In Florida, Huanglongbing (HLB), also known as citrus greening, has severely impacted citrus production, prompting research into alternative treatment methods such as trunk injection of oxytetracycline (OTC) to manage Candidatus Liberibacter asiaticus (CLas) colonization. This study aims to 1) determine the optimal OTC rate administered by injection to improve the health and productivity of HLB-affected mature sweet orange trees and 2) determine the impact of spring versus late summer injections to maximize benefits and the potential for extending the harvest window. The study was initiated in 2023 in a commercial citrus production setting in Polk County, Florida. The trees were 18 years old and composed of ‘Valencia’ (Citrus sinensis) scion on Swingle (C. paradisi × Poncirus trifoliata) rootstock. Four different OTC rates (0.55 g, 0.852 g, 1.1 g, and 1.65 g active ingredient per tree) and two injection times (June and September) were compared against a control receiving no injections. The experimental design was a randomized block design with eight replications, and each replication consisted of four linear trees. A commercial formulation of OTC registered in Florida was used and administered through FlexInject injectors. A volume of 100 ml per tree was used for the 0.55-1.1 g rates and administered with one injector. The highest rate (1.65) was administered using two injectors on opposite sides of the tree in a volume of 75 ml per injector. Tree health was assessed by visual ratings of foliar HLB symptoms and canopy densities, while CLas titers were quantified using real-time PCR. Fruit quality and yield measurements included total soluble solids, titratable acidity, fruit weight, and juice characteristics. Trees were harvested, and fruit/juice quality was determined in March 2024. Overall, injected trees produced significantly more and larger fruit with better juice color, a higher percentage of juice, and more total soluble solids than control trees. The highest rate of OTC produced the highest yield and highest-quality fruits compared to the other rates. We did not observe any difference between spring (June) and summer (September) injections on any measured variables. Bacterial titers were reduced in the injected trees compared to the non-injected trees, and injected trees had fewer HLB symptoms and denser canopies than control trees. This research provides important insights into the best practices for managing HLB using trunk injection of OTC.
