Phenotypic and Etiological Differences Between Psyllid Yellows and Zebra Chip Diseases of Potato

Both potato psyllid yellows and zebra chip (ZC) potato diseases are associated with the potato psyllid, Bactericera cockerelli (Sulc). Aboveground plant symptoms of both diseases are similar but there is a difference in symptoms in potato tubers. ZC has recently been associated with a new species of the bacterium liberibacter, 'Candidatus Liberibacter solanacearum', also known as 'Ca. Liberibacter psyllaurous'. Mechanisms by which the potato psyllid might cause either ZC or potato psyllid yellows symptoms are not understood. Insect transmission studies were conducted to demonstrate psyllid vectoring of both diseases and to compare symptoms and development of the two diseases. Potato plants were exposed to both liberibacter-free and liberibacter-carrying potato psyllids and later evaluated for plant and tuber symptoms. These plants and tubers were then tested for liberibacter by polymerase chain reaction (PCR). In addition, potato plants exhibiting severe psyllid yellows/ZC-like symptoms were collected from a commercial potato field heavily infested with the potato psyllid and tested for liberibacter. PCR detected 'Ca. Liberibacter solanacearum' in ZC symptomatic plants and tubers resulting from exposure to liberibacter-carrying psyllids. Despite development of foliar symptoms that resemble those of ZC in plants exposed to liberibacter-free psyllids, no liberibacter was detected in these plants with psyllid yellows. Moreover, tubers from these plants with psyllid yellows did not exhibit any symptoms of ZC infection and tested negative for the bacterium. No liberibacter was detected in plants or tubers collected from the psyllid-infested potato field, suggesting that the observed symptoms were due to psyllid yellows. Furthermore, potato plants that were infected with liberibacter died sooner than plants that were infected with psyllid yellows. Although an association between liberibacter and ZC has been established, no pathogen is yet associated with potato psyllid yellows and mechanisms by which psyllid yellows symptoms are induced by the potato psyllid remain unclear.