The single greatest threat to the long-term survivability of susceptible cultivars is Pierce’s disease. Pierce’s disease (PD) is caused by a xylem-limited bacterium that clogs the vascular tissue of susceptible grape cultivars. The causal organism is a gram-negative, rod-shaped bacterium named Xylella fastidiosa that is indigenous to the Gulf Coast region of the United States. Although different races of this organism cause similar diseases in other crops, they appear to be host specific, i.e., the grape strain does not appear to infect peach and the peach strain does not appear to cause symptoms in grape. Grapevines become infected when a sharpshooter that carries the bacterium feeds on tender tissue. These insect vectors are very efficient at transferring the bacterium during feeding and infection is likely.
Once a grapevine is infected, the bacteria multiply and colonize the xylem, or water conducting tissue of the plant. This vascular constriction inhibits the movement of water through the grapevine and often results in first visible symptoms noted during periods of heat or drought stress.
Electron micrographs of Xylella fastidiosa in xylem vessels of grapevine
It is important to distinguish between two groups of grapevines: susceptible cultivars and tolerant cultivars. Once a susceptible cultivar is infected, there is no known, approved method of treating the infection and the disease will most probably be fatal to the vine. Cultivars vary in the length of time it takes the pathogen to cause vine death. Tolerant cultivars appear to have internal mechanisms of suppressing bacterial numbers to the point that the vine can live and be productive even in the presence of the bacteria. There is preliminary evidence that some non-susceptible cultivars may in fact be resistant to infection. All native Texas species of Vitis are believed to be tolerant of PD, which potentially makes them carriers of the bacterium. As a consequence, removal of adjacent, wild grapevines is imperative to disease management.
Pierce’s disease infection is dependent upon the presence of a susceptible host, a source of the bacteria, and an insect vector to inoculate the susceptible host. In addition to native grapevines, there are other indigenous plant species that harbor the bacteria without visual symptoms. Surveys in California have identified several alternate host, but in our area, where the disease is endemic, there are undoubtedly many more plant species capable of supporting the causal agent.
Although a serious problem to commercial grape growers on the West Coast, PD is not native to California, but was probably
introduced from the Gulf Coast through infected grapevines. Three or four species of sharpshooters are believed to be the most important vectors in those areas. It is likely that there are numerous species of sharpshooters that can potentially transmit the bacteria in Texas. Work continues to identify these insects, determine their preferred habitat, and understand population dynamics. Sharpshooters do prefer certain habitats. Bermudagrass, perennial rye, fescue grass, blackberry, willow, and elderberry provide important food sources or egg-laying sites for some sharpshooters. Sources of water are also essential to supporting sharpshooter populations, so choosing sites away from these rivers, creeks or ponds can aid in insect management.
Images of several Sharpshooters know to transmit PD.
Photographs by Jim Medley, Beaumont Agricultural Research and Extension Center.
Most, if not all, sharpshooter species go through five larval, or instar stages in which they apparently loose the ability to transmit the disease with each molt. In areas of rampant infection, it is assumed that alternate sources of the bacterium are widely available. Keeping vineyards and adjacent areas free of potential alternate hosts is essential for long-term management of Pierce’s disease. Monitoring insect populations, especially after habitat disturbance such as cutting of adjacent hay fields, can greatly assist growers in the judicious use of insecticides.
There are numerous symptoms expressed by susceptible cultivars after infection. The first symptom is usually uneven marginal leaf necrosis that often appears near the point of infection. Since the disease inhibits water movement in the vine, symptoms often appear during heat stress or near veraison (color change) in the cluster.
Leaf scorch caused by PD
The clusters of heavily infected vines may actually collapse during this time of high water and carbohydrate movement.
