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

The availability of powdery mildew-resistant raisin accessions 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 F-1 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-resistant 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 those of commercial raisin cultivars.