Joy Hollingsworth, Karina Elfar, Tian Tian, and Akif Eskalen*
*University of California, Davis, 1 Shields Ave, Davis, CA, 95616 
(aeskalen@ucdavis.edu)

Sour rot (often known as summer bunch rot) is a disease complex that can be caused by multiple fungal species, including several Aspergillus spp., Botrytis cinerea, Cladosporium spp., Penicillium spp., and Rhizopus spp. or by yeasts, bacteria, and Drosophila flies. It is commonly found in the central and southern San Joaquin Valley of California due to the typically warm climate. Because this complex involves multiple causal organisms, it isn’t easy to control and multiple strategies are likely needed. To help determine the efficacy of disease control options, a field trial was conducted in two commercial table grape vineyards, in Delano on Red Globe and in Bakersfield on Autumn King, comparing seven spray treatments to an untreated control. Each treatment was replicated five times per site and sprays were repeated four times (bloom, pre-close, veraison, and preharvest) during the 2023 growing season. Shortly before harvest, each plot was evaluated for disease incidence and severity. A representative number of harvested bunches were stored in commercial cold storage (0°C, 90 to 95% RH) for five weeks, then rated for disease incidence and severity to determine the treatment efficacy during cold storage. Differences in natural disease pressure were observed between the two vineyards, with greater pressure in the Red Globe vineyard than in the Autumn King vineyard. The Elevate/Luna Experience/Switch/Vacciplant treatment was shown to be the most effective at reducing disease incidence in Autumn King, but there were no significant differences seen in Red Globe. After five weeks in cold storage, however, no product was significantly different from the control in incidence or severity in Autumn King, while the Pristine/Luna Experience/Elevate/Scala treatment resulted in significantly less incidence of infected bunches than the untreated control in Red Globe. Based on these findings, we plan to renew the project for another year to continue product evaluations.

Funding Support: California Table Grape Commission