The systemic delivery of oxytetracycline (OTC) by trunk injection has emerged as a viable strategy to diminish the impacts of the bacterial disease Huanglongbing (HLB) in Florida. This study aimed to determine the optimal OTC dose and best time of delivery to enhance tree health and productivity in mature HLB-affected citrus trees. It also investigated whether OTC effects persist after one single injection compared to two consecutive annual injections. The study began in 2023 at a commercial citrus grove in Polk County, Florida, using 18-year-old ‘Valencia’ (Citrus sinensis) trees grafted onto Swingle (C. paradisi × Poncirus trifoliata) rootstock. Four OTC doses (0.55 g, 0.825 g, 1.1 g, and 1.65 g of active ingredient per tree) were evaluated alongside two application times, spring and late summer, with a non-injected control for comparison. The experiment was a randomized complete block design with eight replications containing four trees. A commercial OTC formulation (ReMedium TI, 95% oxytetracycline hydrochloride) registered in Florida was applied using commercial tree injectors (FlexInject). Doses were applied in 100 ml acidified water through a single injector, except for the highest dose, which was split between two injectors and applied in a volume of 75 ml on opposite sides of the trunk. All trees received their designated doses in year 1, except for the non-injected controls. Only half of the trees injected in year 1 received a second dose in year 2. Trees were harvested in March 2024 and February 2025. Fruits were collected for fruit and juice quality analysis and to determine OTC fruit residue levels. Regardless of the dose, all OTC-treated trees produced more, larger-sized fruits, more total soluble solids (TSS), and a higher TSS-to-acid ratio compared to non-injected controls. The best results were obtained with the highest dose applied by split injections. There were no major differences between spring and summer injections except for the TSS content, which was higher and the percent acid, which was lower when trees were injected in late summer. However, fruit OTC residues were above the allowed maximum after late summer injections, suggesting that injections should be completed by the end of spring. Trees injected for two consecutive years produced significantly more, better-quality fruits than trees injected only once in the first year. These results confirm the efficacy of OTC trunk injections to enhance productivity and fruit quality under HLB-endemic conditions and provide important guidelines regarding dose and timing of injections.
