Abstract
Zebra chip (ZC) disease is a threat to the potato industry because it reduces production and quality of tubers. The pathogen is associated with the bacteria “Candidatus Liberibacter solanacearum” (Lso) that is transmitted by the potato psyllid Bactericera cockerelli Šulc (Hemiptera: Triozidae). Since its detection in the United States in 2000, studies have been conducted to address different aspects of the ZC pathosystem. Sources of resistance has been investigated but characterization of resistance has been studied based on the interaction between the potato psyllid and potato or based on ZC symptom severity in fresh tubers at harvest. Moreover most of those studies have been conducted under controlled conditions. However, there are still critical questions to address that are directly relevant to ZC management and epidemiology. The overall objective of this dissertation was to study interactions between the potato psyllid and a range of potato genotypes, Lso and host genotypes and the potato psyllid and Lso. Although host plant resistance has been proposed as an effective approach in integrated pest management (IPM) program to ZC, none of the commercial potato cultivars in the U.S. are known to be resistant or tolerant consistently. In the first chapter we present a literature review of the zebra chip pathosystem. In chapter second chapter we studied Lso development and ZC symptoms in eight selected potato genotypes in the field, and in the third chapter we evaluated the progress of Lso development and ZC symptom progress post-harvest. Potato genotypes from the A07781 family exhibited relative resistance and/or tolerance to ZC because they showed low susceptibility to Lso and expression of ZC symptoms at harvest and after storage. Therefore, they were proposed as potential source of resistance to ZC. The fourth chapter was aimed at studying the effect of cold temperatures on potato psyllid mortality in relation to their Lso status and the vector haplotype. Lso may promote its vector’s ability to acclimate to cold temperatures but it was not influenced by the potato psyllid genotype (haplotype). We also evaluated the effect of Lso on potato psyllid respiration and we have found that Lso affects the respiration of potato psyllids negatively. Finally, Potato virus Y (PVY) and Zebra Chip (ZC) pathosystems can coexist in within fields, and they affect the potato industry productivity. Monitoring of potato fields indicates that potato psyllids arrive later in the growing season, when PVY may be already established. The objective was to evaluate the impact of PVY on the ZC pathosystem. Pre-existing PVY infection did not affect Lso development, but it negatively impacted Lso vectors by reducing their oviposition. Therefore, PVY may limit the spread of Lso by affecting its vector. Findings from the present dissertation contribute to our understanding of the ZC pathosystem, and represent the first reported study of its interaction with another pathosystem (PVY) in potato host.