Impact of potato psyllid density and timing of infection on zebra chip disease expression in potato plants

The Zebra chip (ZC) syndrome is an important and emerging potato disease and a major threat to the potato industry. Chips from infected tubers develop dark brown streaks when processed, leading to rejection. The potato psyllid, Bactericerca cockerelli (Sulc) is believed to be a vector of the ZC pathogen, which is now thought to be Candidatus Liberibacter, a bacterium. To increase the understanding of the relationship between the psyllid infection and ZC expression, healthy potato plants at different growth stages (4, 5 and 7 weeks after germination) were exposed separately to four different potato psyllid densities (0, 5, 20 and 40 psyllids per cage) in the field and in the greenhouse using cages. ZC symptoms, tuber yield, leaf photosynthetic rates and total nonstructural carbohydrate accumulation in leaves and tubers of healthy and psyllid-infested plants were monitored and recorded. Typical ZC symptoms were observed in leaves and tubers of all plants exposed to psyllids, especially at 4 weeks after germination. Incidence of ZC symptoms in foliage and tubers increased and fewer tubers with small size harvested when plants exposed to psyllids with the increasing of density. Caged potato plants without psyllids (controls) did not show ZC symptoms. Tubers from psyllid-infested plants had significantly higher level of reducing sugars (glucose) and lower level of starch than those from healthy plants, suggesting that psyllid infestation and density interfered with carbohydrate metabolism in either leaves or tubers, resulting in ZC expression.