Damon Smith, Oklahoma State University
Pierce’s disease was first described in California in 1892. Since that time, the disease has been identified in vineyards bordering the Gulf Coast and the Mid-Atlantic, including Virginia and North Carolina, and also the southern Midwest as far north as Oklahoma and Missouri.
Symptoms of Pierce’s disease. Figure 1. Wilting and premature defoliation. Figure 2. Dry, brown leaf scorch. Note the yellow border between green and brown tissue. Figure 3. ‘Matchsticks’ created where the leaves have dropped but petioles remain attached. Figure 4. Late August symptoms, again with yellow border between green and brown tissue. Photos by Damon Smith, Oklahoma State University.
The symptoms of Pierce’s disease are perennial and will appear late in the summer when weather conditions are predominantly hot and dry, or when plants are under drought stress. Plants typically exhibit shoot wilting and premature defoliation (Figure 1). Plants may produce no fruit, or have limited fruit production with poor quality. Fruit may shrivel prior to normal harvest period. Chlorosis and green fading colors will develop at leaf margins, which dry and turn brown (Figure 2). Some vines will have a ‘matchstick’ symptom where the leaves have dropped from the plant, but petioles remain attached (Figure 3). Marginal browning can take on an undulating appearance as it moves toward the veins of the leaf. A yellow to red-brown band may be present between the green and scorched areas of the leaves (Figures 2 and 4). Leaf symptoms of Pierce’s disease can look very similar to drought stress symptoms; however, the yellow or red-brown band between green and scorched areas will be absent in vines suffering from drought stress.
The disease is caused by the bacterium Xylella fastidiosa. Xyllela fastidiosa is a gram-negative bacterium that survives and multiplies in the water-conducting elements (xylem) of grapevines. The bacterium has a broad host range with more than 100 known plant hosts. However, strains of X. fastidiosa show specialization for certain hosts. Therefore, only specific strains of the bacterium are capable of infecting grapevines.
Cultural Management Options
Traditional dogma has suggested that locations that experienced freeze events were not conducive for the survival of the pathogen. Recent research in North Carolina and Georgia examined the low-temperature thresholds as they related to risk of Pierce’s disease in grapevines. From those multi-year/multi-site studies they concluded that winter temperatures at or below 10ºF for two to three days were required for a vineyard to be considered at low-risk for Pierce’s disease. Otherwise, vineyards were considered at medium- to high-risk (Anas et al., 2008).
If Pierce’s disease is identified in a vineyard, affected vines should be removed and destroyed to limit spread of the pathogen to healthy plants. Xylella fastidiosa is typically transmitted from plant to plant by an insect vector (Purcell and Hopkins, 1996). Vectors include spittle bugs, leafhoppers, cicadas, and tube-building spittlebugs. Transmission of the pathogen can occur mechanically, although the risk of this happening is considered low. However, recent studies have shown that inoculation of trees by needle injection is possible. The risk of spreading the bacterium through pruning practices is also considered low during the winter months, when major dormant pruning practices are occurring. The risk is much higher when pruning practices occur during periods of active vine growth. Regardless of the timing of pruning, it is recommended that pruning equipment be sanitized after working on each vine. This will help reduce the risk of mechanical transmission from infected vines to healthy vines. A 10% solution of household bleach is sufficient for killing the bacterium. Maintain a set of pruners in a bucket containing the bleach solution while you use another set of pruners. The set in the bucket will be “sanitizing” while you work. When you move to the next vine, simply switch to the set of pruners that have been sanitized. This will not only reduce the risk of spreading X. fastidiosa, but also reduce the risk of transmitting other types of pathogens that can cause disease in grapevines.
The bacterium can also be transmitted through use of infected propagation material taken from infected grape vines (Robacker and Chang, 1992). Spread of the disease through the use of contaminated propagation material presents many challenges for growers in areas where Pierce’s disease is a problem. Many rely on propagating their own plants from cuttings; however, it is suggested that this practice be avoided in order to reduce the risk of spreading X. fastidiosa. If you insist on propagating your own planting stock, mother plants should be tested prior to propagation to ensure that the plants are free of X. fastidiosa. Growers should purchase plants from reliable grapevine nurseries and resist the temptation to root cuttings received from friends and neighbors. By purchasing clean stock from a reliable source, the likelihood of introducing X. fastidiosa into the vineyard is greatly reduced.
Grape varieties vary in their response to Pierce’s disease. It is generally accepted that European grape varieties (Vitis vinifera) will be more susceptible or intolerant to Pierce’s disease. The bacterium that causes Pierce’s disease is native to North America where there are native grape species. Therefore, grape cultivars that are not native to North America will typically have little or no resistance or tolerance to the disease. Some American cultivars, or cultivars with large portions of their genetic background comprised of native American species, will typically be more resistant or tolerant to Pierce’s disease. For example, in the Gulf Coast region of Texas, ‘Black Spanish’ (syn. ‘Lenoir’) is considered tolerant, meaning vines can be infected with the bacterium but remain symptomless or only slightly affected. Cultivars of Vitis vinifera origin such as ‘Chardonnay’ and ‘Viognier’ are highly susceptible to Pierce’s disease in Texas (David Appel, Texas A&M University, personal communication). Growers who have removed vines that have succumbed to Pierce’s disease should consider replanting with resistant or tolerant cultivars.
Growers should also focus on a sound weed control program in the vineyard. There is circumstantial evidence in Texas that wide (100 ft or more) “buffer zones” of weed-free areas surrounding the vineyards substantially reduce the likelihood of initial infections within the vineyard (David Appel, Texas A&M University, personal communication). Also, maintaining herbicide strips around vines within the vineyard and using a cover crop such as cool-season grasses (e.g., annual ryegrass) between rows may help to reduce spread of the pathogen via insect vectors.
In vineyards diagnosed with Pierce’s disease, a systemic insecticide may be necessary to reduce secondary spread of the pathogen between vines. Several systemic products are available to control insect pests in vineyards. Be sure to check with your local cooperative extension office for approved products for your area.
Chemical Management Options
No chemical management options currently exist for Pierce’s disease.
Anas, O., U.J. Harrison, P.M. Brannen, and T.B. Sutton. 2008. The effect of warming winter temperatures on the severity of Pierce’s disease in the Appalachian mountains and piedmont of the southeastern United States. Plant Health Progress doi:10.1094/PHP-2008-0718-01-RS.
Purcell, A. H. and D. L. Hopkins. 1996. Fastidious xylem-limited bacterial plant pathogens. Annual Review of Phytopathology 34:131-151.
Robacker, C. D. and C. J. Chang. 1992. Shoot-tip culture of muscadine grape to eliminate Pierce’s disease bacteria. Horticultural Science 27:449-450.
Growing Pierce’s Disease Resistant Grapes in Central, South, and East Texas, Texas A&M University
Pierce’s Disease: Overview and Management, Texas A&M University
French-American Hybrid Grapes
Reviewed by Stephen Jordan, University of Wisconsin-Madison and Ed Hellman, Texas AgriLife Extension