ASHS 2025 Conference

Huanglongbing (HLB, aka citrus greening) has devastated citrus production around the world, especially in Florida where the disease has been endemic since 2013. HLB is associated with phloem-limited bacteria, Candidatus Liberibacter asiaticus, vectored by the Asian citrus psyllid, Diaphorina citri. Elimination of the disease vector is difficult because of its abundance and the high costs of frequent insecticide applications. Foliar applied therapies to treat infected trees are largely ineffective because of the systemic distribution of the bacteria. Trunk injection of oxytetracycline (OTC) has emerged as a possible therapy to systemically deliver and target the pathogen and restore tree health. Since the registration of trunk injection of OTC for commercial use in Florida, our lab has conducted numerous field studies to assess the efficacy of this therapy on a large scale and study the influence of various factors such as tree age, rootstock, OTC dose, and time and method of application. Overall, all studies revealed significant improvements in fruit production along with increases in fruit and juice quality after one year of application. Yield improvements after two years of application were cumulative, reaching up to two-fold or more. The magnitude of improvements depended on various factors. Young trees generally responded better than older trees, and tree responses varied based on the rootstock. The efficacy of the therapy also depended on the dose of OTC administered to the trees, with higher doses generally generating larger effects. Moreover, delivering OTC on multiple sides of a tree instead of on only one side enhanced its distribution and therefore, efficacy. Late season injections improved the juice quality more but increased the OTC residue content in the fruits. The currently allowed residue level is 10 ppb, which was not exceeded when injections occurred in spring or early summer. OTC is phytotoxic and can have adverse effects on the tree. Efforts are underway to reduce or eliminate these negative effects and improve efficacy. Alternative therapies to be used instead of, or in combination with OTC, are also under evaluation. Taken together, the systemic delivery of OTC by trunk injection has emerged as a viable and economically feasible therapy to manage HLB in Florida and has been widely adopted. Nevertheless, it is regarded as a short-term solution until other solutions become available.