Matthew Fidelibus and Stephen Vasquez, University of California
Large berry size is a highly valued characteristic of table grapes, but few table grape cultivars produce berries of sufficient size to be marketed in their natural state. However, berry size of many seedless table grape cultivars can be substantially increased by applying gibberellic acid (GA3) to clusters of grape berries about two weeks after bloom. The GA3 stimulates the division and elongation of the berries’ cells, thereby increasing berry size.
Applying Gibberellic Acid
The amount of GA3 needed to optimize berry size depends on the cultivar and many other factors, but 10 to 60 g/acre is a typical application rate. The GA3 may be applied once or several times; on some cultivars, multiple in-season applications may increase efficacy. Application timing is critical and also varies by cultivar. For many cultivars, the first application is made at berry set, or when berry diameter is about 3-5 mm. Later applications can delay fruit maturity and inhibit color development in red- and black-fruited cultivars. Breeding programs have continuously selected for grapes that produce naturally large berries, so many newer cultivars require less GA3 to attain optimal berry size than older cultivars. Seeded grapes are generally less responsive to GA3 treatment than seedless grapes, so using GA3 to increase the size of seeded grapes is generally not recommended.
Other Plant Hormones
Forchlorfenuron (CPPU), a synthetic cytokinin, also may be applied at fruit set to increase berry size. It can be applied by itself, or in combination with GA3, as CPPU and GA3 may have a synergistic effect on berry size. Berries treated with CPPU tend to develop a rounder shape than non-treated berries. This compound is highly potent and normally applied at very low doses (1 to 3 mg/L); excessive doses can delay maturity, retard coloring, and have a negative influence on taste.
Caution: Plant growth regulators (PGRs), such as GA3, are classified as pesticides and are subject to the same rigorous regulatory framework. Plant growth regulators may not be approved for grapes in certain regions, or for particular cultivars of grapes. The unapproved use of PGRs on grapes may result in contaminated crops with illegal pesticide residues, so readers should seek guidance from a qualified local pest control adviser before applying any PGR to their grapes.
Table Grapes – Potential for Maryland, University of Maryland
Table Grapes: A Potential Alternative Crop,Texas A&M University
Reviewed by Eric Stafne, Mississippi State University and Bruce Bordelon, Purdue University