Impact of potato psyllid density and timing of infestation on Zebra chip disease expression in potato plants

The impact of vector density and timing of infestation on potato were investigated. Healthy potato plants at different growth stages (4, 5, and 7 weeks after germination) were exposed separately to four different B. cockerelli densities (0, 5, 20, and 40 psyllids per cage) in field cages and Zebra chip (ZC) symptoms, leaf photosynthetic rates, tuber yield, and total nonstructural carbohydrate accumulation in leaves and tubers of healthy and B. cockerelli-infested plants were monitored. Potato psyllid nymph and egg populations reached a seasonal peak at 6 weeks after the exposure to insect. Younger plants at 4-week growth stage after germination were more susceptible to B. cockerelli infestation and ZC expression than older plants. As few as five B. cockerelli adults were enough to transmit the ZC pathogen and cause ZC expression both in foliage and tuber. At the density of 20 psyllids per cage, more than 50% of plants showed ZC symptoms in tubers. Furthermore, B. cockerelli infestation reduced leaf photosynthesis rates (Pn), resulting in less starch and more reducing sugars in tubers, and hence reduced tuber weight and yield, especially when psyllid infestation occurred at the early growth stages. The results indicate that early B. cockerelli infestation of younger plants was associated with more severe ZC expression in both foliage and tubers, leading to earlier dead plants. The data suggest that strategies for controlling B. cockerelli during early potato crop development could thus lessen the severity of ZC development.