The press of the recent resurgence of Pierce's Disease (PD) in North Coast grown grapevines has gotten some foothill grower's attention. But I have never seen a case of PD in grapes grown here. Why? We think that grapevines grown in the colder climate of the foothills benefit from a little understood phenomenon called "cold curing". What is this "cold curing"? The idea is that freezing temperatures cause vine stress that triggers an increase of some phenolic compounds in the xylem, or water conducting tissue. PD is a disease caused by the bacterium Xylella fastidiosa that resides in the xylem. But in cold-cured vines, we think the increased levels of xylem phenolics interfere with the growth of the Xylella bacterium such that disease 
Cold curing phenomenon has been studied by the late UC Davis plant pathologist Bruce Kirkpatrick, who contributed much to the science of plant diseases like PD. Bruce, along with Richard Bostock (UCD Plant Pathology) and Andy Waterhouse (UCD Vit and Enology) did work back in 2009 to try to identify the compounds in grape xylem sap responsible for cold curing. That work, funded by the American Vineyard Foundation, was continued in 2010 by Waterhouse, visiting scholar Blandine Cretin, and Mauri Anderson, who works in Andy's lab. I collaborated for a part of the study by locating El Dorado growers to provide five "cold" vineyard sites for comparison (same variety, clone and rootstock) with warmer climate sites in Yolo county that were also sampled. We sampled the canes of vines seasonally, and the Waterhouse lab analyzed the cane xylem sap for phenolic compounds. We looked at Cabernet Sauvignon, Barbera, Pinot Noir, Petite Sirah, Chardonnay and Merlot. The Waterhouse group focused on the 'flavonol' group of phenolics, and found that indeed, the vines grown in colder El Dorado county had 50-120% higher yearly concentration of flavonols than their warmer counterparts grown in Yolo, as reported in a poster presented at the American Society for Enology and Viticulture (ASEV) conference in 2012 (Anderson et al. 2012). Unfortunately, this work was not funded further, so the exact mechanism of cold-curing remains unknown. Interestingly, the current resurgence of PD in the North Coast region is thought to be associated with last season's mild winter temperatures-affecting either the vectors and/or the growth of Xylella.