Graduate Thesis Or Dissertation
 

Pyramiding quantitative trait loci conditioning partial resistance to Sclerotinia sclerotiorum in bush blue lake green beans (Phaseolus vulgaris)

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  • Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic pathogen capable of causing white mold, a severe disease in common bean. White mold is of particular concern to the Oregon snap bean 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 combined two quantitative trait loci (QTL) for physiological resistance to white mold: a QTL located on linkage group 7 from G122 and a B8 QTL from NY6020. The B7 QTL is linked to phaseolin for which a sequence characterized amplified region (SCAR) marker phaseolin 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 locus which conditions the white-seeded trait. While most snap beans have T phaseolin seed protein, the OSU bush blue lake (BBL) materials have the S form of phaseolin, facilitating the use of T phaseolin as a selectable marker in breeding for white mold resistance. Thus, transfer of this QTL has to be coupled with breaking the linkage between colored seed and the resistance QTL. The B8 QTL is linked to the SS18₁₆₅₀ SCAR and AW19₁₂₀₀ random amplified polymorphic DNA (RAPD) markers. Oregon State University BBL bean germplasm originally developed with single QTL were crossed 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, OSU 6230, and OSU 6241 are advanced breeding lines that have the SS18₁₆₅₀ allele from NY6020 and show statistically significant higher levels of resistance in the field and greenhouse (straw) test compared to susceptible cultivars. White-seeded, T phaseolin types were selected from a OR 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 selection in the straw test. One hundred and forty eight families were planted in a randomized complete block design (RCBD). All families had been previously genotyped using the PHAS and SS18₁₆₅₀ SCAR molecular markers. Plants were inoculated using actively growing mycelium of S. sclerotiorum and scored using a modified straw test to test for genetic additivity among marker classes. None of the lines were statistically more resistant than G122, a QTL donor and standard resistant check. In a separate study, data collected in NY6020-5 x OR 91G and NY6020-5 x OSU 5613 populations suggest that NY6020-5 has a B7 QTL equivalent to G122. Other researchers have presented evidence that G122 has a B8 QTL equivalent to NY6020. Finally in a mixed linear model study we identified two markers, B18₁₅₀₀ and C8₁₂₀₀, which should prove useful in breeding for white mold resistance. While the material that we developed does not show significantly higher levels of resistance than the resistant parents, we have transferred the resistance QTL into a bush blue lake background, and the lines derived from this work should have significantly higher levels of resistance than existing commercial cultivars. We also present evidence of a QTL not previously identified in NY6020-5.
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