In Florida and worldwide, citrus production is threatened by Huanglongbing (HLB), associated with phloem-limited Candidatus Liberibacter asiaticus (CLas). Oxytetracycline delivery by trunk injection has been approved to treat commercial citrus groves affected by this endemic disease. However, the commercially available formulations require acidification of the active ingredient (oxytetracycline hydrochloride, OTC) to a pH of 1.8-2.0 to dissolve and stabilize the antibiotic, which may cause trunk damage, chloride toxicity, and limits compatibility with other compounds. A novel adjuvant has been developed, using EPA-approved ingredients to dissolve and stabilize OTC at a neutral pH while maintaining efficacy. This study compares the efficacy of neutralized and acidified solutions of OTC with and without the addition of ZnSO4 to optimize HLB management. The efficacy of Streptomycin (STM) as an alternative to or combined with OTC was also assessed. A field trial was conducted in a commercial citrus orchard in southwest Florida using seven-year-old ‘Valencia’ sweet orange trees grafted on X-639 rootstock. The experimental design was a randomized block with ten treatments and eight replications, each containing four trees. Treatments included combinations of OTC, STM, and ZnSO4 with or without the adjuvant, and appropriate controls. Trees were injected in June 2024 with 100 mL of treatment solution on one side of the trunk using commercial (FlexInject, TJ BioTech) injectors. Leaves were collected to assess CLas titers and bacterial suppression. Phytotoxic effects in the canopy and trunk damage at the injection site were evaluated. Treatments containing the adjuvant and/or OTC caused significant phytotoxicity and more trunk damage. Trees were harvested in March 2025. Fruit yield was higher for all treatments containing OTC compared to the controls . The highest yield was measured when trees were injected with a combination of OTC and STM. The findings of this study led to the redesign of the neutral pH adjuvant to minimize tree damage and enhance OTC efficacy. The optimized formulation will be tested using the same experimental design in upcoming studies.
