Phenolic compounds, which may provide tolerance to Pierce's disease [caused by Xylella fastidiosa (Xf)], were examined in non-infected and Xf-infected Chardonnay, Merlot, and Rubired grapevines at three different times during the growing season. Xylem sap and tissue phenolic levels were substantially affected by phenology, with greatest levels occurring in August. Merlot grapevines generally possessed greater phenolic levels than Chardonnay or Rubired, and also had greater symptoms than Rubired. Infection status affected phenolic levels little or not at all. Therefore, the potential induction of phenolics in response to Xf infection did not affect phenolic levels as much as phenology or cultivar. Published by Elsevier Ltd.