Cluster collapse at veraison caused by PD
Another diagnostic symptom of PD is the abscission of leaf blades from shoots with retention of leaf petioles. In addition, as winter approaches, new shoots become woody and develop periderm on one-year-old shoots. This periderm formation usually begins at the basal portion of a shoot and progresses toward the growing tip. In infected grapevines, periderm formation is not uniform, usually resulting in green “islands” at the nodal area while the internodal portion of the stem becomes brown.
Irregular patches on infected stem tissue.
While each of these symptoms can be confused with one or more other non-related factors, the occurrence of several symptoms together provides strong suspicion of infection in a susceptible host.
Pierce’s disease Probability in Texas
In the mid 1970’s, Dr. Ron Perry published a bulletin entitled A Feasibility Study for Grape Production in Texas which included the following figure detailing the expected presence of Pierce’s disease in Texas.
Expected Probability of Pierce’s Disease in Texas
At that time, it was postulated that the range of the disease was limited by the natural range of insect vectors. Cold temperatures have been shown to be therapeutic to plants infected with Xylella, but exact duration and absolute temperatures have not been identified. It is now believed that P.D. is limited to areas which do not receive severe winter temperatures. After a series of warm winters in Texas, outbreaks of PD were confirmed in vineyards previously thought to be in low probability areas.
The Texas Hill Country, long thought to be a transition zone between high and low probability for PD, experienced several warm winters in the mid 1990’s after which several vineyards were found to be completely infected. Prospective growers should realize that this disease is cyclic and that infections are likely to occur. Vineyard survival will ultimately depend on site selection, cultural practices that reduce the risk of widespread economic loss, weather and to some degree, luck.
1996 Positive Elisa Tests Results for Pierce’s Disease
Conventionally, an” ELISA”, or antiseral reaction test is used to confirm suspected cases of Pierce’s disease in Texas. Problems encountered a few years ago were due to defective antiserum distributed by the manufacturer. This procedure is still recommended for growers wishing to confirm Pierce’s disease infection in grapevines.
For research purposes, polymerase chain reaction technology (PCR) is used detect the presence of the causal bacteria. This test, which is approximately 10,000 times more sensitive that the ELISA test will be helpful in confirming suspected insect vectors and alternate hosts of the bacteria.
MANAGEMENT OF PIERCE’S DISEASE OF GRAPE
Because there is no known control for Pierce’s disease, the act of planting susceptible cultivars in areas where P.D. is known to exist assumes an inherent risk.
Remove Wild Grapevines
In Texas, wild hosts of the grape pathogen have not been identified. In other states, grape strains of Xylella fastidiosa have been isolated from wild grape, ragweed, alfalfa and almond trees. As a precaution, it is recommended that wild grapes be removed from around the vineyard
Remove Diseased Vines
Based on foliage and cane symptoms confirmed by laboratory diagnosis, diseased plants should be immediately destroyed. Regardless of varietal tolerance, any vine with symptoms of this disease should be pulled up or cut off at the ground and removed from the vineyard. Since observations indicate that the disease can spread from vine to vine within the vineyard, removal of diseased vines reduces the potential sources of inoculum that could be transmitted by insect vectors.
The disease is vectored by certain kinds of xylem-feeding insects, mainly the leafhopper group known as sharpshooters. All of the insect species responsible for vectoring PD in Texas are not known at this time. There are species of leafhoppers that inhabit Texas vineyards and adjacent wild hosts that look like sharpshooters that are not known to vector P.D. Sharpshooters tend to be significantly larger than other species of leafhoppers found in and adjacent to vineyards.
The difficulty of vector management as a means to manage P.D. is the inability to identify all potential vectors within and adjacent to the vineyard, so chemical control of vectors is tenuous at best. Nonetheless, the current thinking in California is that vector transmission occurs primarily from host plants adjacent to the vineyard, so California growers practice vector control in areas adjacent to the vineyard. Growers should use caution when choosing insecticides to insure that specific pesticide labels permit such use.
The pattern of PD spread in Texas more closely parallels that observed in Florida where significant vine-to-vine spread of the disease occurs. This would indicate that insecticidal control of vectors within the vineyard may also be needed..
