1. Please state project objectives and what work was done this quarter to address them:Objective 1: Investigate efficacy of bactericide treatments for preventing new infections. Objective 2. Determine the effect of bactericide application frequency on Las infection of citrus. Objective 3: Quantify the effect of repeated inoculation of the efficacy of bactericides. During the final quarter of the project, we completed two final treatment applications for experiments addressing objectives 1-3. Samples (leaf, ACP, flush) were collected as in previous quarters. All remaining leaf samples from previous quarters and the final quarter were processed (DNA extraction, qPCR) to determine the CLas titers in trees in response to treatments. 2. Please state what work is anticipated for next quarter: No additional work is expected. The project is complete, and this is the final report. 3. Please state budget status (underspend or overspend, and why):The budget for the project was underspent. The underspent funds were is a result of savings on the treatment materials used in this project (antimicrobials, insecticides. A portion of these materials were donated; therefore, the projected budget for these materials was lower than initially anticipated. Another factor contributing to the underspend was a reduction in activities during spring 2020. Personnel were unable to come in and work in the labs (April and May 2020) or the field (April 2020). One field treatment was missed as a result.. The reduction in salary and material spending contributed to the reduced funds spent. Executive Summary AbstractThe purpose of this project was to identify the most efficacious use of antimicrobial treatments that are commercially available in order to reduce Las inoculation pressure and prevent infection in young trees. Overall, the data indicate that bactericidal treatments did not prevent CLas colonization in young citrus trees. However, the results suggest that bactericides in combination with insecticides could be useful in preventing CLas infections in the first year of new citrus tree plantings. As the trees mature, additional protection should be considered. Additionally, citrus trees receiving monthly Firewall/Fireline applications had higher numbers of ACP adults and eggs and lower flush production across treatments. The use of Tree Defender enclosures and insecticide alone showed better and more prolonged (more than one year) protection of young citrus trees against CLas infection compared with antimicrobial treatment after one year. More frequent (monthly) bactericidal applications in combination with insecticides reduced CLas titers on mature trees as compared to quarterly Firewall/Fireline applications or insecticide only. Additionally, the number of eggs in flush declined in response to antimicrobial treatment. Further studies should evaluate bactericidal application frequency on younger and productive citrus trees (commercial citrus groves rather than in research groves) as a preventative and active method against CLas infections. Additionally, the data suggest that bactericides in combination with insecticides should be used when flush is highly abundant to reduce the numbers of eggs in flush for disruption of CLas transmission.