Workneh, Fekede

‘Candidatus Liberibacter solanacearum’ (Lso), transmitted by the potato psyllid (Bactericera cockerelli), is the putative causal agent of potato zebra chip disease. The bacterial pathogen infects a wide range of solanaceous plants (both wild and cultivated species), among which are peppers, potatoes, and tomatoes. Currently there are two commonly detected, genetically distinct haplotypes of Lso (A and B) identified from potatoes in the United States. To determine whether there are interactions between Lso haplotypes and different solanaceous hosts, experiments were conducted in the greenhouse in which pepper, potato, and tomato plants were infested with psyllids carrying Lso A, B, or an A and B mix (AB) or with psyllids free of Lso. Host plants were grown in pots in cages on the greenhouse benches and infested with six psyllids per plant. In addition, eight pepper cultivars were similarly infested for deeper understanding of host–haplotype interactions. Approximately 7 weeks after infestation, adult psyllids in each cage were counted to determine the impact of Lso haplotype–host interactions on psyllid survival and plants were sampled and tested molecularly for Lso. Individual psyllids carrying haplotypes B or AB and those free of Lso copiously reproduced on all three hosts, and leaf tissue from each plant tested positive for the respective Lso except those infested with Lso-negative psyllids. However, psyllids carrying Lso A did not survive on peppers but survived and abundantly reproduced on potatoes and tomatoes. In addition, samples from peppers infested with psyllids carrying Lso A tested negative for Lso. However, peppers infested with individual psyllids carrying Lso AB tested positive for Lso A, indicating that the presence of B may be required for infection by Lso A and psyllid survival on peppers. The different pepper cultivars infested with psyllids carrying Lso A showed similar results to the haplotype–host interaction tests, suggesting that cultivar may not be a factor in Lso A–pepper host interactions. Results from these studies suggest that Lso A may affect host selection by psyllids either for nutrition or laying of eggs. Mechanisms involved in preventing psyllid reproduction on peppers, once identified, will have significant implications for potential psyllid management.

Zebra chip (ZC) disease of potato is associated with the putative pathogen ‘Candidatus Liberibacter solanacearum’, which is transmitted by the potato psyllid Bactericera cockerelli (Hem., Triozidae). The present study was initiated to investigate ‘Ca. L. solanacearum’ development during and following typical commercial storage practices. Using bacteriliferous psyllids, Russet Norkotah potato tubers were infested in field cages 14, 10, and 4 days before harvest. Changes in ‘Ca. L. solanacearum’ detection rate, ‘Ca. L. solanacearum’ titer, and concentrations of phenolic compounds were documented throughout storage. ‘Ca. L. solanacearum’ titer continued to increase during storage. Although significant increases in the frequency of ‘Ca. L. solanacearum’ detection were observed in all infestation treatments, the impact of ‘Ca. L. solanacearum’ infection on tuber quality remained comparatively low in plants infected 4 days before harvest, because the majority of the tubers remained asymptomatic. Minimizing storage and retail chain movement durations would help to limit ‘Ca. L. solanacearum’ impact on tuber quality in tubers infected 14 and 10 days before harvest. This study also demonstrated that ‘Ca. L. solanacearum’ can relocate from a newly infected leaf to a tuber in as little as 4 days. Psyllid management is recommended until at least 4 days before green harvest, when psyllid pressure is high in fields in which tubers are destined for commercial storage.

Zebra chip (ZC) is a disease of potato, putatively caused by the vectorborne bacterium ‘Candidatus Liberibacter solanacearum’. Although ZC has been a major concern due its significant negative impact on both potato yield and quality, its effect on seed potato sprouting has been the subject of recent evaluations. The present study was conducted to determine whether variation in emergence is affected by the infection duration of ‘Ca. L. solanacearum’-infected seed potato prior to harvest. Furthermore, changes in pathogen detectability and titer levels in late-season-infected plants also were evaluated during and after cold storage. The rate of ZC-affected seed potato emergence following cold storage was not affected by the time of infection in the field, and the majority of ZC-infected tubers failed to sprout. Time to “seedling” emergence also was significantly longer in seed potato from plants infected ≥2 weeks before harvest. The small percentage of plants that emerged from ZC-affected seed potato produced stunted, nonvigorous plants that often died after a few weeks. The rate of successful ‘Ca. L. solanacearum’ detection increased during cold storage, suggesting a continued ‘Ca. L. solanacearum’–tuber interaction postharvest. After tubers were removed from cold storage and held at room temperature, ‘Ca. L. solanacearum’ titer started to increase. Although none of the tubers from plants infected 1 week before harvest exhibited any disease symptoms or tested positive for ‘Ca. L. solanacearum’ at harvest, up to 38% of these tubers tested positive following placement at room temperature after cold storage. Results of this study suggest that the role of seedborne ZC in disease epidemiology is likely to be insignificant. Furthermore, the findings of this study emphasized the importance of continued control measures until at least a week before harvest, and highlighted the need for improved methods of ‘Ca. L. solanacearum’ detection at harvest, especially in tubers infected late in the season.