Influence of potato psyllid colony source and timing of infection on zebra chip disease expression in potato plants

The Zebra chip (ZC) syndrome is an emerging disease of potato (Solanum tuberosum L.) and a major threat to the potato chip industry. Chips from infected tubers develop dark brown streaks when processed, leading to rejection. Although the causal agent(s) of this disease are still unknown, the psyllid, Bactericera cockerelli (Sulc) is suspected to be a vector of the ZC pathogen. To further understand the relationship between the psyllid infection and ZC expression, healthy potato plants at different development stages were exposed separately to four different potato psyllid colonies each reared on either potatoes, tomatoes, eggplants or peppers. Leaf photosynthesis rates (Pn) and total nonstructural carbohydrate (TNC - glucose, fructose, sucrose and starch) accumulation in leaves and tubers of healthy and psyllid-injured plants were measured. Typical ZC symptoms were observed in leaves and tubers of all plants exposed to psyllids regardless of colony source, and this was accompanied by significant reductions in Pn. Caged potato plants without psyllids (controls) did not show ZC symptoms. Psyllid damage and ZC expression were most severe in plants exposed to psyllids at four weeks after germination or earlier. Plants older than 4 weeks were more tolerant of psyllid damage and showed less expression of ZC symptoms in foliage and tubers. Tubers from psyllid-injured plants had significantly higher levels of reducing sugars (glucose) than those from healthy plants, suggesting that psyllid infection and ZC expression interfered with carbohydrate metabolism in leaves or tubers.