Ashraf El-kereamy,* Matthew Fidelibus, and David Obenland
*University of California Cooperative Extension, Kern County, 1031 South Mount Vernon Ave., Bakersfield, CA 93307 (aelkereamy@ucanr.edu)

Autumn King is a late-season, white seedless table grape developed by David Ramming and Ronald Tarailo of the USDA-ARS in Fresno, CA. This variety naturally produces large berries, but gibberellic acid (GA3) and forchlorfenuron (CPPU) can be used to manipulate berry size and cluster quality. However, there is a lack of information on the optimum GA3 and CPPU treatments for Autumn King. Moreover, Autumn King may be spur- or cane-pruned, with possible effects on fruit quality. Therefore, studies were conducted in a commercial Kern County table grape vineyard with mature, uniform own-rooted Autumn King grapevines with two different training systems: head-trained, cane-pruned, or quadrilateral cordon-trained, spur-pruned vines. Vines were subjected to one of four different GA3 thinning sprays (0, 0.5, 1, or 2 ppm GA3) in combination with varying treatments of sizing using GA3 at 2 or 10 ppm and CPPU at 2 or 6 ppm. We found that all of the bloom GA3 treatments reduced the number of berries per cluster on spur-pruned vines, but 1 ppm GA3 was needed to adequately thin cane-pruned vines. The greatest cluster and berry weight, size, and Brix were obtained using 1 ppm GA at bloom, followed by GA at 10 ppm or CPPU at 6 ppm. No treatment combination caused excessive cluster compactness or preharvest bunch rot. Our data showed that none of the treatments had any negative effect on vine fruitfulness during the last two seasons. Autumn King is a protected variety, and the California table grape commission is the exclusive licensee.

Funding Support: California Table Grape Commission