Zebra chip disease development in potato in relation to vector density and time of infection

Zebra chip (ZC) is an economically important disease of potato, caused by a psyllid-borne bacterium, Candidatus Liberibacter solanacearum (Lso). Late–season infections are important as this is the time when psyllid populations are the highest in Idaho. A one-year greenhouse study was conducted to quantify disease development, both pre- and post-harvest, in relation to vector densities and the time of infection before vine removal. Potato plants were inoculated with different densities of infective psyllids (1, 2, and 5/plant) at different inoculation times (84, 21, and 2 days before vine removal, dbvr). ZC symptom severity and Lso titer were evaluated over time in foliar tissue as well as in tubers 3 weeks after vine removal. Photosynthetic rate was recorded at the scoring times of pre-harvest visual symptoms. Soil respiration measurements were performed after vine removal. In the 84-dbvr treatment, 10%, 40% and 60% of plants infested with 1, 2 or 5 psyllids developed foliar symptoms, respectively. Plants infested 2 and 21 days before vine removal did not develop any foliar symptoms; however, symptoms were present in tubers. Only a few tubers showed ZC symptoms in the 2dbvr treatment, but qPCR will be needed for verification. Tubers from plants infested with 1, 2, and 5 psyllids did not show significant differences in severity in either 21- or 84-dbvr treatments. Inoculated plants exhibited lower photosynthetic rates compared to non-inoculated plants, but no significant relationship was detected between tuber symptom severity and soil respiration. Our results to date indicate that monitoring and management of potato psyllids ZC should continue until harvest.