THE SUDDEN OAK DEATH (SOD) EPIDEMIC CAUSED BY THE INVASIVE PLANT PATHOGEN PHYTOPHTHORA RAMORUM IS DEVASTATING OAK FORESTS ON THE US WEST COAST AND SIGNIFICANTLY IMPACTING ORNAMENTAL NURSERIES. SOD IS A REEMERGING THREAT TO US NURSERIES AND FORESTS. THE RECENT DISCOVERY OF A NEW POPULATION IN ASIA PROVIDES FURTHER SUPPORT FOR THIS IMMINENT THREAT. THE NEWLT DISCOVERED POPULATIONS IN ASIA ARE DISTINCTLY DIFFERENT FROM US POPULATIONS. WHILE US POPULATIONS ARE CLONAL, ASIAN POPULATION APPEAR TO BE SEXUAL AND ARE GENETICALLY DIVERSE. THESE POPULATIONS MIGHT BE THE ORIGINAL SOURCE FOR INTRODUCTIONS OF THIS PATHOGEN INTO US FORESTS AND NURSERY OPERATIONS. THIS WORK WILL COMPARE THE ASIAN PATHOGEN POPULATION TO PREVIOUSLY CHARACTERIZED POPULATIONS FOUND IN NORTH AMERICA AND EUROPE. WE WILL TEST THE HYPOTHESES THAT ASIA IS THE CENTER OF ORIGIN, HARBORS SEXUAL POPULATIONS, AND THAT ASIAN POPULATION ARE POTENTIALLY MORE AGGRESSIVE. A RANGE OF APPROACHES INCLUDING GENOME SEQUENCING AND EVOLUTIONARY ANALYSIS WILL BE USED TO STUDY THE EVOLUTIONARY HISTORY OF SAMPLES FROM ASIA, NORTH AMERICA AND EUROPE. ASIAN POPULATIONS WILL ALSO BE COMPARED TO US POPULATIONS TO DETERMINE IF THE PATHOGEN IS MORE AGGRESSIVE IN ASIA. THIS RESEARCH WILL ALLOW MITIGATION OF FURTHER EMERGENCE OF P. RAMORUM, UNDERSTANDING MECHANISMS OF EMERGENCE IN P. RAMORUM, IDENTIFYING SOURCE POPULATIONS OF POTENTIAL, NOVEL MIGRANTS, AND ASSESSMENT OF THE THREAT OF NEWLY FOUND POPULATIONS OF THE PATHOGEN TO US AGRICULTURE AND FOREST ECOSYSTEMS. THIS WORK WILL ALSO ENABLE DEVELOPMENT OF NOVEL GENOMIC RESOURCES AND IMPROVED DIAGNOSTIC DETECTION METHODS. THIS RESEARCH WILL CONTRIBUTE TO ENHANCING ENVIRONMENTAL QUALITY AND TO SUSTAIN THE ECONOMIC VIABILITY OF US FORESTRY AND AGRICULTURE.