Sap-feeding insects in the order Hemiptera associate with obligate endosymbionts that are required for survival and facultative endosymbionts that can potentially modify resistance to stress, enemies, development, and reproduction. In the superfamily Psylloidea, the jumping plant lice (psyllids), less is known about the diversity and prevalence of their endosymbionts compared to other sap-feeding pests such as aphids (Aphididae). To address this knowledge gap, using 16S rRNA sequencing we identify symbionts across divergent psyllid host lineages from around the world. Taking advantage of a new comprehensive phylogenomic analyses of Psylloidea, we included psyllid samples from 44 species of 35 genera of five families, collected from 11 international locations for this study. Across psyllid lineages, a total of 91 OTUs were recovered, predominantly of the Enterobacteriaceae (68%). The diversity of endosymbionts harbored by each psyllid species was low with an average of approximately 3 OTUs. Two clades of endosymbionts (clade 1 and 2), belonging to Enterobacteriaceae, were identified that appear to be long term endosymbionts of the psyllid families Triozidae and Psyllidae, respectively. We also conducted high throughput metagenomic sequencing on three Ca. Liberibacter infected psyllid species (Russelliana capsici, Trichochermes walkeri, and Macrohomotoma gladiata), initially identified from 16S rRNA sequencing, to obtain more genomic information on these putative Liberibacter plant pathogens. The phylogenomic analyses from these data identified a new Ca. Liberibacter species, Candidatus Liberibacter capsica, that is a potential pathogen of solanaceous crops. This new species shares a distant ancestor with Ca. L. americanus, which occurs in the same range as R. capsici in South America. We also detected the first association between a psyllid specializing on woody hosts and the Liberibacter species Ca. L. psyllaurous, which is a globally distributed pathogen of herbaceous crop hosts in the Solanaceae. Finally, we detected a potential association between a psyllid pest of figs (M. gladiata) and a Ca. Liberibacter related to Ca. L. asiaticus, which causes severe disease in citrus. Our findings reveal a wider diversity of associations between facultative symbionts and psyllids than previously reported and suggest numerous avenues for future work to clarify novel associations of ecological, evolutionary, and pathogenic interest.

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. Plant pathogen emergence is a major threat to food security 1 and occurs through multiple, non-mutually exclusive pathways 2. For example, a plant pathogen may be a previously unknown or undetected organism, or an organism that was once non-pathogenic to its host but has evolved pathogenic traits over time. Alternatively, the plant pathogen may already be known, but has spread to, and proliferated within, a new geographic area or host population. An outbreak may also represent the re-emergence of a plant pathogen whose incidence had declined significantly in the past but, more recently, increased over a short period. These pathways of disease emergence are complex, and thus require consideration of microbe-plant interactions in both managed and unmanaged systems. With advances in sequencing technologies and development of methods to overcome detection challenges in non-crop hosts 3 , inclusion of wild vegetation in microbial diversity and pathogenicity studies is enabling the discovery and characterization of novel strain types 4 , and expanding our understanding of the processes underlying pathogen emergence 5-8. Among plant pathogens that have emerged as serious threats to food production in the last 100 years are those within the taxon 'Candidatus'(Ca.) Liberibacter 9 (Fig. 1). Ca. Liberibacter species are largely unculturable members of the Alphaproteobacteria class in the Proteobacteria phylum 10. Over the last decade, eight species of Ca. Liberibacter have been identified around the world following emergence as phloem-limited plant pathogens in mostly crop systems (cited within Fig. 1). Each known Ca. Liberibacter species is also associated with one or more herbivorous insect hosts (psyllid...

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