The Florida citrus industry has seen a steady decline in production since the arrival of Huanglongbing (HLB), or citrus greening disease, in Southern Florida in 2005. Following infection, trees experience a steady decline in health and productivity. HLB has since spread throughout all of Florida’s citrus producing regions resulting in nearly 100% infection rates in traditional field settings. Lamentably, no cure has been found for HLB, so research efforts have focused on mitigating the symptoms associated with this disease. Among the myriad of symptoms associated with HLB, the increased rates of mature fruit (preharvest fruit drop) is a major concern for growers. Not only do more fruit drop in HLB-affected trees, but fruit also begin dropping earlier in affected trees as well. This loss of mature, and potentially marketable, fruit in the months leading up to harvest represents a visual loss of revenue for the growers. The authors have previously reported that the likelihood of a fruit to drop during preharvest fruit drop is related to the size of that fruit; the relatively smaller fruit on the tree are more likely to drop during the preharvest fruit drop window. As plant growth regulators (PGRs) have had promising results in preventing fruit drop in many crop species, the efficacy of PGR applications in reducing preharvest fruit drop was evaluated. Thirty small and thirty large fruit were tagged on four 12-year-old ‘Valencia’ on ‘Swingle’ rootstock trees. Small fruit were those that were a ½ standard deviation below the average fruit size for that tree whereas large fruit were those that were a ½ standard deviation above the average. Ten of the small tagged fruit and ten of the large tagged fruit were then dipped into ProGibb® (33 ppm Gibberellic Acid [GA]), Citrus Fix® (106 ppm 2,4-Dichlorophenoxyacetic Acid [2,4-D]), or left untreated (control). Citrus Fix® improved retention in both small and large fruit whereas ProGibb® only improved retention in the large fruit. As GA prevents drop by delaying the senescence process, the lack of effect in the small fruit may suggest that they have already entered the senescence process. To further explore why the small and large fruit differ in their likelihood to drop and why they respond differently to PGR applications, additional biochemical and molecular analyses are underway.
