Citrus Huanglongbing (HLB), also known as citrus greening, is associated with Candidatus Liberibacter asiaticus (CLas), a phloem-limited bacterium transmitted by an insect vector, Asian Citrus Psyllid (ACP). Currently, there are no commercially viable treatment options available for HLB management. Trunk injection of Oxytetracycline (OTC) has been widely adopted as a treatment option in Florida. Currently, OTC is not approved for use in Texas citrus, and no OTC efficacy data is available. This study evaluated the efficacy of OTC in mitigating HLB and managing tree health in nine and 17-year old ‘Rio Red’ grapefruit (Citrus paradisi Macfadyen) trees on sour orange (Citrus aurantium L.) rootstock, on a randomized complete block design in two commercial orchards. RECTIFY™ (95% Oxytetracycline hydrochloride) was injected into tree trunks at two rates (0.55g and 1.1g per tree) using Chemjet® tree injectors during June-July 2023 and May 2024. Trees were evaluated for various attributes, including periodic (pre-treatment, three, six, and nine months after OTC injection) bacterial titer in leaf and root tissues and tree health monitoring (canopy color, canopy density, and HLB severity) for the same time intervals. Fruits collected at harvest maturity were evaluated for yield, fruit size, and juice quality (brix, titratable acidity, flavonoids, carotenoids). Additionally, pre-harvest fruit drop and trunk injection site damage were recorded. OTC residue was measured in whole fruit and juice using Ultra-High-Performance Liquid Chromatography coupled with tandem Mass Spectrometry (UHPLC-MS-TOF). Nine-year-old trees treated with 1.1g of OTC/tree in one of the sites showed a significant reduction of bacterial titers in leaf tissue. OTC treatment did not significantly improve yield. Canopy color showed noticeable improvement after the second OTC injection in the sites, whereas canopy density remained largely unaffected. After the second OTC injection, HLB severity and pre-harvest fruit drop were reduced, and improvement in fruit size was observed in 9-year-old trees. There is no significant improvement in juice quality. Damage caused by trunk injections healed rapidly in older trees than in younger trees. No OTC residue was detected in the juice and whole fruit at six months after the OTC injection. This is the first report on the efficacy of OTC for HLB management in Rio Red grapefruit in South Texas.
