Development of a field bioassay for rapid detection of Candidatus Liberibacter solanacearum (Lieft.) in potato (Solanum tuberosum L.) leaves and tubers

Abstract

Candidatus Liberibacter solanacearum (CaLso) is vectored by Tomato/Potato Psyllid (Bactericera cockerelli Sulc.). The bacterium spreads throughout the phloem and eventually infects underground tubers, which exhibit brown flecking throughout the perimedullary tissue. Upon deep-drying of tuber cross-sections in oil, this results in a characteristic zebra pattern, giving rise to the common name, Zebrachip. Currently, the only way to determine whether plants are infected with CaLso is by time-consuming and expensive PCR testing. Here, we developed an inexpensive bioassay that can be performed rapidly in the field. CaLso is known to degrade starch into amylose and amylopectin. The latter stains dark purple in iodine. Potato laminas were sliced parallel to, but 2 mm away from, the midrib. The smaller piece of the lamina was immersed in 3% iodine solution for 30 sec., rinsed for 5 sec in distilled water and cut surfaces photographed immediately. Similarly, cross-sections of tubers were stained in 3% iodine for 10 sec., rinsed in distilled water for 5 sec and photographed. Cut surfaces that stained dark purple were thought to be infected with CaLso. 100% of plants with dark flecked tubers had positively stained leaves and tubers. Of the healthy-looking plants, 25% of them were suspect for CaLso as those leaves and tubers exhibited partial staining of cut surfaces. PCR of all leaves and tubers were undertaken to confirm the presence or absence of CaLso. Results of the PCR study will be presented, along with a confidence interval for accuracy of the bioassay.