A combination of gibberellic acid (GA) and 2,4-D reduced preharvest fruit drop by 18% and improved fruit size but produced no statistical effect on yield, researcher Tripti Vashisth reported Aug. 30. The combination “seems promising,” the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) horticulturist stated. The report was based on first year results from a trial of 15-year-old Valencia orange trees on Swingle rootstock.
Vashisth’s presentation was part of an update on the 2,4-D product Citrus Fix, manufactured by AMVAC. Mike Herrington, AMVAC senior field manager, said the product has been used in California citrus. “We don’t know the applications in Florida completely,” he said.
According to Vashisth, 2,4-D is a synthetic auxin, a naturally occurring plant hormone needed by for growth. She said that auxin is involved in fruit growth and development and that it retards abscission.
Vashisth reported that:
- HLB-associated fruit drop is related to reduction in levels of auxin and GA.
- The signal for fruit drop starts early in the season.
- Small fruit are more likely to drop. The signal for fruit drop is linked to fruit development, and treatments that can improve fruit size are likely to help with fruit drop reduction.
- Hormonal treatments applied early in the season have potential of reducing fruit drop.
- Repeated hormone application can keep the fruit drop signals below threshold.
UF/IFAS horticulturist Fernando Alferez said for 2, 4-D to increase fruit size, “It is not about the date of application; it is about the physiological stage.” His rule of thumb for mandarins is to apply the product when fruit diameter is between 15 and 20 millimeters. For oranges, the rule of thumb is to apply when diameter is between 25 and 30 millimeters.
Vashisth has made numerous presentations about GA over the past year. See a recent Tip of the Week article here that she wrote about when to apply GA.
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