** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** THE PRIMARY OBJECTIVE IS TO UNDERSTAND MECHANISMS OF CLIMATE ADAPTATION IN A GLOBALLY RELEVANT PLANT PATHOGEN. XYLELLA FASTIDIOSA IS A BACTERIAL PATHOGEN THAT INFECTS OVER 600 PLANT SPECIES IN DIVERSE CLIMATES AROUND THE WORLD. IN CALIFORNIA, STRAINS OF GRAPEVINE-INFECTING X. FASTIDIOSA PERSIST FROM WARMER REGIONS IN THE SOUTH CENTRAL VALLEY (BAKERSFIELD) THROUGH REGIONS WITH COLD WINTERS IN NORTHERN CALIFORNIA (HOPLAND). HOPLAND IS AT THE NORTHERN EDGE OF THE GRAPEVINE-INFECTING X. FASTIDIOSA RANGE IN CALIFORNIA AS THE BACTERIUM IS SENSITIVE TO COLD WINTERS; VINES CAN BE CURED OF THE DISEASE WHEN EXPOSED TO COLD WINTERS. OUR RESULTS FROM AN ONGOING FIELD EXPERIMENT IN HOPLAND INDICATE THAT A LOCAL STRAIN OF X. FASTIDIOSA SURVIVES THE WINTER AT MUCH GREATER RATES THAN A NON-LOCAL STRAIN FROM BAKERSFIELD. TO VERIFY THAT THIS IS LOCAL ADAPTATION, I PROPOSE USING RECIPROCAL TRANSPLANT EXPERIMENTS (SITES IN HOPLAND AND BAKERSFIELD) WITH MULTIPLE STRAINS OF EACH GENETIC CLUSTER (HOPLAND CLUSTER, BAKERSFIELD CLUSTER) TO COMPARE OVERWINTER SURVIVAL RATES ACROSS SITE AND GENETIC GROUP. THEN, USING A RECENTLY SEQUENCED POPULATION OF STRAINS FROM HOPLAND, I WILL BUILD GENE-ENVIRONMENT ASSOCIATION MODELS TO IDENTIFY CANDIDATE GENES ASSOCIATED WITH COLDER CLIMATES. FROM THIS LIST OF CANDIDATE GENES, I WILL CONSTRUCT KNOCKOUT AND COMPLEMENTED STRAINS FROM A REPRESENTATIVE HOPLAND WILDTYPE ISOLATE. I WILL VALIDATE THE FUNCTION OF CANDIDATE GENES BY TESTING KNOCKOUT STRAINS IN PLANTA USING FIELD EXPERIMENTS IN HOPLAND. THIS PROJECT WILL ENHANCE UNDERSTANDING OF THE GENETIC MECHANISMS OF CLIMATE ADAPTATION IN BACTERIAL PLANT PATHOGENS.