Trunk injection is a targeted delivery of pesticides, insecticides, nutrients, or other plant protection materials into the stem or trunk of woody plants as an alternative to spraying or soil drenching. Trunk injection has historically been used for disease and pest management of high-value forest tree species or ornamental plants when aerial applications are problematic due to spatial problems and health-related concerns. An interest in using the injection technique for protection of agricultural crops in commercial production systems has emerged more recently, where foliar applications and soil drenches have proven ineffective or pose environmental hazards. This review provides an overview of the basic principles of trunk injection and the plant physiological implications, its current use in commercial agriculture and other plant systems, and associated risks.

Huanglongbing (HLB) is a devastating bacterial disease associated with ‘ Candidatus Liberibacter asiaticus’. The location of the pathogen within the vasculature of the tree has left growers with limited options for the effective management of the disease. Trunk injection is a crop protection technique that applies therapeutics directly into the xylem of woody tree species and allows for their systemic uptake and transport, which may provide more effective management of vascular diseases such as HLB. In this study, mature ‘Valencia’ and ‘Hamlin’ sweet orange ( Citrus sinensis) and ‘Duncan’ grapefruit ( C. paradisi) trees were injected with oxytetracycline (OTC) in the spring and/or fall to evaluate the effects of injection timing and response to injection. In addition to seasonal evaluations of tree health and bacterial titer, preharvest fruit drop, yield, and fruit quality were measured at harvest to determine the effects of OTC injection. The benefits associated with injection included a reduction in fruit drop, an increase in fruit yield and fruit size, and improvements in juice quality. However, results varied due to the timing of injection and were not consistent across all three varieties. Residue analysis at different time points after injection suggests that trunk injection effectively delivers therapeutics to mature citrus trees. This study provides fundamental information on the short-term benefits associated with trunk injection of OTC for HLB management in citrus groves. The potential for use of trunk injection at the commercial scale and the possible risks are discussed.

Traditional foliar spray and soil drench applications of crop protection compounds have been ineffective at managing huanglongbing (HLB) in citrus. Trunk injection is a technique that delivers crop protection compounds directly into the tree vasculature, which optimizes compound availability while minimizing drift, runoff, and damage to non-target organisms. Five-year-old HLB-affected ‘Valencia’ sweet orange (Citrus sinensis) trees were injected with the insecticide imidacloprid or the antibacterial oxytetracycline in October 2020 and April 2021. Trees were monitored for tree health, psyllid colonization, bacterial titers, fruit quality, fruit drop, and yield for two production seasons. Imidacloprid injection caused 63% mortality of psyllid adults within one week and reduced progeny survival by 80%, though the efficacy waned within two months. Injection with oxytetracycline significantly improved tree health, reduced bacterial titers, and reduced preharvest fruit drop by more than 3-fold with corresponding increases in yield. Residue dynamics varied by injected compound and tissue analyzed. These results suggest trunk injection could be an effective delivery method for existing or novel therapeutics targeting either the insect vector or the pathogen causing HLB.