UC Riverside

Insect-transmitted plant pathogens threaten crop production and conservation of natural habitat. Despite decades of research aimed at controlling these pathogens in crops, our understanding of vector behavior and ecology and genetic diversity beyond the borders of agriculture is lacking, limiting the efficacy of disease management efforts. This dissertation explores these three factors with regard to the transmission of viruses and fastidious bacterial pathogens by phloem-feeding insects in the suborder Sternorrhyncha. In chapter 1, I report how treating cultivated melon with a synthetic plant hormone analog affects symptom induction by two phylogenetically distant viruses, and how this alteration in symptoms may influence vector behavior in ways that decrease pathogen spread. This work demonstrates the potential utility of disease management tools that mitigate symptom development and disrupt transmission-enhancing vector behaviors, rather than solely controlling vector populations or breeding host plants for resistance. In Chapter 2, I pivot to investigate the ecology and population genetics of the bacterial pathogen Candidatus Liberibacter solanacearum (CLso) and its vector, the potato psyllid (Bactericera cockerelli), in natural plant communities in southern California. I report that CLso and potato psyllids commonly associate with the native perennial Solanum umbelliferum, but these CLso and potato psyllid populations are both genetically distinct from and more diverse than their counterparts in agricultural fields. This suggests that multiple sympatric, but genetically isolated populations of potato psyllid may exist in California, with members of these psyllid populations, and CLso, only rarely moving between crop and wild habitats. In Chapter 3, I tested this hypothesis by sampling potato psyllids from both native and crop plant hosts throughout the state of California. I used restriction enzyme-associated DNA-sequencing (RADseq) to assess genetic connectivity between these psyllid populations and evaluated their CLso infection status, to better understand the importance of different potato psyllid genotypes in spreading CLso. Results of this study greatly improve our understanding of CLso and potato psyllid distribution and genetic diversity in an understudied portion of the country. They confirm that studying plant pathogens and their insect vectors in natural habitats could drastically enhance our ability to forecast and manage future outbreaks of plant disease.