Impact of Vine Kill on Zebra Chip Severity and Incidence of ‘Candidatus Liberibacter Solanacearum’ in Potato Tubers

Potato zebra chip (ZC) is putatively caused by the bacterium ‘Candidatus Liberibacter solanacearum’ (Lso), which is transmitted by the potato psyllid Bactericera cockerelli Sulc. The most commonly recognized symptom of the disease is internal browning of tubers, which when sliced and fried turn dark brown, affecting marketability of fried potato products. Infection can occur at all stages of plant development, requiring multiple insecticide applications to control the vector during the season. Many growers routinely kill potato vines a few weeks prior to harvest for skin setting to minimize bruising during transit and storage that could potentially lead to rotting. The question arose as to how this practice might impact development of ZC in tubers following late-season infections. To address this question, experiments were conducted over a two-year period in which plants were infested with bacteriliferous psyllids at seven and two days before vines were killed, and two and seven days after vine kill. Tubers were harvested approximately a month later. In both years, over 85% of tubers from plants infested seven days before vine kill tested positive for Lso and developed symptoms of ZC. Symptoms of ZC also developed in an average of approximately 50% of the tubers from plants infested just two days before vine kill. However, no tubers from any of the plants infested two days or seven days after vine kill developed symptoms of ZC and Lso was not detected in any of these tubers. This study showed that infestation of potatoes with bacteriliferous psyllids as late as two days before vine kill can result in significant infection and ZC symptom development in unharvested tubers. It also highlighted the importance of late-season vector management.