Technical Abstract: Pierce’s disease of grapevine (PD), caused by the bacterial pathogen Xylella fastidiosa (X.f.), remains a serious problem for grape production in California and elsewhere. This research examined induction of phenolic compounds in grapevines (cv. Thompson Seedless) infected with X.f. over a six month period. Two months post-inoculation with X.f., catechin, digalloylquinic acid, and astringin were found at greater levels in xylem sap; multiple catechins, procyanidins, and stilbenoids were found at greater levels in xylem tissues; and precursors to lignin and condensed tannins were found at greater levels in xylem cell walls. However, such large-scale induction of phenolic compounds were not observed four months after inoculation. Six months after inoculation, infected plants had significantly reduced phenolic levels in xylem sap and tissues when compared to control plants, including lowered levels of lignin and condensed tannins. At six months, PD symptoms were severe in infected plants and most photosynthetic tissue was abscised. These results suggest that even though grapevine may initially respond to X.f. infection with increased production of phenolic compounds, ultimately Pierce’s disease causes grapevines to enter a state of decline whereby diseased hosts no longer support secondary metabolite production, including defense-associated phenolic compounds.