The Evaluation of Different Cultural Practices on Scarlet Royal Table Grapes to Determine Postharvest Quality

Scarlet Royal is a newer red seedless table grape variety developed by USDA–ARS, which ripens mid- to late August. As a late season variety, Scarlet Royal is subject to rain, which increases its susceptibility of Botrytis bunch rot. In addition, the variety lacks a thick epicuticular wax and seems to be susceptible to berry crack and rot infections where the berries touch. This problem is also evident and even enhanced in the cold storage environment where the grapes become unattractive for shipping and marketing. Table grape growers in the San Joaquin Valley use plastic covers that are mounted on top of the grapevine trellis system later in the season to protect the grapes from rain. These covers however, induce higher humidity in the grapevine canopy that also contributes toward rot infections. The aim of this project is to evaluate Scarlet Royal under different combinations of cultural practices to mitigate rotting problems due to rain and to maintain optimal postharvest quality under plastic covered grapevines. The cultural practices applied included bunch thinning and removal of bunch shoulders, GA (Gibberelic Acid) and GA + CPPU (Forchlorfenuron) treatments. The experimental layout consisted of four treatments with six replicates each. Clear plastic was used to cover all the vines in the trial before harvest for rain protection. Postharvest quality did not show any significant differences due to the absence of rain during the 2012 growing season. Berry size showed significant differences for the GA + CPPU treated clusters where the berry diameter and berry length were greater than the untreated control bunches. No significant differences were found in juice values, berry size, berry color, or postharvest defects when manual bunch thinning was compared to no bunch thinning. For Botrytis infections, the control and GA treatments with no manual thinning had a higher percentage of infection. Post-storage evaluations showed that GA applications with no thinning had lower titratable acid levels and no significant differences in juice pH levels. Regarding quality parameters, loose berry percentage was significant for the GA + CPPU treatment. Berry color presented no differences in lightness, chroma, or hue when the different treatments were compared. This study was only conducted for one season but will continue for two additional seasons. This study is dependent on late season rainfall in the San Joaquin Valley.