Foliar and drench applications of the plant growth regulator abscisic acid (ABA) were applied to 1-year-old potted Vitis vinifera 'Pinot Noir' and 'Cabernet Sauvignon' vines infected with Xylella fastidiosa, the bacterial pathogen that causes Pierce's disease (PD). A naturally occurring ABA and a synthetic ABA were applied, and both materials showed some effectiveness at increasing curing rates of PD-affected grapevines. Pinot Noir grapevines treated with the drench ABA treatments had significantly greater disease curing effects than the unsprayed control plants. It has been shown that plant phenolics have antimicrobial properties, and we found a positive correlation between effective ABA treatments and the total phenolic compound content of xylem sap extracted from Pinot Noir vines.