|RASHED, ARASH - Idaho State University|
|WORKNEH, FEKEDE - Texas A&M University|
|PAETZOLD, LI - Texas A&M University|
|RUSH, CHARLES - Texas A&M University|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2016
Publication Date: 3/15/2016
Citation: Rashed, A., Wallis, C.M., Workneh, F., Paetzold, L., Rush, C. 2016. Variations in Zebra Chip disease expression and tuber biochemistry in response to vector density. Phytopathology. 106(8):854-860. doi: 10.1094/PHYTO-01-16-0026-R.
Interpretive Summary: Zebra chip disease (ZC) is a major threat to potato production in the western United States and elsewhere. ZC is caused by infection by the bacterium ‘Candidatus Liberibacter solanacearum’ (Lso), which is transmitted by potato psyllids. Although studies have shown as little as one infectious psyllid can inoculate a susceptible potato host, it was unknown whether greater psyllid populations would result in greater ZC symptoms when tubers were harvested weeks after inoculation events. Thus, potato plants from two cultivars were exposed to five, ten, or thirty infectious potato psyllids for one week. Tubers were harvested eight weeks after exposure and evaluated for ZC symptoms, Lso titers, and changes in tuber biochemistry previously associated with symptom development. This study demonstrated that increasing numbers of infectious potato psyllids on a host not only resulted in more severe tuber ZC symptoms, but also observed were greater Lso titers and ZC-associated host physiology shifts. These findings suggest that five or less psyllids per plant are likely tolerable, but greater numbers could result in unacceptable ZC severity upon harvest. Growers therefore can raise action thresholds for pesticide sprays against psyllids accordingly, thereby reducing costs and delaying onset of pesticide resistance in insect populations. Results also suggest that minimizing host responses to Lso infections could result in Lso-infection tolerant germplasm that does not develop ZC symptoms.
Technical Abstract: This study examined effects of the number of ‘Candidatus Liberibacter solanacearum’ (Lso)-positive psyllids feeding on potatoes to Lso titers, zebra chip disease (ZC) symptom severity, and levels of amino acids, carbohydrates, and phenolics in tubers harvested weeks later. Red La Soda and Russet Norkotah seed potatoes were planted in field cages and later inoculated with Lso using five, ten, or thirty Lso-positive potato psyllids. Exposure to psyllids lasted one week, and eight weeks later tubers were harvested. Lso titer and ZC symptom severity in tubers from both cultivars increased when a greater number of infectious psyllids were released on the plants. Amino acids and reducing sugar levels generally increased when plants had greater numbers of psyllids, although changes in the levels of specific compounds varied according to cultivar. Phenolic compound levels were consistently positively associated with the number of Lso-positive psyllids placed on the plants. These results suggest reducing vector numbers within potato fields would significantly minimize quality losses from Lso-infections by reducing biochemical changes and corresponding ZC symptoms. Furthermore, based on observations from this study that further linked physiological responses to disease development, a viable strategy to develop Lso-tolerant potato genotypes would be to focus on interrupting host defense pathways that are associated with observed physiological shifts.