By Tripti Vashisth
Since 2021, a number of Florida growers have adopted the use of gibberellic acid (GA) in their grove management programs. This widespread adoption is due to the fact that GA-treated trees show canopy rejuvenation and reduction in preharvest fruit drop, resulting in improved yields. The research on Valencia shows that the effect of GA improves over time. Therefore, repeated application is desirable.
Recently collected harvest data on Hamlin shows a significant improvement in yield efficiency of GA-treated trees as compared to the control. Hamlin trees were treated with GA three times from August through November at 45-day intervals. The GA recommendation for HLB-affected trees has been developed from a Valencia field trial in which five monthly applications from September through January were found to be useful. However, as growers are venturing out with GA use in other varieties as well as trying to reduce the number of applications, different application timings are being explored.
GA application can potentially enhance vegetative growth. Therefore, many growers have indicated a desire to time GA application with the natural flushing cycle. However, growers are strongly discouraged from GA application in the spring season. A simple rule of thumb is to avoid GA application from January (once the trees have attained high floral bud induction) to early June (when the “June drop” has ended). The rationale behind this advice is that once the tree has attained sufficient floral bud induction hours, the GA has minimal effect on flower suppression.
Moreover, GA application at the time of fruit set can enhance the fruit set. Under normal conditions, more fruit set is good. However, HLB-affected trees have poor canopies and roots, so an increase in fruit set more than what these trees can handle is not ideal. Excessive fruit set can further stress a tree’s carbohydrate requirement, and excessive fruit may result in excessive preharvest fruit drop later in the season. Therefore, avoiding the window of flowering, fruit set and early fruit growth is recommended.
As researchers develop more information on the timing of gibberellic acid use, growers are encouraged to only use GA within the recommended window of August through December. June and July application also seems promising but needs further scrutiny before developing a recommendation. The chance of negative impact in the June–July timeframe seems fairly low in comparison to potential benefits.
Tripti Vashisth is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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