Using gibberellic acid to manage crop load in cherries
To help manage crop load in cherries, growers should apply gibberellic acid to cherries when terminals have five to seven leaves.
Gibberellic acid is a plant hormone that promotes growth and elongation of cells. In tart and sweet cherries, gibberellic acid has been used successfully to reduce flowering during the early years of an orchard's life. The reduced flowering and subsequent reduced fruiting helps young trees increase vegetative growth. Minimizing flowering in early years slows the transmission of pollen-borne viruses in young trees. Furthermore, preventing fruit development on non-bearing trees could help to reduce localized population build up of spotted wing Drosophila and other direct pests of cherry fruit. We have also shown that gibberellic acid used in mature tart cherry orchards can increase fruiting capacity by stimulating formation of lateral shoots and spurs.
When gibberellic acid is applied to cherry trees in late spring, a percentage of the flower buds forming for the following season will be converted to vegetative buds. Therefore, gibberellic acid application in 2016 influences flowering in 2017. The effectiveness of gibberellic acid is dependent on rate, timing and temperature. Surfactants have also been shown to influence gibberellic acid applications. As a rule of thumb, high gibberellic acid rates are required to prevent young trees from fruiting, whereas much lower rates are used to keep bearing trees in a good balance between vegetative and fruit production.
Gibberellic acid applications should be made when daily high temperatures are expected to be above 70 degrees Fahrenheit for two to three days, if possible. We have observed poor results when applications are made when daily high temperatures are below 60 F, as is the case with most growth regulators. We hypothesize that the temperatures of 2015 may have been too cool when gibberellic acid was applied for a good response from the application. As a result, some young trees have a lot of flowers on them when growers were trying to use gibberellic acid to minimize flowers and fruit on the trees. In this situation, growers should really try to optimize gibberellic acid use when temperatures are warm this spring to reduce the crop load for 2017.
If orchards have five to seven fully expanded leaves, Michigan State University Extension suggests growers begin gibberellic acid applications this week to improve response. Currently, temperatures are predicted to be in the 70s for the remainder of the week with the chance for a temperature drop into the 60s early next week. Gibberellic acid will be less effective in cooler temperatures, so growers should try and make gibberellic acid applications when temperatures are warm, particularly if they had lack of response in young blocks that were the result of the cool temperatures last spring.
Applying gibberellic acid to non-bearing trees
Gibberellic acid is typically applied to non-bearing cherries with a handgun, so rates are applied on a dilute basis. The best results are generally achieved with two applications of 50 parts per million (ppm) (20 fluid ounces of 4 percent 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 a half to three weeks later. An alternative method, though slightly less effective, is to apply a single treatment of 100 ppm (40 fluid ounces per 100 gallons of water) at about three to four weeks after bloom.
Gibberellic acid should not be applied to trees during the year of planting due to possible phytotoxicity. Vigorously growing trees in their second leaf do not need gibberellic acid, as these trees naturally produce little fruit the following year. Gibberellic acid application often starts in year three, but may be desirable in year two if trees start off poorly. These high rates should continue until the year prior to first harvest or year of production.
Applying gibberellic acid to early bearing trees
To bring young cherries into bearing following gibberellic acid treatments with high rates, growers should phase down gibberellic acid rates rather than discontinuing gibberellic acid use all at once. A sudden drop of gibberellic acid from high rates to nothing will result in oversetting of fruit and potential tree stunting. Trees that have been kept vegetative with gibberellic acid use have a tremendous capacity to set (overset) fruit.
The year prior to when growers first desire fruiting, they should apply gibberellic acid at 30-40 ppm if spraying dilute (12-16 fluid ounces per 100 gallons of water) or 20-24 fluid ounces per acre if applied at a concentrated rate. This rate per acre for concentrate spraying takes average tree size into account; therefore, growers should not reduce the rate further based on tree row volume. The next year, decrease this rate to 15-20 ppm applied dilute (6-8 fluid ounces per 100 gallons of water) or 10-12 fluid ounces per acre concentrate. The following year, 10 ppm is optional, but often not required. In orchards where growth is weak, growers should continue annual gibberellic acid applications at 10-15 ppm as described for bearing trees.
Applying gibberellic acid to bearing trees
Growers should apply gibberellic acid three to four weeks after bloom or when trees have five to seven leaves (three to five fully expanded) on terminal growth. Gibberellic acid should be used at rates of 10-20 ppm or 4-8 ounces per 100 gallons of ProGibb 4 percent (or equivalent) when applied dilute. For concentrate application to full-sized tart cherries, use 6 ounces per acre of product to achieve a 10 ppm response or 12 ounces per acre for a 20 ppm response. Lower rates are typically used on more vigorous orchards or those with previous successful use of gibberellic acid. Adding surfactants has caused varied responses – everything from increased phytotoxicity to no gibberellic acid-related effects. Therefore, adding a surfactant is not suggested unless a grower has enough experience with a product to have confidence in the response.
Applying gibberellic acid on Balaton trees
Balaton appears to have less need for gibberellic acid during non-bearing years to maintain good tree growth, but as it matures, the variety produces a lot of blind wood. Therefore, using gibberellic acid is strongly encouraged on bearing Balaton trees. Figure 1 shows the successful use of gibberellic acid to increase lateral shoots and spurs in a Balaton orchard at the Northwest Michigan Horticulture Research Center. However, we cannot conclude that gibberellic acid applications improve Balaton yields although gibberellic acid does appear to increase crop load based on a trial conducted at the Northwest Michigan Horticulture Research Center (Figure 2).