Breeding Common Bean for High Levels of Resistance to White Mold.

White mold caused by Sclerotinia sclerotiorum (Lib.) de Bary is a highly destructive disease in common bean in the United States. Quantitatively inherited partial resistance occurs in the primary and secondary gene pools. A three-pronged strategy is essential for breeding for high levels of resistance: (1) introgressing resistance from relatives, (2) pyramiding or combining resistance from across gene pools, and (3) transferring resistance in cultivars. Our objectives were to: (1) introgress high levels of resistance from the secondary gene pool, P. coccineus PI 439534, (2) pyramid high levels of resistance from across the primary and secondary gene pools, (3) compare the resistance of new breeding lines with parents and known sources of resistance, and (4) assess the composition of SCAR markers linked with resistance QTL. Susceptible pinto ‘Othello’, three interspecific breeding lines (IBL) derived from UI 320*2/PI 439534, four pyramided breeding lines (PBL) from three inter-gene pool crosses, and resistant parents were evaluated in the greenhouse inoculation with ARS12D, ND710, CO467 and NY133 S. sclerotiorum isolates. Othello, UI 320, and ICA Bunsi with WM2.2 QTL were susceptible to the four isolates (mean scores 6.6 to 8.9). Pinto VC13 series IBL with WM2.2 QTL were significantly more resistant (3.7 to 5.3) than PI 439534 (5.6 to 6.2) to ARS12D, ND710, and CO467 isolates. Also, they were more resistant than IBL I9365-31 with WM7.3 QTL and 92BG-7 (5.7 to 7.3) to the four isolates. Pinto SE 153 series PBL with WM2.2 and WM7.1 QTL, and Andean SE 155-9 with WM2.2, WM7.1, and WM8.3 QTL had significantly lower scores (3.7 to 4.8) than their parents USPT-WM-1 with WM2.2 QTL and 92BG-7 (5.3 to 8.3) to all isolates. The direct disease screening with multiple pathogen isolates was crucial for development of IBL and PBL with broad spectrum high levels of resistance.