Joy Hollingsworth and Tian Tian*
*University of California Cooperative Extension, Kern County, 1301 South Mount Vernon Ave, Bakersfield, CA 93307 (titian@ucanr.edu)

Well-developed color, desirable level of firmness, and sufficient sugar content are important quality attributes of table grapes. However, in warm production areas like the San Joaquin Valley, CA, these three qualities are often unbalanced. As a result, growers have observed that berries of some table grape varieties become overly soft before they are well colored, even though the concentration of total soluble solids (TSS) reached harvest standards. To better understand the onset of berry softening and investigate potential mitigation solutions, we conducted two experiments in 2022 with a midseason red proprietary variety. In the first experiment, we sampled fruit periodically in three vineyards (two in Kern County and one in Fresno County) during fruit ripening (June to August) to assess the correlation between TSS, berry color, and berry firmness. We found that berry color improved with increased TSS, and ideal color was found when the berries reached ~20 to 21 Brix. However, some berries started to turn soft when TSS was >19 Brix. The results suggest a possible disconnect between color development, firmness, and sugar accumulation. Thus, in the second experiment, we investigated the possibilities of using ethephon ((2-chloroethyl)-phosphonic acid)) spray, basal leaf removal, and cluster thinning at 10 Brix to mitigate this disconnect. We monitored TSS, berry firmness, and berry color weekly from the onset of veraison to harvest. Then we collected yield data. We found that none of the treatments promoted color development or influenced berry softening. Interestingly, cluster thinning did not decrease harvestable yield, but seemed to enhance ripening uniformity. Based on knowledge gained in 2022, we plan to look into this issue in the coming year while adjusting treatments to include earlier leaf removal and products that may increase firmness or accelerate coloration.

Funding Support: UC ANR