Technical Abstract: Both xylem sap and stem tissue are key components in the Xylella fastidiosa-grapevine interaction. In this research we investigate protein expression in xylem sap and stem tissue of highly tolerant and susceptible grape genotypes. Ten sequential samplings of stem tissues have been conducted from infected and noninfected grapevines ranging from 1 day to 12 weeks post inoculation. Protein extraction and analyses on these tissues has recently been initiated. Plants for xylem sap extraction are currently being grown in the greenhouse. Xylem sap will be extracted multiple times post inoculation from Xf and water-inoculated plants. Differentially expressed proteins in both stem tissue and xylem sap will be identified and investigated in more detail in the coming months. Results obtained will deepen our understanding of host-pathogen interactions, a key component in fighting PD.