Rootstock Effects on Deficit-Irrigated Winegrapes in a Dry Climate: Vigor, Yield Formation, and Fruit Ripening

A rootstock field trial was conducted in the Yakima Valley, southeastern Washington, with three Vitis vinifera cultivars (Merlot, Syrah, Chardonnay). Vines were grown on their own roots or field-grafted to the rootstocks 5C, 99R, 140Ru, 1103P, 3309C, and an unnamed rootstock from Cornell University (here termed 101CU) that is a likely sibling or seedling of 101-14 Mgt. Repeated scion dieback due to cold injury to 99R led us to abandon this rootstock. Vine phenology, vigor, water status, yield formation, and fruit ripening and composition were evaluated during three years beginning in the vineyard's ninth year. Own-rooted Merlot and Chardonnay grew more shoots than grafted vines, and 140Ru and 1103P tended to reduce pruning weights. However, 3309C was the rootstock associated with the highest pruning weights in Syrah and the lowest in Chardonnay. Rootstocks usually did not impact vine phenology, fruit set, and plant water status, although there was a trend for stem water potential to be highest with 3309C and lowest with 5C. The rootstock effect on yield formation depended on the scion cultivar, and variations in different yield components often cancelled out each other, but 3309C (Merlot and Syrah), 5C (Merlot and Chardonnay), and own roots (Chardonnay) were often associated with high yields. Nevertheless, the rootstocks had only minor effects on fruit ripening and did not consistently alter soluble solids, TA, K+, or anthocyanin pigments, but the pH was higher in fruit from own-rooted vines compared with grafted Merlot and Chardonnay. Overall, scion effects and differences due to yearly climate variation far outweighed any differences due to rootstock.