Timing of gibberellic acid applications to manipulate flowering and improve fruit production in HLB-affected sweet orange

Since the onset of Huanglongbing (HLB) in Florida, citrus production has declined significantly while simultaneously increasing the cost of production. While HLB is known for decreasing fruit yields among other devastating symptoms, how HLB affects flowering is not fully understood. Extended periods of bloom and increased off-season bloom have been observed in many commercial orchards around the state, presumably due to HLB-induced stress and Florida’s warm winter climate. Gibberellic acid (GA) is a plant growth regulator that is known for its inhibitory effect on flowering in citrus and has garnered a lot of attention for its potential in alleviating some devastating HLB symptoms. Recent research has shown that 4-5 monthly applications of GA during flower bud induction period can successfully manipulate flowering in HLB-affected citrus, but due to high cost of multiple applications growers are looking for more cost-effective application strategies. A 2-year field trial was initiated in 2017 in a commercial citrus orchard to evaluate if 1 or 2 GA applications could achieve similar results in manipulating flowering and improve fruit production. All treatments received the same rate of GA at each application, 33 mg L^‑1, and single application treatments were applied either once in November, December, January, or February and double application treatments were applied in November and December, December and February, or January and February. An untreated control was included for comparison, and bloom intensity, fruit set, and final yield were evaluated. The single December application produced significantly more yield in 2019 and cumulatively, 140.2 and 248.2 kg tree^‑1, respectively, while stabilizing the percentage of buds that differentiate into flowers in both years. All GA treatments were successful in synchronizing bloom to some degree while control exhibited more prolonged flowering and reduced fruit production. This research suggests that a single application in December can be adopted if multiple sprays are not affordable.