Efficacy of Gibberellic Acid Treatment on HLB-affected ‘Hamlin’ Sweet Orange

Huanglongbing (HLB) is a severely destructive disease of citrus. Gibberellic acid (GA), a plant growth regulator, has garnered a lot of attention for the possibility it may improve the health of HLB trees as past research has shown that GA has potential in mitigating some symptoms such as preharvest fruit drop and increasing canopy density by promoting vegetative growth in ‘Valencia’ sweet orange. Gibberellic acid also inhibits flowering, which could prove beneficial in saving resources for increased productivity as flowering is a resource-costly event for a stressed HLB-affected tree. However, its effect on Hamlin is unknown. Grower collaborated field trials evaluating the use of GA on HLB-affected ‘Hamlin’ were initiated at two locations in August of 2021 and repeated in 2022. Gibberellic acid was applied 3 times from August to November at 45-day intervals. Yield results from year 1 showed that GA-treated trees retained on average 15.6 kg more fruit than control trees. In year 2, after Hurricane Ian (category 3) devastated Hardee County, the GA trees still retained on average 22.7 kg more fruit than the control. In year 1, fruit drop was significantly reduced by GA treatments at both sites by 11.1% in Hardee County and 4.7% in Polk County. Significant improvements in canopy density of the GA-treated trees are also evident at both sites. Results from this trial suggest that use of GA on ‘Hamlin’ can improve canopy health and reduce pre-harvest fruit drop caused by HLB and inclement weather.