Pierce's disease of grape—Xylella fastidiosa
Pierce's disease is caused by a bacterium that debilitates and kills grapevines. Different strains of the bacterium cause diseases in other hosts that include almond leaf scorch, mulberry leaf scorch, oleander leaf scorch, olive leaf scorch, and sweet gum dieback.
Identification
The initial symptom of Pierce's disease in spring commonly is yellowing between the veins on the first leaves produced on shoots. Later in the growing season these leaves appear normal. Once the disease is well established, during summer leaves on at least a portion of a vine become yellow, then reddish and brown beginning at leaf margins then spreading inward. During hot weather much of the foliage on an infected grapevine may turn brown without any red or yellow. Diseased vines flush leaves later than normal and the vines remain undersized in comparison with healthy grapevines. However as summarized below several other maladies cause similar symptoms on grape.
When Pierce's disease is the cause of unhealthy grapevines, four symptoms in mid- to late summer indicate its presence:
- Leaves become slightly yellow or red along margins in white and red varieties, respectively. Eventually leaf margins dry or die in concentric zones.
- Dried leaves drop prematurely, leaving the petiole (leaf stem) attached to the cane. The petioles gradually die back from the tip, a symptom called "matchsticks."
- Fruit clusters shrivel and berries become raisinlike before the normal time of harvest and beginning anytime after fruit set.
- Wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark, a symptom called "green islands."
All four symptoms are not necessarily present when Pierce's disease is present. When in doubt about the presence of Pierce's disease, samples of vines can be sent to a plant diagnostic laboratory to determine whether they are infected with X. fastidiosa.
Lookalikes. Symptoms of several other maladies of grape can be confused with Pierce's disease. Anything that weakens vines or slows vine growth in late summer or fall such as drought stress, an early frost, or salt toxicity can lead to delayed or stunted growth the following spring. However, symptoms of drought stress, frost, and salt toxicity tend to occur uniformly across large areas when there are groups of vines grown together and these maladies can affect other species of plants nearby. Pierce's disease initially begins in localized, scattered locations. Other causes of similar symptoms include:
- Armillaria root rot, or oak root fungus, which can cause foliage discoloration and wilting, along with drying of foliage and fruit. Removing some bark around the root crown generally reveals the presence of whitish mycelia mats of Armillaria when it is the cause of disease.
- Eutypa dieback that causes spring symptoms including stunted shoots and small, chlorotic, distorted leaves. Pruning wound cankers are commonly the initial location of Eutypa disease and cankers are not associated with Pierce's disease.
- Grape phylloxera, an aphidlike insect that feeds on roots and causes a decline of vines. Use a hand lens to examine roots and look for these oval, insects that are about 1/25 inch (1 mm) long and vary in color from brown, light brown, olive green, orange, and yellow to yellowish green.
- Phytophthora crown and root rot, which is a relatively uncommon disease of grape that causes leaves to discolor and drop prematurely. When Phytophthora is the cause, removing some bark at and just below the root crown reveals darkly discolored cambium and wood. Small roots are sparse and at least some of the rootlets are dark and decayed.
Life cycle
Pierce's disease is caused by the xylem-inhabiting bacterium X. fastidiosa. There are numerous strains of the pathogen that can occur in numerous host plants, but only certain strains cause disease in certain hosts. The bacterium is spread by leafhoppers (family Cicadellidae) known as sharpshooters (subfamily Cicadellinae). Some closely related insects such certain spittlebugs (family Cercopidae) can also spread the pathogen.
Important vectors of X. fastidiosa in California's Central Valley are the green sharpshooter, Draeculacephala minerva, and the red-headed sharpshooter, Carneocephala fulgida. In coastal regions such as the Napa Valley the blue-green sharpshooter, Graphocephala atropunctata, is the most common vector. In Southern California and southern parts of the San Joaquin Valley the glassy-winged sharpshooter, Homalodisca vitripennis, is the most important vector; it spreads Pierce's disease more effectively than the other sharpshooters.
Sharpshooters pick up the bacterium by feeding on infected hosts and spread it when they move to feed on healthy grapevines. Once injected into plants with the salivary secretions of the sharpshooter, the bacterium moves quickly upward and slowly downward in the plant's xylem system. Bacterial aggregates develop and plug the xylem vessels, inhibiting and eventually stopping the flow of water and dissolved nutrients upward from the roots.
Damage
The extent of disease development and death of grapevines varies greatly according to many factors. These include the abundance, transmissions efficacy, and species of the vectoring insects, environmental conditions, and grape cultivars differing susceptibility to the pathogen. Sometimes the pathogen is naturally eliminated from a host for reasons that are unclear but apparently can include cold winters. Grapevines often die within 1 or several years of developing Pierce's disease.
Solutions
Prevention is the only effective management strategy. There are no known actions that can eliminate the pathogen from vines infected in the field. Obtain planting stock from a reliable commercial supplier that provides disease-free plants. Consider planting grape varieties less susceptible to Pierce's disease, such as Chenin Blanc, Ruby Cabernet, Sylvaner, and Thompson Seedless.
When growing multiple grapevines with some having developed Pierce's disease, remove the diseased vines to reduce the likelihood of them becoming a source of the pathogen that can spread to nearby grapevines. Replant with less susceptible cultivars.
Reducing the number of insect vectors that can move to grapevines is an important strategy. This can include applying an insecticide such as the systemic imidacloprid to overwintering, evergreen hosts of the sharpshooters such as citrus and oleander. Applying insecticides to grapevines is not an effective control. A sharpshooter that migrates in may be able to cause infection of the vine before it is killed by the insecticide.
Because certain plants including bermudagrass, blackberry, and willow are alternative hosts of the grape-infecting strain of X. fastidiosa and its insect vectors, removing the alternative hosts near grapevines if feasible helps reduce the incidence of Pierce's disease. Note that where host plants occur in riparian areas there are regulations that limit vegetation removal. Before managing vegetation along waterways, contact the California Department of Fish and Wildlife to learn of any rules that may apply to your situation.
For more information, see the Pest Notes: Glassy-Winged Sharpshooter and Oleander Leaf Scorch. Adapted from the publications above, Grape Pest Management Third Edition from UC Agriculture and Natural Resources also available as an eBook, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
First leaves on a shoot (bottom) that have interveinal chlorosis, a spring symptom of Pierce's disease.
Chlorosis and necrosis of a leaf that begins in the outer margin due to Pierce's disease.
Severe chlorosis and necrosis of a leaf late in summer due to Pierce's disease.
Symptoms of Pierce's disease include irregular patches of green bark and mature brown bark, grapes that have dried up before harvest time, and leaves that have dropped prematurely but left behind leaf petioles that are dying back from the tip.
Adults of glassy-winged sharpshooter (below) and blue-green sharpshooter, both vectors of Xylella fastidiosa.
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