Our hypothesis is that application of antibacterial-producing bacteria directly to citrus root could suppress Las population in the roots and control Las. Application of antibacterials in this manner will avoid the strict restrictions of application of antibiotics on crops and ease public concerns since those bacteria are naturally present in the soil and are associated with plant roots. In order to achieve the goal, the following objectives will be conducted: Test antibacterial-producing bacteria against Liberibacter crescens and other Rhizobiaceae bacteria which are closely related to Las. We tested 27 antibacterial compound producing bacteria including Bacillus cereus, B. licheniformis, Paenibacillus polymyxa, Pseudomonas spp., Streptomyces aureofaciens, Streptomyces fradiae, Streptomyces fradiae, Streptomyces garyphalus, Streptomyces griseus, Streptomyces kanamyceticus, Streptomyces niveus, Streptomyces pristinaespiralis, Streptomyces virginiae, Streptomyces ribosidificus, Streptomyces venezuelae, Streptomyces vinaceuse, and Streptomyces capreolus. We have isolated 327 bacteria from Florida citrus groves. The antagonistic activity against Agrobacterium, Sinorhizobium meliloti, L. crescens and Xanthomonas citri pv. citri was determined. 21 strains including bacteria belonging to Paenibacillus, Burkholderia, Bacillus, and Streptomyces showed good antagonistic activity. Those isolated bacteria showing high antimicrobial activities have been sequenced to help us understand the mechanism and for identification purpose. Currently, the genome sequencing was finished and genome analysis is ongoing. Because Las infection also affects host resistance to Phytophthora, one common citrus pathogen in Florida, we tested the antimicrobial activity of the bacterial isolates against Phytophthora nicotinae and P. palmivora, multiple bacterial isolates showed antimicrobial activities against Phytophthora spp. Four bacterial strains: two Burkholderia, one Pseudomonas geniculata, and one Rhodococcus strains have been tested for their activity in controlling citrus HLB and canker and all showed induced plant defenses and control effect against infection by Xanthomonas citri. The HLB result is shown below. To further study the antimicrobial producing bacteria, tow Burkholderia strains have been labeled with GFP tag. Seven other strains are being labeled with GFP or RFP tag. We also investigated the antibiotic genes in the 21 antimicrobial producing bacteria that we isolated previously. These strains were inoculated to citrus roots and the colonization was determined by inoculation and recover method in lab condition using small citrus seedlings. Around 10E8 cfu were inoculated to each seedling. Approximately 10E4 cfu were recovered from roots 20 days after inoculation (dpi). In a separate experiment, two Burkholderia strains were tested and up to 10E5 cfu/g soil was recovered at five days post inoculation. Four antimicrobial producing bacterial strains belonging to Paenibacillus, Bacillus sp., and Pseudomonas geniculata were tested in field trial via a soil drench method applied every two months for one year. The treated trees were divided into the following categories based on the disease index of 0-5: 1) No symptoms or few symptoms (0-2); 2) Trees with severe HLB symptoms (3-5). One gallon of bacterial culture was applied per tree at the three concentrations: 106, 107, and 108 CFU/ml. Water treatment was used as negative control. The result demonstrated the applied bacteria survived better in rhizosphere soil than applied via irrigation, but the overall survival in the soil is still limited and the bacteria did not establish high population on the root surface. Application of beneficial bacteria slowed down the disease index increase and Las titers for the asymptomatic or trees showing few symptoms compared to the control, but it did not prevent the disease index and Las titers from increasing. For the trees showing severe symptoms, the applied bacteria did not show any effect on disease index and Las titers. One manuscript is being prepared to report the findings.