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Pierce’s disease (PD) caused by Xylella fastidiosa (Xf) is a major threat to the rapidly growing Vitis vinifera/French-American hybrid winegrape industry in the southeastern United States. The bacterium, which is transmitted by xylemfeeding insects, is unable to survive low winter temperatures, and infected vines often recover the next year. Maps available from the 1970s indicate that PD is not a serious threat in the southeastern US, where the average minimum January temperature is ≤ 1.7°C. However PD symptoms developed in many vineyards planted in the late 1990s in areas identified as low risk. Surveys conducted in North Carolina and Georgia confirmed the presence of Xf in symptomatic vines using ELISA kits. Weather data for November to March from 84 weather stations from 1972-2005 for Alabama, Georgia, North Carolina, South Carolina, Tennessee, and Virginia were used to construct new maps using ArcGIS 9.1, and PD survey data from vineyards were superimposed on the maps. Areas for low risk for PD corresponded most closely with a minimum winter temperature of ≤ 12.2°C for 2 to 3 days or ≤ -9.4°C for 4 to 5 days. Warm winter temperatures during the last 8 years have resulted in a significant shift in the isotherms towards the north and west, increasing the risk of PD in the Piedmont region of the Southeast, and have extended the threat into Virginia and Tennessee. Introduction The Vitis vinifera/French American hybrid winegrape industry has experienced rapid growth in North Carolina as well as other southeastern states over the past 10 years, but further expansion of the industry may be limited by Pierce’s disease (PD). PD is caused by Xylella fastidiosa Wells et al. (Xf) (19), a gram-negative fastidious xylem inhabiting bacterium which is spread from plant-to-plant by xylem feeding insects with piercing mouth parts such as sharpshooters, leafhoppers, and spittle-bugs (15). Xf is geographically limited to areas with mild winters where PD can be the principal limiting factor in the production of vinifera/French American hybrids and Vitis labrusca (10). In areas with cold winters and freezing temperatures, infected vines have been known to recover the following spring (7,16), because populations of Xf within the grape xylem tissue decline under cold conditions (6). This "cold curing" is related to biochemical changes in vines exposed to cold temperatures (12). In North Carolina, the disease is well documented in the milder Coastal Plain region on V. labrusca and V. rotundifolia (14). Currently available maps [(9), UC Berkeley X. fastidiosa website, www.cnr.berkeley.edu/ xylella/overview/page2.html] based primarily on anecdotal information, indicate that the disease is more sporadic or not a serious threat in the cooler Piedmont region. The map on the UC Berkley Xylella fastidiosa website defines areas in North Carolina, South Carolina, Georgia, and Alabama, where the disease is rare, by an isotherm based on an average minimum January temperature of ≤ 1.7°C (Fig. 1). Most of the expansion of the industry in the Southeast has been in the Piedmont and Mountain regions of the states inside this isotherm or "blue" zone (Fig. 1). However, soon after vineyards were planted in this zone, vines symptomatic of Pierce’s disease were observed in some 18 July 2008 Plant Health Progress vineyards. Because of the high cost of establishing a vineyard, it is essential to warn growers not to plant in areas where the likelihood of PD is great. Thus the goal of this research was: (i) to conduct surveys to confirm the presence of PD in newly planted vineyards in the Piedmont and Mountain regions of North Carolina and Georgia; (ii) to relate the presence of PD to winter temperatures within the regions; and (iii) to identify and map low, moderate, and high-risk areas for PD in the Southeast. These maps should be useful to growers as a guide for making planting decisions in the southeastern United States; and may also help to explain the spread of the disease into areas which had been believed to be disease free. Fig. 1. Map of United States showing probable occurrence of Pierce’s disease. From UC Berkeley X. fastidiosa website. Surveys for Pierce’s Disease In late summer of 2001, 22 vineyards of V. vinifera and French-American hybrid grapes were selected arbitrarily within the winegrape growing region of the Piedmont and Mountains of North Carolina. Vines were scouted in each vineyard for three characteristic symptoms of PD: leaf scorch showing marginal chlorosis and necrosis; leafless petioles (match-sticks); and "green islands" on the canes (Figs. 2 A, B, and C). Symptomatic leaves or leafless petioles which exhibited the typical "match-stick" symptom for PD were collected from symptomatic vines. One to four samples were tested for Xf according to procedures described for Agdia DAS ELISA test kits (Agdia Incorporated, Elkhart, IN) (Table 1). In 2002, 14 additional vineyards were scouted and symptomatic vines tested for Xf as described above. One additional vineyard was sampled in both 2003 and 2004 and is included in the survey results (Table 1). Between 2000 and 2006, eight vineyards where symptomatic vines were observed were surveyed in north Georgia. Samples were tested for Xf as described above and were confirmed through isolation on PW medium. Vineyards which tested positive for Xf had a range of symptoms from one or two leaves showing marginal chlorosis to complete vines showing "match-stick" symptoms. 18 July 2008 Plant Health Progress Fig. 2. Pierce’s disease symptoms on grape vines showing (A) marginal chlorosis and necrosis, (B) leafless petioles "match-stick" symptoms, and (C) "green island" symptoms. A B

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The Effect of Warming Winter Temperatures on the Severity of Pierce's Disease in the Appalachian Mountains and Piedmont of the Southeastern United States