Zebra Chip Progression: From Inoculation of Potato Plants with Liberibacter to Development of Disease Symptoms in Tubers

Zebra chip (ZC), a new and serious disease of potatoes, has caused millions of dollars in losses to the potato industry in the United States, Mexico, Central America, and New Zealand. The disease has been associated with the bacterium "Candidatus Liberibacter solanacearum" transmitted to potato by the potato psyllid, Bactericera cockerelli (ulc). The most characteristic symptoms of ZC develop in potato tubers and include browning of vascular tissue concomitant with necrotic flecking of internal tissues and streaking of the medullary ray tissues, all of which can affect the entire tuber. Upon tuber frying, these symptoms become more pronounced and potato chips or fries processed from ZC-affected tubers show very dark blotches, stripes, or streaks, rendering them commercially unacceptable. Field experiments were conducted to determine how rapidly ZC symptoms develop in potato tubers following plant exposure to liberibacter-infective potato psyllids and to assess how the disease affects the overall potato yield and tuber processing quality over time. Results indicated that ZC symptoms developed in potato tubers 3 weeks following plant exposure to psyllids. Tuber development ceased upon the onset of ZC symptoms, resulting in substantial yield loss. Levels of tuber solids decreased as soon as initial disease symptoms were observed. In contrast, reducing sugar levels in tubers increased dramatically upon the onset of ZC symptoms, significantly affecting potato processing quality. This information, in combination with effective psyllid monitoring and control, will assist potato producers make harvest timing decisions following infestations of potato psyllids in their fields to minimize damage caused by ZC.