By Tripti Vashisth, Mary Sutton and Taylor Livingston
Preharvest fruit drop remains a major concern for HLB-affected sweet oranges. Research shows that both gibberellic acid (GA) and 2,4-D can help reduce fruit drop, but the combination of GA and 2,4-D provides the greatest reduction. In a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) study, the combination reduced fruit drop by about 18% compared with untreated trees. In comparison, 2,4-D alone reduced fruit drop by about 14%, while GA alone resulted in a smaller reduction.
WHY IT WORKS
The combination likely works better because GA and 2,4-D affect fruit drop differently. GA delays fruit maturation and senescence, helping fruit remain on the tree longer. However, the response to GA depends on the stage of fruit development at the time of application. If the application is made too late, after the fruit has already advanced toward senescence, GA may provide less benefit.
In comparison, 2,4-D acts more directly on the abscission process. It reduces the fruit’s sensitivity to ethylene at the point where the fruit separates from the stem. This makes 2,4-D more consistent for reducing drop. When GA and 2,4-D are used together, GA helps delay fruit aging, while 2,4-D directly delays fruit separation. This complementary activity is likely why the combination gives better control than either product used alone.
WHEN TO APPLY
For Valencia oranges, applications should be made duringNovember–December. The key is to start the program before heavy fruit drop becomes visible. GA needs to be applied early enough to delay fruit maturation, while 2,4-D helps maintain fruit attachment as the crop moves closer to harvest.
CONSIDER FRUIT USE
Fruit destination should also be considered. GA can delay peel color development and may keep the fruit greener for a longer time. Therefore, the GA + 2,4-D combination may be better suited for fruit grown for processing, where peel color is generally not a major concern.
For fruit intended for the fresh market, 2,4-D alone may be the better choice, since it can reduce fruit drop without the same concern about delayed color development. Another option for fresh oranges is that GA can be applied in early fall, and a 2,4-D application can be made in December.
Overall, the GA + 2,4-D combination offers the greatest potential for reducing preharvest fruit drop in sweet oranges. Please follow the label. The label is the law!
Tripti Vashisth is associate center director and professor, and Taylor Livingston is a biological scientist, both at the UF/IFAS Citrus Research and Education Center in Lake Alfred. Mary Sutton, previously with UF/IFAS, is now a University of Georgia assistant professor and citrus Extension specialist.
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