**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** INFECTIOUS DISEASES OF PLANTS ARE EMERGING WITH INCREASING FREQUENCY, BUT WE OFTEN LACK GENOMIC AND ECOLOGICAL DATA TO UNDERSTAND HOW THIS OCCURS. THIS IS PARTICULARLY TRUE FOR UNCULTURABLE MICROBES THAT CAN ONLY BE STUDIED INSIDE PLANT AND INSECT HOSTS, SUCH AS CANDIDATUS LIBERIBACTER SOLANACEARUM (CLSO) WHICH IS TRANSMITTED BY INSECTS CALLED PSYLLIDS. SINCE 2008, AT LEAST NINE CLSO VARIANTS HAVE EMERGED AS CAUSAL AGENTS OF DISEASES SUCH AS ZEBRA CHIP IN POTATO AND YELLOWING IN TOMATO, PEPPERS, CARROTS, AND RELATED CROPS. TENS OF MILLIONS OF DOLLARS ARE SPENT YEARLY MITIGATING LOSSES FROM CLSO IN THE US AND ABROAD. THE RAPID EMERGENCE AND EXPANSION OF CLSO SUGGESTS WE ARE ILL-PREPARED TO REDUCE IMPACTS OR PREDICT THREATS, DUE IN LARGE PART TO HAVING NO INFORMATION ABOUT CLSO FROM BEFORE WE COULD SEQUENCE THE GENOMES OF PATHOGENS, OR ON BROADER CA. LIBERIBACTER SPECIES ASSOCIATIONS WITH THE PSYLLIDS THAT TRANSMIT THEM. THE PROPOSED PROJECT WILL ADDRESS THESE GAPS THROUGH CHARACTERIZATION OF HISTORIC GENETIC DIVERSITY OF CLSO IN THE US, IDENTIFICATION OF CA. LIBERIBACTER FROM US PSYLLID SPECIES, AND SEQUENCING OF WHOLE GENOMES OF SELECT NEW CA. LIBERIBACTER DETECTIONS. TO COMPLETE THESE OBJECTIVES, WE WILL USE APPROACHES VALIDATED IN OUR PRIOR WORK. THESE INCLUDE USING MOLECULAR TECHNIQUES TO DETECT CLSO AND RELATED PATHOGENS IN PRESERVED PLANT SAMPLES HOUSED IN MUSEUM COLLECTIONS AS WELL AS USING SIMILAR TECHNIQUES TO DETECT PATHOGENS INSIDE THE BODIES OF INDIVIDUAL PSYLLID INSECTS. USING THESE TECHNIQUES, WE DEMONSTRATED THAT A VARIANT OF CLSO WAS PRESENT IN THE US DECADES BEFORE EMERGENCE IN CROPS, AND THAT PSYLLIDS HARBOR NOVEL LIBERIBACTER SPECIES THAT POSE THREATS TO US AGRICULTURE. THE DATA WE WILL GENERATE WILL BE USED TO DETERMINE GENETIC FACTORS UNDERLYING DISEASE (AND HOW TO DISRUPT THEM), IMPROVE MONITORING FOR NEW LIBERIBACTER THREATS, AND PREDICT PSYLLIDS AND LIBERIBACTER OF POTENTIAL CONCERN. THESE RESULTS WILL BE COMMUNICATED TO TARGET END USERS, SUCH AS GROWERS, USDA-APHIS PERSONNEL, AND THE SCIENTIFIC COMMUNITY THROUGH DIVERSE METHODS CUSTOMIZED TO EACH AUDIENCE. ADDITIONALLY, THIS PROJECT WILL HELP TRAIN GRADUATE AND UNDERGRADUATE STUDENTS TOWARD CAREERS IN AGRICULTURAL SCIENCES.