The purpose of this project is to reveal the mechanisms of bactericide uptake and transport in citrus plant and establish a theoretical basis for developing technologies to improve the efficacy of bactericides, which is helpful to provide potential solution to the development of effective chemotherapeutic tools for HLB management. Achieving this outcome will require progress in the following three tasks: (1) to compare the delivery efficacy of bactericides with three application methods (foliar spraying, truck injection, and root administration) based on the uptake and dynamic movement/distribution of the bactericide within the tree; (2) to clarify the systemic movement and transportation mechanisms of bactericides within the phloem of tree; and (3) to investigate the effects of citrus variety and age on the delivery efficacy of bactericides. This project requires a combination of greenhouse studies and field trials. Prior to conducting these experiments, a sensitive and accurate quantifying method of bactericides (oxytetracycline and streptomycin) in citrus tissues is needed. This project officially started on December 1, 2018. This is the 8th quarterly progress report covering September 01 to November 30, 2020. During this period, we have conducted and/or completed the following work/research tasks:1) The samples from the trunk injection and root administration experiments in greenhouse were analyzed. The results show that after trunk injection, oxytetracycline (OTC) was detected in leaves on day 7, 15 and 30; the concentration of OTC in leaves was highest on day 7 and decreased with time. The OTC in stems and roots was detected only on day 7, whilst streptomycin (STR) was not detected in leaves, stems or roots. The results of root administration using a root-split technique showed that both OTC and STR could be horizontally transported from the treated roots to untreated roots. The absorption efficiency of STR was higher than that of OTC. Both antibiotics could be detected in leaves and roots on day 30.2) Among the three tested application methods, root administration by the split technique (RS) has the highest delivery efficiency for OTC, followed by trunk injection (TI), and least with foliar spraying: RS > TI >FS. The results for STR are inconsistent. The results from greenhouse experiments remain to be confirmed by the field trials.3) The field trials with three application methods, i.e. foliar spraying, trunk injection and root administration, were conducted in two different citrus groves, one grapefruit and one orange. So far we have completed the treatments and collection of leaf, stem and root samples on the 7th, 15th and 30th days after the application of OTC or STR.The work planned for the next quarter (December 1, to February 28, 2021):The major research tasks for the next three months include: 1) Continuing the field experiments.2) Preparation of the samples from the field trials prior to analysis of OTC and STR by LC-MS, including grinding, extraction, purification and concentration.3) Analysis of the pretreated samples for concentration of OTC and STR using the LC-MS.4) Processing of the data from the field trials.5) Preparation of reports and/or manuscripts from the data obtained from the greenhouse experiments.