Huanglongbing (HLB), associated with phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), is a major threat to citrus production worldwide, severely reducing tree health, yield, and fruit quality. In Florida, trunk injection of oxytetracycline (OTC) has emerged as a promising management strategy for HLB. Under this context, this study aimed to enhance the efficacy of OTC injection by evaluating different injection volumes, modes of injection (one or two-sided), and injection timing in HLB-affected ‘Valencia’ orange (Citrus sinensis) trees in southwest Florida. Two experiments were conducted with 8-year-old trees. The first experiment, implemented in two locations, assessed four combinations of different OTC volumes and modes of injection: 1) 50 mL (one-sided), 2) 100 mL (one-sided), 3) 100 mL (two-sided, 50 mL each side), and 4) 200 mL (two-sided, 100 mL each side), all delivering 0.825 g (a.i.) OTC (Rectify, AgroSource, Inc.) per tree. The second experiment compared early (May) and late (July) injections using 100 mL (two-sided, 50 mL each side), delivering 0.825 g (a.i.) OTC (Rectify) per tree. Injectors were placed on opposite sides of the trunk for the two-sided treatments. OTC solutions were prepared right before injection. In both experiments, treatments were compared against an untreated control. The experimental design was completely randomized, with 10 single-tree replications for the first experiment and 8 replications of 4 trees each for the second. Leaf samples were collected over time to analyze bacterial (CLas) titers and OTC residues, while canopy health, yield, and fruit quality were evaluated annually. For the second experiment, fruit development (size and peel color) was monitored monthly after OTC injection, and fruit yield and quality were assessed annually. Our results demonstrate that two-sided injection increases fruit yield more than one-sided injection and that the combination of two-sided injection and a larger volume of the injected solution may further enhance fruit yield compared to the untreated control. This improvement is attributed to better OTC distribution within the canopy, leading to greater bacterial suppression and tree recovery. Moreover, early (May) injection promotes fruit size by extending the fruit growth period, while late (July) injection enhances juice soluble solids content with a more compact fruit size. However, early injections result in greener fruit, which requires a longer maturation period to achieve the desired peel color and sugar content. These findings offer valuable insights for optimizing OTC trunk injection strategies to improve citrus health and productivity in HLB-affected citrus orchards.
