IN RECENT YEARS, THE RATE OF EMERGENCE OF ECONOMICALLY IMPORTANT CROP PATHOGENS HAS ACCELERATED. THIS TREND IS LIKELY TO CONTINUE AS CLIMATE CHANGE AND GLOBAL TRADE BRING NOVEL COMBINATIONS OF PATHOGENS, INSECT VECTORS, AND HOST PLANTS INTO MORE FREQUENT CONTACT. ONE CROP PATHOGEN THAT HAS EMERGED AS A SERIOUS THREAT TO AGRICULTURE IN THE US AND GLOBALLY IS CANDIDATUS LIBERIBACTER SOLANACEARUM (HEREAFTER, CLSO) WHICH IS SPREAD AMONG CROP HOSTS BY AN INSECT: THE POTATO PSYLLID. CLSO COLONIZES THE CIRCULATORY SYSTEM OF PLANTS THAT MOVES NUTRIENTS AROUND TO DIFFERENT TISSUES, CAUSING STUNTING, MALFORMATION, AND PREMATURE DEATH. IN NORTH AMERICA, CLSO CAUSES SERIOUS LOSSES IN CROPS IN THE NIGHTSHADE FAMILY (POTATOES, TOMATOES, PEPPERS), BUT IS BEST KNOWN AS THE CAUSAL AGENT OF ZEBRA CHIP (ZC) DISEASE OF POTATO. THE ZC OUTBREAK THAT LED TO THE DISCOVERY OF CLSO BEGAN IN MEXICO IN THE MID 1990S AND FIRST APPEARED IN THE US IN THE YEAR 2000. HOWEVER, BECAUSE CLSO BACTERIA CAN ONLY LIVE INSIDE PLANTS OR INSECTS, AND CANNOT BE CULTURED, THE CAUSAL AGENT WAS NOT IDENTIFIED UNTIL 2008. BY THIS TIME, CLSO HAD ALREADY SPREAD ACROSS MUCH OF THE WESTERN US AND HAD BEEN ACCIDENTALLY INTRODUCED INTO NEW ZEALAND. SINCE THEN, SEVERAL NEW GENETIC VARIANTS OF CLSO HAVE BEEN LINKED TO DISEASE IN CROPS IN OTHER KINDS OF CROP PLANT HOSTS (E.G. CARROTS).THIS PATTERN OF REPEATED, RAPID CLSO EMERGENCE EVENTS IN CROPS WORLDWIDE HAS RAISED THE KEY QUESTION OF HOW LONG THIS PATHOGEN MAY HAVE BEEN PRESENT, ADAPTING TO THE CROP HOST ENVIRONMENT, BUT OVERLOOKED BECAUSE IT COULD NOT BE CULTURED OUTSIDE OF THE HOST OR POSITIVELY IDENTIFIED. THROUGH OUR RECENT EFFORTS, WE ADDRESSED THIS KNOWLEDGE GAP BY LOOKING FOR CLSO INFECTIONS IN PRESERVED SPECIMENS OF WILD PLANTS (NIGHTSHADES) KNOWN TO BE ATTACKED BY THE POTATO PSYLLID VECTOR. THROUGH THIS, WE DISCOVERED A NEW GENETIC VARIANT OF CLSO IN NON-SYMPTOMATIC SPECIMENS COLLECTED 48 YEARS BEFORE THE FIRST APPEARANCE OF ZC DISEASE IN THE USA. THE CLSO IN THESE SPECIMENS IS DISTINCT FROM THE GENETIC VARIANT THAT INFECTS CROPS, BUT STILL CLOSELY RELATED. FOLLOWING THIS DISCOVERY, WE SURVEYED PRESENT-DAY POPULATIONS OF THE SAME NIGHTSHADE HOST AND FOUND THAT THE NEW VARIANT OF CLSO IS STILL PRESENT. ADDITIONALLY, SURVEYS OF POTATO PSYLLIDS ON THE NIGHTSHADES FOUND THAT THEY TOO ARE GENETICALLY DISTINCT FROM THOSE TYPICALLY FOUND IN CROPS. THESE FINDINGS ARE CRITICAL BECAUSE CLSO CANNOT BE GROWN IN ISOLATED CULTURE. THEREFORE, WE HAVE LITTLE KNOWLEDGE OF GENETIC FACTORS THAT CONTROL HOW SICK THE PATHOGEN MAKES ITS HOSTS. BEING ABLE TO COMPARE THE BIOLOGY AND GENOMES OF CLSO GENETIC VARIANTS THAT CAUSE DISEASE WITH THOSE OF CLOSELY RELATED, BUT NONPATHOGENIC GENETIC VARIANTS COULD RAPIDLY INCREASE OUR UNDERSTANDING IN THIS AREA.TO BUILD ON THESE RESULTS AND DETERMINE THEIR IMPLICATIONS FOR THE FUTURE EMERGENCE OF ADDITIONAL CLSO VARIANTS, THIS PROJECT WILL COMPARE THE HOST RANGES AND DISEASE SYMPTOMS OF THE NOVEL CLSO VARIANT (CLSO G) TOTHE MAIN VARIANT NOW CAUSING DISEASE IN CROPS (CLSO B). EACH VARIANT WILL BE INOCULATED TO CROP HOSTS AND WILD HOSTS, AFTER WHICH WE WILL TRACK INFECTIONS AND SEVERITY OF DISEASE. TO DETERMINE IF GENETIC VARIATION IN PSYLLID VECTORS PLAYS A ROLE IN MOVING CLSO VARIANTS TO AND FROM WILD HABITATS, WE WILL ALSO COLLECT POTATO PSYLLIDS FROM CROP AND WILD HABITATS THROUGHOUT THE SOUTHWEST. THE PSYLLIDS WE COLLECT WILL BE ANALYZED TO MEASURE VARIATION IN THEIR GENOMES AND CONNECT THIS WITH THE HOST PLANTS ON WHICH THEY ARE COLLECTED. THE KNOWLEDGE WE WILL GENERATE FROM THIS PROJECT WILL HELP TO IDENTIFY 1. WHY CLSO CAUSES DISEASE, AND 2. WHETHER POTATO PSYLLID HOST PLANT PREFERENCES AFFECT THE LIKELIHOOD OF CLSO BECOMING A DISEASE-CAUSING PATHOGEN IN CROPS. THIS KNOWLEDGE WILL BE SHARED WITH STAKEHOLDERS THROUGH OUTREACH AND WITH OTHER RESEARCHERS THROUGH PUBLICATIONS AND PROFESSIONAL PRESENTATIONS. THE PROJECT WILL ALSO ALLOW THE GRADUATE STUDENT PROJECT DIRECTOR TO GAIN VALUABLE EXPERIENCE IN MENTORING AND PROJECT MANAGEMENT, AND PROVIDE TRAINING FOR UNDERGRADUATE RESEARCHERS IN ENTOMOLOGY AND RELATED FIELDS.