How to Manage Pests
UC Pest Management Guidelines
In vines that are infected in spring, symptoms of Pierce's disease first appear as water stress in midsummer, caused by blockage of the water-conducting system by the bacteria. The occurrence of the following four symptoms in mid- to late summer indicates the presence of Pierce's disease: (1) leaves become slightly yellow or red along margins in white and red varieties, respectively, and eventually leaf margins dry or die in concentric zones; (2) fruit clusters shrivel or raisin; (3) dried leaves fall leaving the petiole (leaf stem) attached to the cane; and (4) wood on new canes matures irregularly, producing patches of green, surrounded by mature brown bark. Delayed and stunted shoot growth occurs in spring following infection even in vines that did not have obvious symptoms the preceding year.
Leaf symptoms vary among grape varieties. Pinot Noir and Cabernet Sauvignon have highly regular zones of progressive marginal discoloration and drying on blades. In Thompson Seedless, Sylvaner, and Chenin Blanc, the discoloration and scorching may occur in sectors of the leaf rather than along the margins.
Usually only one or two canes will show Pierce's disease symptoms late in the first season of infection, and these may be difficult to notice. Symptoms gradually spread along the cane from the point of infection out towards the end and more slowly towards the base. By mid-season some or all fruit clusters on the infected cane of susceptible varieties may wilt and dry. Tips of canes may die back; roots may also die back. Vines of susceptible varieties deteriorate rapidly after appearance of symptoms. Shoot growth of infected plants becomes progressively weaker as symptoms become more pronounced.
Climatic differences between regions can affect the timing and severity of symptoms, but not the type of symptoms. Hot climates accelerate symptoms because moisture stress is more severe even with adequate soil moisture.
A year after the vines are infected some canes or spurs may fail to bud out, and shoot growth is stunted. New leaves become chlorotic (yellow) between leaf veins, and scorching appears on older leaves. From late April through summer infected vines may grow at a normal rate, but the total new growth is less than that of healthy vines. In late summer leaf burning symptoms reappear.
The bacterium that causes Pierce's disease lives in the water-conducting system of plants (the xylem) and is spread from plant to plant by sap-feeding insects that feed on the xylem. Symptoms appear when a significant amount of xylem becomes blocked by the growth of the bacteria. (This bacterium is also responsible for alfalfa dwarf disease and almond leaf scorch in California.) Insect vectors for Pierce's disease belong to the sharpshooter (Cicadellidae) and spittlebug (Cercopidae) families. The blue-green sharpshooter (Graphocephala atropunctata) is the most important vector in coastal areas. The green sharpshooter (Draeculacephala minerva) and the red-headed sharpshooter (Carneocephala fulgida) are also present in coastal areas but are more important as vectors of this disease in the Central Valley. Other sucking insects, such as grape leafhoppers, are not vectors.
A new Pierce's disease vector, the glassy-winged sharpshooter, has recently become established in California. This vector is a serious threat to California vineyards because it moves faster and flies greater distances into vineyards than the other species of sharpshooters. The glassy-winged sharpshooter occurs in unusually high numbers in citrus and avocado groves and on some woody ornamentals. Until now, these plants have not been sources of Pierce's disease vectors.
Since the early 1990s, the glassy-winged sharpshooter has been seen in high numbers in citrus along the coast of southern California. It subsequently has become locally abundant further inland in Riverside and San Diego counties. In 1998 and 1999, high populations on citrus and adjacent vineyards were seen in southern Kern County and in 2001, hundreds of vines had Pierce's disease. The glassy-winged sharpshooter is expected to spread north and eventually become a permanent resident of various habitats throughout northern California.
Glassy-winged sharpshooter feeds and reproduces on a wide variety of trees, woody ornamentals, and annuals in its region of origin, the southeastern United States. Crepe myrtle and sumac are especially preferred. It reproduces on Eucalyptus, coast live oaks, and a wide range of trees in southern California. But because glassy-winged sharpshooter is a relatively new arrival to the state, it is not clear yet which regions and habitats it will become permanently established in.
The principal breeding habitat for the blue-green sharpshooter is riparian (riverbank) vegetation, although ornamental landscape plants may also harbor breeding populations. As the season progresses, these insects shift their feeding preference, always preferring to feed on plants with succulent growth. In the Central Valley, irrigated pastures, hay fields, or grasses on ditch backs are the principal breeding and feeding habitats for the green and red-headed sharpshooters. These two grass-feeding sharpshooters also occur along ditches, streams, or roadsides where grasses and sedges provide suitable breeding habitat.
