Frateuria defendens (Frd) is a recently discovered endophytic bacterium belonging to the family of Rhodanobacteraceae. The application of Frd by foliar spraying and soil drenching was documented to reduce the symptoms associated with phytoplasmas and liberibacters in various crops, including grapevine and carrots. It is thought that Frd has multiple modes of action including secretion of antimicrobials and priming of host defenses. This project was part of a collaborative effort between US (UF/IFAS SWFREC) and Israel (ARO) to develop an environmentally safe, affordable, and effective solution for the management of citrus HLB. It aimed to test whether Frd can effectively reduce CLas titers and HLB disease symptoms in citrus trees and can be developed as a commercial biocontrol agent. The project was originally intended as a 3-year study but was revised to conduct a 1-year feasability study. The overall goal of the project was to test the efficacy of Frd in reducing HLB disease effects in Florida citrus. The specific objectives were 1) to test the curative effects of Frd against HLB under greenhouse conditions, and 2) to optimize delivery of Frd and determine suitability for large-scale control of HLB in field conditions. Objectives had been modified slightly from the original proposal to make the study feasible within the reduced time frame of the funding period.Our studies showed that, under the conditions of our study, Frd had difficulties establishing in citrus leaves. This effect may have been associated with Frd-unfavorable biochemicals in the leaves and seemed to have been exacerbated by CLas infection.There was no clear evidence that Frd interacted with CLas directly (i.e., Frd applications did not reduce CLas titers) and/or had a curative effect against HLB disease. However, Frd applications had positive effects on plant growth and seemed to be able to mitigate some of the damage caused by HLB or to retard disease progression of severly affected plants under greenhouse conditions.The direct delivery of Frd via leaf infiltration or trunk injection was more effective than foliar sprays.