Xylella fastidiosa is the causal agent of Pierce's disease in grapevines. The mechanisms of pathogenicity are largely due to occlusion of xylem vessels by aggregation of X. fastidiosa and biofilm formation. Xylella fastidiosa was subjected to xylem fluids with varying chemistries to examine the effects of nutritional components on bacterial growth in vitro. The exposure of X. fastidiosa to xylem fluids collected from different Vitis genotypes resulted in highly significant differences in both planktonic growth and biofilm formation. Planktonic growth of X. fastidiosa in Vitis xylem fluid was correlated to the concentration of citric acid, amino acids (glutamic acid, glutamine, histidine, valine, methionine, isoleucine and phenylalanine) and inorganic ions (copper, magnesium, phosphorus and zinc). Biofilm formation was correlated to many amino acids at 1 h of incubation. Xylem fluid from Vitis rotundifolia cv. Noble (fluid that supported low planktonic growth) was supplemented with the compounds that were correlated above to levels found in Vitis champinii cv. Ramsey (fluid that supported high planktonic growth) to determine the direct impact of xylem constituents on the growth characteristics of X. fastidiosa. Augmentation of fluid from Noble with the amino acids listed above, citric acid, calcium and magnesium resulted in increased planktonic growth and aggregation.