By Tripti Vashisth
Over the past decade, HLB-associated fruit drop has become a significant problem in Florida groves. Many growers opt to harvest the fruit before the peak fruit drop begins. However, the downside to this strategy is that the fruit does not get enough time for optimal Brix accumulation.
Plant growth regulators, when applied at the right time, can provide some respite to growers by reducing preharvest fruit drop. Gibberellic acid (GA) and 2,4-D are two tools that are available to Florida citrus growers. GA and 2,4-D both have different modes of action in reducing preharvest fruit drop so they cannot be used interchangeably. GA slows down the peel aging whereas 2,4-D counteracts the fruit abscission signals.
It is critical that these PGRs are applied before the actual fruit drop has begun. Once the signal of fruit drop ramps up, it is difficult to halt it. University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers have found that the fruit drop signal arises at least eight to 10 weeks before the physical drop starts. In general, it will be September–October for Hamlin (harvested in December–January) and November–January for Valencia (harvested in late March–April).
In 2023, a UF/IFAS field trial was initiated to assess the effect of 2,4-D and 2,4-D + GA in mitigating fruit drop in Valencia with one spray. 2,4-D (Citrus Fix, 3 ounces per acre) and GA (ProGibb LV Plus, 10 ounces per acre) both were applied with an adjuvant in November 2023. The harvest was in March 2024. The combination of 2,4-D and GA reduced the fruit drop by more than 15% (35% versus 19%, as seen in Figure 1).
One application of GA and 2,4-D each appears to be sufficient in reducing preharvest fruit drop, thus improving yield. The combination of both GA and 2,4-D provides the best of both worlds. This spray regime has no effect on canopy growth as the time of GA application does not overlap with the vegetative growth period.
Tripti Vashisth is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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