Breeding Common Bean for High Levels of White Mold Resistance.

White mold caused by Sclerotinia sclerotiorum (Lib.) de Bary is among the most severe diseases of common bean, Phaseolus vulgaris L. in North America and similar environments elsewhere. Under favorable cool and humid conditions 100% crop loss may occur on susceptible cultivars. The fungus is endemic, seed-transmitted, and its sclerotia may survive in soil for 5 or more years. Low levels of white mold avoidance/resistance occur in the Middle American common bean. However, higher levels of resistance occur in the Andean common bean and Phaseolus species of the secondary gene pool such as P. coccineus, P. polyanthus, and P. costaricensis. White mold resistance and avoidance traits are quantitatively inherited with low to moderate heritability, and controlled by >20 QTL. Of which 13 QTL influencing plant architectural avoidance traits are co-located with 13 white mold resistance QTL. Also, major dominant and recessive resistance genes in common bean, and a single dominant gene in P. vulgaris/P. coccineus interspecific populations were reported. A three-pronged breeding strategy involving (1) introgression of resistance from distantly related germplasm, (2) pyramiding resistance from across Phaseolus species, and (3) combining high levels of resistance with other desirable traits into high yielding high quality cultivars would be essential to combat white mold in the USA and elsewhere. Some form of backcrossing for introgresion, use of multiple-parent crosses for pyramiding resistance and cultivar development, and use of multiple S. sclerotiorum isolates of different aggressiveness, multiple-inoculations/plant, weekly evaluations beginning at 7 d post-the first inoculation and verification of the resistance response at harvest would be required. The above methodology has successfully been used to introgress high levels of white mold resistance from the secondary gene pool (e.g., VCW 54, VCW 55, VCP 13, VRW 32), pyramid resistance (e.g., PRA 152, PRA 154, PRA 155, PRP 153), and common bean cultivar development.