The bacterial pathogen associated with citrus huanglongbing (HLB) resides in the phloem of affected trees. The widespread abundance of the vector in Florida, the Asian citrus psyllid (Diaphorina citri), a sap-sucking, hemipteran bug, and the location of the pathogen in the tree vascular tissue limits the efficacy of foliar-applied therapies.
Trunk injection is a crop protection strategy that applies therapeutic compounds directly into the tree vascular system, enabling their systemic distribution within the tree. However, limited information is available on the most effective methodology for implementing trunk injection at the commercial scale and the extent of damage inflicted by the injection. In this study, 5-year-old HLB-affected 'Midsweet' sweet orange (Citrus sinensis) trees were injected with the insecticide imidacloprid, the antibacterial oxytetracycline, or water. Injections occurred in June and Oct 2020 using three trunk injection techniques. Trees were monitored for external wounding and internal damage associated with injection, as well as tree health, bacterial titers, and yield for two production seasons. Low-pressure injection caused the least damage; however, it was less effective at delivering the tested compounds than medium- or high-pressure injection.
Despite causing the greatest extent of external and internal damage at the injection site, injection of oxytetracycline significantly improved tree health, reduced bacterial titers, and increased yield in the two seasons of this study. Imidacloprid injection caused less wound damage but did not result in any lasting benefits to the trees.
Determining the long-term effects of injections on tree health is necessary before widespread adoption of this technology. Nevertheless, because of the devastation HLB has wreaked on the Florida citrus industry, trunk injection of OTC was approved for commercial use in Oct 2022 (and final clearance was given in Jan 2023) and has since been widely adopted. This study confirmed the efficacy of trunk-injected OTC for management of HLB in young citrus trees. OTC injections reduced bacterial titers, which led to notable decreases in fruit drop and, consequently, increases in yield for two harvests after injection. Whether the increase in productivity outweighs the risks and costs associated with injections at the commercial scale will need to be determined.
According to Dr. Albrecht, "We conducted this research because of the devastation the bacterial disease huanglongbing (HLB, also known as citrus greening) has wreaked on the Florida citrus industry and other citrus production areas around the world. Our research has helped elucidate the benefits and risks associated with the delivery of HLB therapies by trunk injection. Our findings have contributed to the approval of the technology for commercial use in Florida citrus and to its widespread adoption by citrus growers (a recent survey indicated an adoption rate of 84%). Our research has also gained attention around the world, prompting researchers and growers from countries where HLB is a major concern to visit our center and learn about the technology. The research was part of a larger project funded by USDA."
Dr. Albrecht is Associate Professor of Plant Physiology, University of Florida-IFAS, Southwest Florida Research & Education Center
Read the entire article on the HortScience electronic journal website at: https://doi.org/10.21273/HORTSCI17172-23
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