The purpose is to evaluate the control effect of bactericides via trunk injection. Objective 1. To illustrate whether application of bactericides via trunk injection could efficiently manage citrus HLB and how bactericides via trunk injection affects CLas and HLB diseased trees.Three field trials have begun to investigate how the application of bactericides via trunk injection affects citrus growth, production, HLB symptom development, and CLas population in different aged trees at different levels of HLB disease severity. We developed a new method for evaluating the effects of oxytetracycline (OTC) treatment on CLas titers in planta, and determined the relationship between OTC residue levels and control levels achieved for CLas using mathematical modeling in greenhouse and field experiments. In either greenhouse or field, OTC spray did not reduce the titers of CLas and produced undetectable or mild levels of OTC residue in leaves within 7 days post application (DPA). In greenhouse, OTC injection at 0.05 g/tree decreased CLas titers to an undetectable level (Ct value = 36.0) from 7 to 30 DPA, and produced a residue level of OTC at 0.68-0.73 µg/g fresh tissue over this period. In the field, OTC injection at 0.50 g/tree resulted in the decline of CLas titers by 1.52 log reduction from 14 to 60 DPA, with residue levels of OTC at 0.27-0.33 µg/g fresh tissue. In both trials, a first-order compart model of OTC residue dynamics in leaves of trunk-injected trees was specified for estimating the retention of effective concentrations. Furthermore, nonlinear modeling revealed significant positive correlations between OTC residue levels in leaves and the control levels for CLas achieved. The results suggested that the minimum concentration of OTC required to suppress CLas populations in planta to below the detection limit is 0.68 and 0.86 µg/g, and the minimum concentration of OTC required for initial inhibition of CLas growth in planta is approximately 0.17 and 0.215 µg/g in leaf tissues under greenhouse and field conditions, respectively. This finding highlights that a minimum concentration of OTC should be guaranteed to be delivered to target CLas in planta for effective control of citrus HLB. Trunk injection is a promising delivery method for bactericides to control HLB. A manuscript entitled: The in planta effective concentration of oxytetracycline against Candidatus Liberibacter asiaticus for suppression of citrus Huanglongbing has been accepted for publication by Phytopathology.Objective 2. To examine the dynamics and residues of bactericide injected into citrus and systemic movement within the vascular system of trees and characterize the degradation metabolites of bactericides in citrus. A field trial has begun to determine the concentrations of bactericides in leaf, stem, root, flower, and fruit using HPLC at the following time points: 2, 7, 14, 28 days, 2, 4, 6, 8, 10, 12 months after injection at different doses. The leaf samples are being collected at different time points for testing. Objective 3. To determine whether trunk injection of bactericides could decrease Las acquisition by Asian citrus psyllids (ACP).We are determining whether trunk injection of bactericides at three different doses could decrease Las acquisition by ACP in greenhouse and in the field. Objective 4. To monitor resistance development in Las against bactericides and evaluate potential side effects of trunk injection of bactericides. Las-specific primers were designed to target the putative binding sites of OTC in 16SrRNA gene of Las. Plant genomic DNA was extracted from citrus trees received OTC injection for three years. PCR were performed with the primers and DNA samples, and the products were purified and subjected to DNA sequencing. No mutation was identified yet. We have collected more samples from multiple citrus groves.