Using GA to adjust cropping in cherries
Gibberellic acid (GA) may be used on tart and sweet cherries to reduce flowering during the early years of an orchard's life. The reduced flowering and subsequent reduced fruiting allows the young trees to increase vegetative growth. Also, minimizing flowering in early years helps slow the transmission of pollen-borne viruses to the young trees. GA is also used in mature tart cherries to increase fruiting capacity by stimulating the formation of lateral shoots and spurs.
The application of GA causes a portion of the flower buds forming for next year to instead be vegetative buds. Therefore, GA application this year influences flowering next year. The effectiveness of GA is dependent on rate, timing and temperature, plus it can be influenced by the use of surfactants. High rates are required to keep a young tree from fruiting, whereas much lower rates are used to keep bearing trees in a good balance between vegetative and fruit production.
Apply when daily high temperatures are expected to be above 70°F for two to three days, if possible. Applications made when high temperatures are expected to be below 60°F have given poor results.
GA is typically applied to non-bearing cherries with a hand gun, so rates are based on a dilute basis. The best results are generally achieved with two applications of 50 ppm (20 fl. oz. of 4% formulated product per 100 gallons of water). The first application should occur three to three and a half weeks after full bloom, followed by a second application two and half to three weeks later. An alternative method, though slightly less effective, is to apply a single treatment of 100 ppm (40 fl. oz. per 100) at about three to four weeks after bloom. Do not apply GA to trees the year of planting, due to possible phytotoxicity. Vigorously growing trees in their second leaf don't need GA, as these trees naturally produce little fruit the following year. GA application often starts in year three, but may be desirable in year two if trees get off to a poor start. Treat at high rates until the year prior to desired fruiting.
To bring young tart cherries into bearing following treatment with high rates during non-bearing years, it is very important to phase down GA rates, rather than discontinue all at once. A sudden drop of GA from high rates to none will result in oversetting of fruit and stunting of trees. Trees that have been kept vegetative have tremendous capacity to set fruit. The year prior to desired first fruiting, apply GA at 30 to 40 ppm if spraying dilute (12-16 fl oz./100 gal.), or about 20 to 24 fl. oz./acre if applied concentrate. This rate per acre for concentrate spraying already takes into account the average tree size of this age tree, therefore do not reduce the rate further based on tree row volume. The next year, decrease this rate to 15 to 20 ppm applied dilute (6-8 fl. oz./100 gal.) or 10-12 fl. oz./acre applied concentrate. The following year, 10 ppm is optional but often not required. In orchards where growth is weaker, it may be desirable to simply continue annual GA applications at 10-15 ppm as described for bearing trees, rather than discontinue at this time.
Apply about three to four weeks after bloom or when trees have five to seven leaves (three to five fully expanded) on terminal growth. Use at rates of 10 to 20 ppm, or 4 to 8 oz/100 gallons of ProGibb 4% (or equivalent) when applied dilute. For concentrate application to full-sized tart cherries, use 6 oz./a of product to achieve a 10 ppm response, or 12 oz./a for a 20 ppm response. Lower rates are typically used on more vigorous orchards or those with previous successful use of GA. Adding surfactants has caused varied responses from over activity with phytotoxicity to no effect. Therefore, adding a surfactant is not suggested unless a grower has enough experience with a product to have confidence in the response.
GA use on Balaton
Balaton appears to have less need for GA during non-bearing years to maintain good tree growth, but as it matures, the variety produces a lot of blind wood. Therefore, using GA is strongly encouraged on bearing Balaton. Rate studies on Balaton are in progress.