Tian Tian* and Joy Hollingsworth
*University of California Cooperative Extension, Kern County, 1031 S Mt Vernon Ave, Bakersfield, CA, 93307 (titian@ucanr.edu)

Tablegrape growers in the San Joaquin Valley, California face a significant challenge: berries of some red varieties lose firmness before harvest. This issue, known as preharvest fruit softening, has inflicted significant economic losses upon the CA table grape industry and appears to stem from disconnection between berry firmness decline and color development during ripening. Potential remedies include enhancing berry color development and extending the period during which berries maintain firmness after reaching maturity (18 to 19 Brix). In 2023, we evaluated whether cluster thinning, foliar nutrient applications (Ca, Mg, and orthosilicic acid), or foliar sprays of plant growth regulators (24-epibrassinolide, ethylene, and abscisic acid) could enhance berry firmness and color during ripening and ultimately increase marketable fruit at harvest. The trial was conducted in McFarland, CA in a Krissy vineyard that previously experienced preharvest fruit softening. We found that foliar sprays of Ca and Mg every two weeks from bloom to veraison resulted in firmer berries, but slower color development. Biweekly applications of orthosilicic acid during the same period improved berry color but had only minor effects on berry firmness. Applying 24-epibrassinolide at bloom, fruit set, and veraison increased berry firmness during ripening without affecting berry color. Application of ethylene with abscisic acid at veraison enhanced berry color, but compromised berry firmness. Cluster thinning at 10% at fruit set did not affect berry color or firmness. Neither foliar sprays nor cluster thinning altered harvestable yield and total yield at the first harvest. Due to an unexpected storm in late August and subsequent severe bunch rot, we were unable to collect more data. Even so, this study showed the potential of applying Ca and Mg, orthosilicic acid, and 24-epibrassinolide to mitigate preharvest fruit softening in red table grapes.

Funding Support: UC ANR and industry donation