Physiological and Horticultural Responses of HLB-Affected ‘Valencia’ Orange Trees to Trunk-Injected Oxytetracycline and Streptomycin Formulations Differing in Adjuvant and pH

Background/Objectives: Trunk injection of oxytetracycline (OTC) can suppress Candidatus Liberibacter asiaticus (CLas) and improve the productivity of citrus trees affected by huanglongbing (HLB), a devastating bacterial disease, but conventional acidic formulations may cause phytotoxicity and exacerbate trunk injury. This study evaluated a proprietary adjuvant intended to stabilize OTC at higher pH and assessed streptomycin (STM) as an alternative or complement to OTC. Methods: Two commercial field trials with HLB-affected ‘Valencia’ sweet orange trees compared a 2 × 4 factorial combination of two formulations, adjuvant/high-pH and no-adjuvant/low-pH, and four antibiotic treatments: none, OTC, STM, and OTC + STM. Tree growth, canopy health, phytotoxicity, CLas load, external wound responses, fruit drop, yield, and fruit quality were evaluated. Results: Tree growth and canopy health were not affected by treatment. The adjuvant/high-pH formulation caused severe canopy phytotoxicity in trial 1 and increased wound width, wound length, and bark cracking in both trials, compared to the control. OTC and OTC + STM generally reduced fruit drop, increased yield, and improved fruit quality attributes though effects on CLas were moderate and not consistent. STM alone provided no consistent therapeutic or horticultural benefit, and OTC + STM were not consistently superior to OTC alone. Conclusions: OTC was the primary effective component to mitigate HLB in affected citrus trees. The adjuvant/high-pH formulations did not affect OTC efficacy but caused tree injury, even in the absence of OTC. Novel trunk-injection formulations must balance chemical stability and delivery with plant tissue compatibility and safety to meet commercial viability.