Pyramiding QTL Conditioning Partial Phenotypic Resistance to Sclerotinia Sclerotiorum Into a Bush Blue Lake Green Bean (Phaseolus vulgaris) Background

White mold caused by Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic pathogen capable of causing severe infection in common bean.  White mold is of particular concern to the Oregon vegetable processing industry, where processors allow less than 3% incidence in harvested shipments.  Breeding for white mold resistance in beans has been difficult due to quantitative inheritance and low heritability.  We pyramideded two QTL for physiological resistance to white mold; a B7 QTL from G122 and a B8 QTL from NY6020.  The B7 QTL is linked to phaseolin for which a SCAR marker (PHAS) has been used successfully to transfer the QTL in dry bean.  The transfer in snap bean is more challenging because this QTL is also linked to the p which conditions white seed.  While most snap beans have T-phaseolin, the OSU bush blue lake (BBL) materials have S-phaseolin, facilitating the use of T-phaseolin as a selectable marker. Simultaneously, the linkage between colored seed and the resistance QTL must be broken.  The B8 QTL is linked to the SS18.1650 SCAR and AW19.1200 RAPD markers.  Oregon State University BBL bean germplasm originally developed with single QTL of interest were crossed in order to pyramid the two resistance QTL.  The assumptions made in combining these two sources of resistance are that the QTL are non allelic and are additive.  OSU 6229, 6230, and 6241 are all advanced breeding lines that have the SS18.1650 allele from NY6020 and show statistically significant higher levels of resistance in the field and straw test compared to susceptible cultivars.  White seeded, T phaseolin types were selected out of a 91G* x G122 BC2F3 population.  The selected lines showed levels of resistance significantly better than the susceptible check cultivars in the straw test.  The two sources were crossed and the progeny were subjected to three or more generations of phenotypic recurrent selection.  Sixty-eight F4:6 families and 74 F5:7 families were planted in three blocks using an RCBD design.  All families had been previously genotyped using the PHAS and SS18.1650 SCAR markers.  Plants were inoculated using active mycelium of S. sclerotiorum and scored using a modified straw test ranking to test for genetic additivity among marker classes.  Results to date show that while lines with markers for both QTL possess similar resistance levels to the resistant parents, none are significantly more resistant.  This suggests that assumptions of additivity of these two QTL may not be correct.