Evidence that the Zebra Chip Disease and the Putative Causal Agent Can be Maintained in Potatoes by Grafting and In Vitro

In the last several years, a disorder of chipping potatoes that causes internal browning of raw tubers and very dark chip color when the tubers are fried has been described from the southwestern United States. The discoloration often shows as rays or stripes and the common name "zebra chip" (ZC) has been used to describe the disorder. Foliar symptoms include chlorosis, purpling of shoot tips, and leaf scorch. Aerial tubers are sometimes produced on diseased plants. The disease has been associated with the potato psyllid, Bactericera cockerelli, but a suspected pathogen that may be associated with the ZC disorder has only very recently been described. In our efforts to identify the causal agent we have been maintaining the disease in the greenhouse by tip grafting symptomatic potato shoots onto healthy plants of several commercially important cultivars of potatoes, including Atlantic, Russet Burbank, and Russet Norkotah. Most of the grafted plants subsequently developed ZC symptoms. Many of the grafted plants produced tubers with symptoms typical of the disease. In cultivar Atlantic, 104 of 138 grafted plants developed foliar symptoms of ZC and 72 of the grafted plants produced ZC-symptomatic daughter tubers. Grafted plants without foliar symptoms did not produce symptomatic tubers. Herein we report the results of these grafting experiments and describe the symptoms produced in grafted plants and daughter tubers. We have also maintained the disease by placing surface-sterilized symptomatic shoots onto growth medium in vitro. In addition, the putative ZC pathogen, a new Candidatus Liberibacter spp., has been detected in symptomatic grafted plants, symptomatic in vitro plants, and potato psyllids by PCR.