Grape Germplasm Research Articles

EVALUATION OF GRAPE GERMPLASM FOR DOWNY MILDEW RESISTANCE

Trials were established in Fayetteville, Ark., in 1993 and in 1994 to evaluate grape germplasm for downy mildew resistance. Accessions were obtained from the national grape repositories in Geneva, N.Y.; Davis, Calif.; and the Univ. of Arkansas fruit substation, Clarksville. The 1993 trial contained 26 cultivars, eight selections, and 24 Vitis species; the 1994 trial contained 37 cultivars. Each trial had four single-plant replications. Plants were rated on a scale of 0 to 5 for sporulation, chlorosis, and necrosis on 23 Aug. and 26 Sept. 1994. Fungicides were not applied to the vineyard to better determine the level of natural downy mildew resistance. The results from both trials indicated that several grape accessions showed little or no symptoms of downy mildew, which suggests these have resistance that would be beneficial to use in a breeding program.

Proposed descriptors for organic acids to evaluate grape germplasm

A total of 259 grapevine accessions consisting of 169 varieties, 81 bred lines, 5 rootstocks and 4 clones of 2 wild species were surveyed for content and composition of organic acids in grape juice. Free acid content ranged from 0.13 g to 2.79 g/100 ml, and total acid content ranged from 0.36 g to 3.95 g/100 ml. The acid contents (both free and total acid) of the varieties and bred lines were lower than those of rootstocks and wild species. Furthermore, ratios of free to total acid of the varieties and bred lines were significantly lower than those of rootstocks and wild species. As tartaric and malic acids were major components in acid composition, the ratio of tartaric to malic acid (designated as β ratio) varied from 0.25 to 7.23, and the varieties and bred lines were superior to the rootstocks and wild species in the β ratio. Varietal difference in the β ratio was also observed in the cultivated species (Vitis vinifera and hybrids of V. labrusca x V. vinifera). The ripening period was associated with sharp decrease in free acid content, while β ratio was continued to increase. Although their change during ripening was remarkable, less variation in free acid content and the β ratio was observed near the ripening stage. The variations in free acid content and the β ratio as the descriptors between years were not negligble, indicating that evaluation for the two descriptors should be made in several years, at least three years.