Common bacterial blight (CBB) caused by (Xanthomonas axonopodis pv. phaseoli) is a major seed-borne disease limiting commercial yield and disease-free seed production of dry bean (Phaseolus vulgaris) worldwide. Genetic resistance is the most effective control method but is difficult to incorporate because of quantitative inheritance and origin in exotic backgrounds. More than 20 QTL conditioning resistance to CBB have been identified which further attests to the complexity of resistance. Two of the QTL conditioning CBB resistance in dry bean derive from tepary bean (P. acutifolius) via interspecific hybridization and have major effect. The QTL are tightly linked with the dominant SCAR markers BC420 and SU91 on linkage groups B6 and B8, respectively. Our objective was to study the effect of combining these two QTL on level of resistance in a near-isogenic background. A population of 116 BC₆F₂ (‘Teebus'*6/XAN-159) plants segregating for both QTL was developed. Teebus is a South African white bean highly susceptible to CBB. XAN-159 is a breeding line from CIAT that possesses both QTL and a high level of resistance to CBB. The individual BC₆F₂ plants were inoculated with the pathogen and assayed for the QTL-linked markers. The BC420 and SU91 SCARs were interpreted as codominant markers by conversion to Taqman probes and assay by Real-Time PCR. Codominant markers enabled the effect of allelic dosage at each QTL to be examined. The QTL expressed recessive epistasis (9:3:4) as BC₆F₂ plants with both QTL had the highest level of resistance (9), SU91 only had intermediate resistance (3), and plants with BC-420 only or neither QTL expressed no resistance (4). This is the first report of recessive epistasis between two QTL for any trait in dry bean.