By Tripti Vashisth
Gibberellic acid (GA) is a naturally occurring plant hormone. GA has been used in citrus production for years to improve fruit set and size, reduce preharvest drop and delay harvest.
Recent findings show that monthly foliar application of GA from September to January can improve productivity of HLB-affected Valencia trees. On average, a 30% increase in yield was observed for GA-treated trees over a period of four years. In addition, GA trees maintained their canopy density while untreated treated trees declined in canopy density. GA favors leaf growth over flowering.
GA use on HLB-affected trees has huge potential in improving yield and productivity of Valencia sweet oranges and possibly other citrus varieties. However, a few aspects of GA use require serious consideration before applying:
1. GA application slows down the fruit peel aging process. As a result, fruit remains green (Figure 1). Color break, increase in Brix and decrease in acidity are interrelated. Hence, GA-treated trees may require more time to change color and develop desired Brix. As a general rule of thumb, approximately a 4-month interval between the last GA application and harvest provides sufficient time to achieve desired internal quality and peel color. For example, in the case of Hamlin, do not apply GA after mid-October. It is expected that with a mid-October GA application, desired fruit quality will be attained at the end of January/mid-February.
2. Weather plays a significant role in peel color change, fruit Brix and acid content. Cold weather enhances color development and internal fruit quality whereas warm temperatures can deter it. Therefore, warm weather can further slow the color change and Brix development in GA-treated trees. When warm weather is anticipated before harvest, GA sprays should be withdrawn sooner rather than later.
3. Since GA application slows down the fruit (peel) aging process, it can reduce fruit drop and improve rind quality. Hence, GA allows for the fruit to hang longer on the tree and delays harvest. This can be a desirable scenario, especially in cases where processors and packinghouses are not accepting fruit early in the season.
4. GA suppresses and synchronizes flowering. This is beneficial for the varieties that flower profusely. However, varieties that do not flower copiously should be avoided for fall GA application.
5. GA affects multiple processes in the tree. Therefore, the beneficial result may not be obvious in short duration. It can require up to two to three years (depending on tree health) of fall GA applications to see improvements.
Overall, GA has potential, but it may not be a fit for every grove or situation. Careful consideration is required before GA application.
Tripti Vashisth is an assistant professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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