Synergistic Action of 2,4-D and Gibberellic Acid Mitigates Fruit Drop in Huanglongbing-Affected Sweet Orange

Economic losses attributable to accelerated preharvest fruit drop caused by Huanglongbing (HLB) in Florida have devastated the citrus industry. Moreover, in recent years, hurricanes have also contributed significantly to crop losses. Plant growth regulators, such as gibberellic acid (GA 3 ) and 2,4-dichlorophenoxyacetic acid (2,4-D), have shown promising potential in mitigating symptoms of HLB, such as preharvest fruit drop. This 2-year field trial of commercially grown ‘Valencia’ trees evaluated whether smaller and more frequent 2,4-D applications improve fruit retention and assessed the efficacy of combining 2,4-D and GA. Treatments included a single label rate of 2,4-D, reduced monthly rates (September–November), and GA combined with a single label rate of 2,4-D. Over the 2 years, the combination of GA + 2,4-D consistently reduced fruit drop, with a 16.21% reduction in year 1 and 14.85% reduction in year 2 after Hurricane Milton. While yield was not different in year 1, all the hormonal treatments were able to produce more yield efficiency in year 2. Phytohormone analysis revealed that GA + 2,4-D had lower abscisic acid and jasmonoyl-isoleucine in the peel, suggesting a hormonal mechanism for reduced fruit drop under stress conditions. This study demonstrates that a single combined application of GA and 2,4-D can effectively reduce fruit drop in HLB-affected ‘Valencia’ sweet orange by targeting distinct hormonal pathways. This strategy offers a cost-effective tool for citrus growers to minimize losses under stress conditions such as HLB and hurricanes.