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An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Over the last century, repeated emergence events within the Candidatus Liberibacter taxon have produced pathogens with devastating effects. Presently, our knowledge of Ca. Liberibacter diversity, host associations, and interactions with vectors is limited due to a focus on studying this taxon within crops. But to understand traits associated with pathogen emergence it is essential to study pathogen diversity in wild vegetation as well. Here, we explore historical native host plant associations and diversity of the cosmopolitan species, Ca. L. psyllaurous, also known as Ca. L. solanacearum, which is associated with psyllid yellows disease and zebra chip disease, especially in potato. We screened tissue from herbarium samples of three native solanaceous plants collected near potato-growing regions throughout Southern California over the last century. This screening revealed a new haplotype of Ca. L. psyllaurous (G), which, based on our sampling, has been present in the U.S. since at least 1970. Phylogenetic analysis of this new haplotype suggests that it may be closely related to a newly emerged North American haplotype (F) associated with zebra chip disease in potatoes. Our results demonstrate the value of herbarium sampling for discovering novel Ca. Liberibacter haplotypes not previously associated with disease in crops.