The Use of Different Plant Growth Regulators in ‘Scarlet Royal' Table Grape Production to Determine Postharvest Quality

Scarlet Royal is a new red seedless table grape variety developed by USDA-ARS, which ripens mid to late August. As a late season variety grown in the San Joaquin Valley, Scarlet Royal is subject to early rain events which increases its susceptibility to Botrytis bunch rot and other diseases during the harvest period. In addition, this variety lacks a thick epicuticular wax on the skin and seems to be prone to berry crack when the clusters set too tight which can favor fungal infections. This problem is sustained through post-harvest storage where the grapes become unsuitable for shipping, marketing and consuming. The aim of this project is to evaluate disease incidence and berry quality parameters on Scarlet Royal after post-harvest storage using different combinations of plant growth regulators (PGR) during the growing season. The experimental layout consisted of three treatments arranged as a complete randomized design with six replicates and three vines per replicate.  The treatments included a Control (no PGR applied), 5 ppm Gibberelic Acid (GA) and a combination of 5 ppm GA + 6 ppm Forchlorfenuron (CPPU). Bunch thinning and removal of bunch shoulders were performed for all treatments. After commercial cold storage was completed, berries were evaluated for Botrytis bunch rot, berry crack, berry shatter, other diseases and other damage. Normal quality parameters were also evaluated including pH, TA, Brix, berry firmness, berry diameter, berry length and skin color. Results showed no significant differences for Botrytis bunch rot incidence, berry crack, other damage or other disease. Berry shatter showed significant differences, the 5 ppm GA treatment presented the highest percentage of loose berries while the 5 ppm GA + 6 ppm CPPU treatment had the lowest percentage. Berry length was significantly larger for the 5 ppm GA + 6 ppm CPPU treatment with a mean length of 28.34 mm while the control treatment had the smallest berries with a mean length of 26.93 mm. Berry firmness and berry diameter were statistically equal for all treatments. No significant differences were obtained for the juice parameters pH, TA and Brix. Berry color presented no differences in lightness, chroma or hue.

The results of this study are dependent on rain events during the harvest period in the San Joaquin Valley. During the past two seasons, the absence of rain may serve as an explanation for the lack of significant differences between treatments.