Based on the best information available, the following vector control recommendations are suggested:
Establish and maintain a 150 foot buffer (minimum) around the vineyard through mechanical or chemical mowing or cultivation.
The California experience would indicate that the greatest danger from transmission of PD through sharpshooter vectors is shortly after budbreak and decreases as the season progresses. Starting at budbreak and continuing for 6 weeks, sample the vegetation in the area outside and adjacent to the buffer, or in the absence of a buffer, sample the vegetation adjacent to the vineyard.
Sampling consists of using a standard sweep net and taking a minimum of eight 25-sweep samples at least twice a week. If adult sharpshooter numbers exceed an average of 1 per 25-sweep sample, insecticidal treatment may be justified.
Treat a 65-foot band adjacent to the buffer or a 130-foot band adjacent to the vineyard in the absence of a mowed buffer. If it is not possible to treat adjacent vegetation, it might be appropriate to treat the vineyard itself. The problem with this approach is that if the alternate host reservoir for the sharpshooter vectors is large and the buffer is small or absent, then within vineyard treatments may be ineffectual in keeping sharpshooters out. Twice a week spraying for 4 to 6 weeks following budbreak may be necessary, but only if sweep samples indicate that a threshold population has been reached.
Care should be exercised in judiciously using insecticides. Unfortunately, the greater the number of sprays, the more likely secondary pest outbreaks will be created, especially with spider mites.
Use an insecticide registered for use for the target area. In most cases (and for all sites external to the vineyard), sharpshooters are not listed as a target pest on the label. Specific use restrictions for grapes and alternate hosts will be found on the label.
Vineyard Floor Management
Because there is limited information as to other species may serve as a source of the P.D. organism, many growers are utilizing clean cultivation to eliminate any possible inoculum source within the vineyard. Weed growth under the trellis can be controlled with cultivation, or herbicides, but management of the vineyard floor between the rows has become problematic. Clean cultivation can have serious drawbacks such as the potential for serious soil loss due to erosion. The use of cover crops in vineyard row centers has several advantages over cultivation including increased equipment mobility, the preservation of soil structure within the vineyard and erosion control.
Because at this point we do not know what plant species constitute propagative alternate hosts of Pierce’s disease, the decision on what plant species growers should plant or encourage on the vineyard floor is still only a guess. In light of these considerations, it may be wise to plant (drill or no-till seed) cool season, annual cover crops such as annual rye grass or oats in October and encourage cover crop growth during the months that grapevines are dormant. These annual plants have a low probability of contracting the causal bacterium and would be growing during a period when transmission to grapevines is not believed to occur.
Cover crop height can be managed by mowing and is easily controlled during the spring with low rate glyphosate applications. This practice keeps cover crop roots in place to support equipment traffic, helps reduce erosion and establishes an organic material layer that inhibits the germination of indigenous weed species. When annual rye grass is used for this purpose, additional suppression of weed seed germination may be observed due to the allelopathic properties of rye. Additional applications of glyphosate or glufosinate can be used throughout the growing season to keep developing weed populations in check. Pre-emergence herbicides can also be incorporated into a vineyard floor management program.
The Management program was formulated by the Pierce’s Disease Advisory Panel. This is an interdisciplinary working group, made up of representatives from the commercial grape industry, as well as members of the Department of Plant Pathology and Microbiology, Department of Entomology and Department of Horticulture, Texas A&M University.
Management of Pierce’s Disease in Texas
Texas AgriLife Extension Service
Key to Leafhoppers and Sharpshooters
Texas A&M AgriLife Research
Grape Pierce’s Disease
University of California Pest Management Guidelines
Pierce’s Disease – Xylella fastidiosa web site
University of California, Berkeley
Glassy-winged Sharpshooter Website
Napa and Sonoma County Agricultural Commissioner’s Offices