Matthew Fidelibus,* George Zhuang, and Rodrigo Espindola
*University of California, 9240 S Riverbend Ave, Parlier, CA 93648 (mwfidelibus@ucdavis.edu)

Sunpreme, a new USDA raisin grape variety, has two key traits: fruitful basal buds, and fruit that dry spontaneously after ripening. These unique traits should make it possible to prune vines and harvest raisins from them with machines, with little, if any, additional labor needed for either practice. Complete mechanization would be a revolutionary change for raisin production practices, which historically have been among the most laborious of any horticultural crop. Basic insight on how cultural practices may affect the viticultural performance of Sunpreme, including possible effects of rootstock and trellising, are lacking, but necessary to fully capitalize on this varieties’ unique traits. It is also necessary to better understand one of Sunpreme’s undesirable traits, preharvest fruit drop, as this problem could limit yields and possibly complicate mechanical harvest operations. Therefore, we established a Sunpreme vineyard at the Kearney Agricultural Center in Parlier, CA, in a split, split plot design, where the main plot factor was trellis (single high wire or 0.8-m wide T), the subplot factor was rootstock (None, Freedom, or 1103P), and the sub-subplot was thinning (0 or 40% cluster removal at fruit set). Vines on T-trellises were more productive than vines on bilateral cordons, due to differences in the number of shoots per vine. Rootstocks strongly affected vine mineral nutrient status, but had little effect on productivity except that the fruit from vines on rootstocks appeared to mature slightly earlier than those on own-rooted vines, resulting in drier raisins at harvest. Thinning increased berry weight, decreased yield, and increased bunch rot. Preharvest fruit drop reduced potential yield by ~10%, and 5 to 10% of the remaining raisins were lost during machine or hand harvest, respectively. Observations on the cause of fruit drop will be illustrated and discussed.

Funding Support: California Raisin Marketing Board