Huanglongbing (HLB), a devastating bacterial disease presumed to be caused by Candidatus Liberibacter asiaticus, disrupts normal plant physiology by inducing hormonal imbalances that impair growth, fruit development, and defense responses. With no known citrus germplasm exhibiting resistance to HLB, effective management strategies are essential for sustaining citrus production. This study focuses on the combined application of methyl salicylate (MeSA), a systemic acquired resistance (SAR) inducer, and gibberellic acid (GA₃), a growth-promoting hormone, as a potential strategy to improve tree health and mitigating symptoms in HLB-affected citrus orchards. The study is being conducted at UF-CREC in Lake Alfred, Florida, on 15-year-old Valencia trees grafted onto Swingle rootstock. Four treatments are being evaluated: Control, GA₃, MeSA, and GA₃ MeSA. Trees were sprayed every 45 days from July to November 2024 with GA₃ and MeSA at 10 ounces per acre. The study assesses key production parameters, including yield, fruit drop, canopy density, and fruit quality. Preliminary results indicate that GA₃ application reduced preharvest fruit drop by 57% compared to the control. The leaf water content was significantly higher in Ga MeSa (58.42%) as compared to Control. No significant differences in canopy density were observed across treatments. Leaves were collected for molecular analysis on Days 7, 14, 30, 46, and 60 after the first treatment application. Differential gene expression analysis revealed significantly lower transcript levels of Rboh and LOX2 within 7 to 14 days post-treatment in GA MeSA, suggesting a potential reduction in oxidative stress. In contrast, CDR1 expression was significantly higher in MeSA treated leaves between days 30 and 45, likely indicating SAR activation. These physiological responses may contribute to improved tree performance under HLB pressure. The study will continue into the next year to refine application strategies and better understand the sustained impact of these treatments on tree health and productivity.