Some vines recover from Pierce's disease the first winter following infection. The probability of recovery depends on the date of infection. Infections that occur until June have the greatest probability of surviving until the following year. Recovery rates also depend on grape variety; recovery is higher in Chenin Blanc, Sylvaner, Ruby Cabernet, and White Riesling, compared to Barbera, Chardonnay, Mission, Fiesta, and Pinot Noir. Thompson Seedless, Cabernet Sauvignon, Gray Riesling, Merlot, Napa Gamay, Petite Sirah, and Sauvignon Blanc are intermediate in their susceptibility to this disease and in their probability of recovery. In tolerant cultivars the bacteria spread more slowly within the plant than in more susceptible cultivars. Once the vine has been infected for over a year (i.e., bacteria survive the first winter) recovery is much less likely.
Young vines are more susceptible than mature vines. Rootstock species and hybrids vary greatly in susceptibility. Many rootstock species are resistant to Pierce's disease, but the rootstock does not confer resistance to susceptible Vinifera varieties grafted on to it. Finally, the date of infection strongly influences the likelihood of recovery: late infections (after June) by blue-green sharpshooters, green sharpshooters, and red-headed sharpshooters are least likely to persist the following growing season. This may not be the case with glassy-winged sharpshooter, however, because it feeds on leaves near the base of the cane, as well as on 2-year old dormant wood.
Insecticide treatments aimed at controlling the vector in areas adjacent to the vineyard have reduced the incidence of Pierce's disease by reducing the numbers of sharpshooters immigrating into the vineyards in early spring. The degree of control, however, is not effective for very susceptible varieties such as Chardonnay and Pinot Noir or for vines less than 3 years old. If a vineyard is near an area with a history of Pierce's disease, plant varieties that are less susceptible to this disease. Monitor and treat for insect vectors as described in the section on SHARPSHOOTERS.
During the dormant season, remove vines that have had Pierce's symptoms for more than one year; they may be chronically infected and are unlikely to recover or continue to produce a significant crop. Also, remove vines with extensive foliar symptoms on most canes and with tip dieback of canes even if it is the first year that symptoms have been evident. From summer through harvest, mark slightly symptomatic vines; reexamine for symptoms the following spring through late summer or fall and remove vines that have symptoms for a second year. Pruning a few inches above the graft union of vines with moderate foliar symptoms (some canes on entire cordons without symptoms or no symptoms at the bases of most canes) may eliminate Pierce's disease and allow vigorous regrowth the following year, but symptoms will reappear in many (30–40%) or most of these severely pruned vines the second year.
For table grapes, examine vines for poor budbreak in spring. Later in the season, look for pests and damage.
Because the glassy-winged sharpshooter feeds much lower on the cane than other sharpshooters in California, late-season (after May-June) infections and infections occurring during dormancy made by the glassy-winged sharpshooter can survive the winter to cause chronic Pierce's disease. This enables vine-to-vine spread of Pierce's disease, which has not been the case in California. Vine-to-vine spread can be expected to increase the incidence of Pierce's disease exponentially rather than linearly over time, as has been normal for California vineyards affected by Pierce's disease. Insecticide treatments of adjacent breeding habitats, such as citrus groves, has been the most effective approach.
Removing diseased vines as soon as possible when Pierce's disease first appears in a vineyard is also critical to help reduce the infection rate. Early and vigilant disease detection and vine removal is recommended for any vineyards that experience influxes of the glassy-winged sharpshooter.
Long-term studies are being conducted on the effect of riparian vegetation management in reducing disease incidence and severity in North Coast vineyards. Riparian vegetation management has proven to be effective in reducing the damaging spring populations of blue-green sharpshooters. Because these areas are ecologically sensitive and regulated by federal, state, and local legislation, the unauthorized removal of vegetation is prohibited or restricted. Vegetation management of these areas must be acceptable or beneficial for wildlife and water quality and maintain the integrity of the riparian habitat. For additional information, contact the California Department of Fish and Game for current regulations and guidelines. For more information, see the complete Riparian Vegetation Management for Pierce's Disease in North Coast California Vineyards.
UC IPM Pest Management Guidelines: Grape