaction of resistant cultivars and advanced breeding lines, and (iii) report on CBB resistance of lines developed from P. vulgaris × P. acutifolius (tepary bean) hybridization and gene pyramiding at CIAT. Between 1994 and 1998, we evaluated, in the field, 162 accessions of tepary, scarlet runner ( P. coccineus ), lima ( P. lunatus ), and common beans, 119 CBB‐resistant cultivars and advanced breeding lines of common bean, and six lines recently developed by interspecific hybridization and gene pyramiding. For inoculation, we used aspersion, surgical blades, and/or multiple needles. The highest levels (scores of 1.2–2.0) of CBB resistance were found in P. acutifolius accessions, G40029 and G40156, followed by P. lunatus (scores of 4.2–6.2), P. coccineus (scores of 4.8–5.5), and P. vulgaris (scores of 4.5–6.4). Resistance available in P. coccineus and P. vulgaris landraces has already been transferred to common bean. But resistance transferred from P. acutifolius was much lower (scores of 3.8–4.5) than those available. Gene pyramiding produced lines with high CBB resistance (scores of 1.5–2.4), and is thus, a suitable method for developing CBB‐resistant cultivars of different market dasses.