Technical Abstract: Zebra chip disease (ZC) is an emerging problem for the potato industry as it causes undesirable symptoms such as increased browning of freshly-cut tubers and brown-striping of fried tuber slices. ZC is putatively caused by infection of the bacterium ‘Candidatus Liberibacter solanacearum’ (CLso). This project examined association of pathogen-induced changes in host physiology, especially in phenolic, amino acid, and defense-associated protein levels, with symptoms. To this end, potato tubers were obtained from a field where ZC was present, visually assessed for ZC disease, and mashed for chemical analysis. High performance-liquid chromatography of methanol extractions of mashed tuber tissue found that tubers with more severe ZC symptoms possessed greater levels of phenolic compounds. A gas chromatography-based amino acid extraction/ derivativization determined that tubers with more severe ZC symptoms also possessed greater levels of five individual amino acids. Finally, colorimetric assays determined that symptomatic tubers possessed greater levels of peroxidase, polyphenol oxidase, chitinase, and ß-1,3-glucanase. These results are consistent with pathogen infection of a plant. Observed increases in phenolics, amino acids, and defense-associated enzymes were significantly correlated with symptom severity ratings, but uncorrelated with bacterial titers. These results verify the hypothesis that increased phenolic and polyphenol oxidase levels result in increased browning of freshly cut tubers, and that increased amino acid levels result in increased browning when tuber slices are fried.