Inheritance of ICA Bunsi‐Derived Resistance to White Mold in a Navy × Pinto Bean Cross

Abstract

Pinto beans (Phaseolus vulgaris L.) of cool subtropical and temperate origins are extremely susceptible to white mold disease caused by Sclerotinia sclerotiorum (Lib.) de Bary. Breeding pinto beans with resistance to white mold is difficult because of the paucity of resistant germplasm in a closely related background. White mold resistant ‘ICA Bunsi’, a small white navy bean, is from the same Middle American gene pool, but it is of warm tropical origin and needs to be exploited in pinto bean improvement. Our objectives were to determine inheritance of ICA Bunsi‐derived resistance in a cross with pinto bean, determine association of the resistance with disease avoidance, and identify white mold resistant pinto beans. White mold reactions of 85 F5:8 recombinant inbred lines (RIL) from the cross ‘Aztec’/ND88‐106‐04 were characterized in the field in North Dakota and Washington in 2001 and 2002. Aztec pinto is susceptible to white mold, and ND88‐106‐04 navy bean has partial resistance derived from ICA Bunsi. Disease severity score (1 = no disease to 9 = completely susceptible), yield, and disease avoidance traits were measured. Disease severity for Aztec and ND88‐106‐04 across environments was 6.9 and 2.5, respectively, compared with 4.6 for ICA Bunsi. Reduced lodging and late maturity enhanced disease avoidance. The RILs with stay‐green stem at harvest, similar to ND88‐106‐04, also exhibited less disease. Normal distribution and moderate heritability (Hns = 56 and 36% for WA and ND environments, respectively) for disease score indicated resistance was influenced by environment and likely conditioned by genes with small effects. Nonetheless, white mold resistance was present in a few RILs with high yield potential and pinto seed type.