We are attempting to characterize the biochemical factors that mediate the cold curing phenomenon of grapevines infected with Xylella fastidiosa (Xf). We are working towards completing project objectives that were initiated in July of 2004. To better understand the cold therapy phenomenon, we examined Pierce’s Disease (PD) disease severity, curing rates and biochemical changes in control and Xf infected Pinot Noir and Cabernet Sauvignon grapevines grown in four locations in Northern California and four cold chamber temperatures. After the cold treatments, xylem sap was extracted using a pressure bomb and the sap analyzed for pH, osmolarity, abscissic acid (ABA), glucose, sucrose, fructose, calcium and magnesium ion concentrations. Differences between varieties and between temperature treatments were observed. In the field and cold chamber experiments, pH and osmolarity of xylem sap from cold treated vines was lower than what is found in PD3 culture medium used to grow Xf. PD severity was lowest and curing rates were highest for the coldest temperatures.