Maintaining product quality in raisins throughout the introgression of powdery mildew resistance

Authors: Ledbetter, Craig; Lee, Steven

Submitted to: Journal of Horticultural Science and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2018
Publication Date: 1/9/2019
Citation: Ledbetter, C.A., Lee, S.A. 2019. Maintaining product quality in raisins throughout the introgression of powdery mildew resistance. Journal of Horticultural Science and Biotechnology. 94(4):460-467. https://doi.org/10.1080/14620316.2018.1561213.
DOI: https://doi.org/10.1080/14620316.2018.1561213

Interpretive Summary: All cultivars of raisin grapes are susceptible to powdery mildew, and control of the disease represents a major production expense for raisin growers. There are wild species of grape that naturally resist infection of the powdery mildew fungus, but these grapes have very poor fruit quality characteristics. A breeding effort was initiated to combine the powdery mildew resistance from wild grape species with high fruit quality raisins that had already been developed in the breeding program. At each cycle of breeding, seedlings were exposed to powdery mildew to identify and eliminate seedlings that were susceptible to the disease. Resistant seedlings were then fruited, and vines possessing the desired combination of fruit quality traits and production characteristics were used in the next cycle of breeding. After two cycles of breeding, raisin selections were identified with fruit quality traits similar to those of commercial raisin cultivars. Elite powdery mildew-resistant raisin selections are currently being trialed under different pruning regimes to identify selections with acceptable yield. Utilization of powdery mildew-resistant raisins in California will significantly reduce fungicide applications in this important commodity.

Technical Abstract: The availability of powdery mildew-resistant raisins will lower grower production costs and enhance the environment through reduced fungicide use. To achieve this objective, backcrossing has been employed with Vitis romanetii as the source of non-race-specific powdery mildew resistance. Initial crosses of powdery mildew-resistant F1 hybrid B36-45 with seeded raisin cultivars Rangspray and Raisin de Palestine yielded seedless powdery mildew- resistant first generation backcross selections C87-41 and C87-106, which were used to develop second generation backcross populations. Principal component analyses consistently identified ‘wrinkle', ‘meatiness', ‘product attractiveness’ and ‘skin persistence’ as being the quality characteristics most important in discriminating among powdery mildew raisin accessions and commercial raisin cultivars. Raisin quality ratings were much improved across most evaluated characters in second generation backcross families as compared with parental powdery mildew- resistant accessions C87-41 and C87-106. After two generations of backcrossing, powdery mildew-resistant raisin selections were identified with product quality characteristics similar to commercial raisin cultivars.