NOTE [A work in progress; last updated 11/2003]
These guidelines cover “traditional” Pierce’s disease as well as considerations for the new vector, the glassy-winged sharpshooter.
The glassy-winged sharpshooter (GWSS) became established in the California’s San Joaquin Valley in the 1990s. GWSS a serious new threat to California vineyards because of its longer distance movements into and within vineyards compared to vectors native to California (“traditional” vectors). GWSS can reach unusually high numbers in citrus and avocado groves and some woody ornamentals that until now have not been sources of Pierce’s disease vectors. GWSS has been seen in high numbers in citrus along the coast of southern California since the early 1990s. During the past few years it has become locally abundant further inland in Riverside and San Diego counties. In the late 1990s high populations of GWSS were seen on citrus and adjacent vineyards in southern Kern County. GWSS will spread north into the citrus belt of the Central Valley and probably will become a permanent part of various habitats throughout northern California.
Experience with GWSS as a Pierce’s disease vector is recent, so guidelines for vineyards that may be impacted by this sharpshooter are still evolving. Vineyards within 1/2 to 1 mile of citrus or avocado groves appear to be at greatest risk. As GWSS adapts to central and northern California, it may inhabit and breed in riparian zones, tree crops or ornamental landscapes as yet unknown.
In addition to its greater dispersal to vineyards and from vine to vine within vineyards, two other characteristics of GWSS may make it a more effective vector for Pierce’s disease. First, adult GWSSs feed on grapevine canes than other sharpshooters in California. Secondly, GWSS adults feed during winter months on dormant vines. How do these characteristics contribute to the effectiveness of GWSS as a vector?
Summer infections by vectors such as the blue-green sharpshooter (BGSS) late season (after mid-June) infections usually do not survive the winter, although the disease recovery rate depends on the cold severity of the following winter, the age and variety of the vine, and how late in the season the infection is introduced by a vector. The colder the winter, the older the vine, the less susceptible the variety (for example, Fiesta and Redglobe are much more susceptible than Thompson Seedless or Flame Seedless), the higher the recovery rate from PD. However, blue-green sharpshooter feeds mainly on leaves, while GWSS feeds mainly (but not exclusively) on stems, even canes with mature (smooth, brown) bark or even branches with scaly bark. When the Pierce’s disease bacterium (Xylella fastidiosa) colonizes these older woody tissues, it may be better able to survive the winter to continue to cause disease the next year (chronic Pierce’s disease). This may be significant because it is the summer months when vectors can best pick up Xylella fastidiosa by feeding on infected vines. The better survival of vector-mediated infections ofXylella fastidiosa made during summer could mean that the vine-to-vine spread of Xylella fastidiosa by GWSS within vineyards in summer is important for establishing new chronic infections. In contrast, “traditional” vectors in California such as the blue-green sharpshooter (riparian areas of the coastal regions), and the green and red-headed sharpshooters (most important vectors in Central Valley) feed mainly on leaves, which may be why summer infections established into vines during summer rarely survive the next winter, even when the Pierce’s disease bacterium moves into canes by autumn. This seems to explain why we have not seen evidence of major vine-to-vine spread of Pierce’s disease in California before the arrival of GWSS. Traditionally, the greatest concentrations of Pierce’s disease in central California have been near breeding habitats of the green and red-headed sharpshooters. These habitats include irrigated pastures, weedy alfalfa fields, and the weedy margins of canals, ponds, or ditches. The green and red-headed sharpshooters are primarily grass-feeders, only rarely observed feeding on vines and usually not venturing far from their breeding habitats (an important exception is where permanent Bermudagrass habitats occur along the vine row). The spatial pattern of where Pierce’s disease occurs in Central Valley vineyards traditionally has mirrored the pattern of insect vectors entering a vineyard from vineyard edges nearest vector habitats. Pierce’s disease incidence in a vineyard is usually highly variable from year to year, but usually approximate a straight line increase over many years.
Where GWSS is a factor in spreading Pierce’s disease, we see two things that are different in the pattern of the disease. First, the tendency of the disease to be highest near breeding habitats of GWSS (mainly citrus so far) is much less pronounced than with traditional vectors. Instead, infected vines appear to be occur in clusters throughout the vineyard. Secondly, PD increases over several years in an “exponential” (or more accurately “logistical”) fashion where GWSS is present in significant numbers. Both of these characteristics suggest that the vine-to-vine phase of vector-mediated infection is more important than with “traditional” PD vectors. The importance of this new situation means that to control Pierce’s disease where GWSS is the main vector, keeping the GWSS out of vineyards during summer months is critical. Secondly, removing PD-vines is important to prevent vine-to-vine spread of the bacterium.
Another difference of GWSS from traditional PD vectors is that GWSS feeds on dormant grapevines during the winter. Recent field experiments in Bakersfield showed that GWSS can infect dormant vines in February and January. This means that growers should try to reduce numbers of glassy-winged sharpshooter present in vineyards at any time of year.
GUIDELINES FOR CENTRAL VALLEY VINEYARDS WITHOUT GWSS
As in coastal valley vineyards, the most effective control measures for Pierce’s disease (PD) are prevention by avoiding planting vineyards near vector breeding habitats.
The principal “traditional” PD vectors in the Central Valley of California are grass-feeding sharpshooters, the green sharpshooter (Draeculacepahala minerva) and the red-headed sharpshooter (Xyphon [formerly Carneocephala]fulgida). The main breeding areas for both sharpshooters are similar — irrigated pastures, weedy alfalfa fields, ditch banks, and the edges of ponds. Both of these sharpshooter species can be found in lawns, but usually not in high numbers unless the lawns are watered frequently or deeply and mowed infrequently enough for the grass to grow over several inches in height. Seasonal grass covers (summer or winter) within vineyards so far has not proven to be a major source of sharpshooters, but many of the various types of cover crops, especially newer types of cover that involve permanent cover, have not been evaluated for their potential to support sharpshooter populations. Growers with cover crops should remain vigilant for sharpshooters by monitoring their vineyard cover crops and covers in adjacent crops for the presence of sharpshooters in cover vegetation. Some tree crops — for example, flood irrigated plums — have permanent cover of grasses such as Bermuda grass that support high populations of sharpshooters.
It is extremely rare to find either sharpshooter species on grape. Both the red-headed and green sharpshooters fly at dusk, and the vectors are presumed to feed on grape only by accident, mostly during darkness. If light winds are present, they are carried into vineyards downwind during flight. Strong winds probably inhibit flight. Because the grass-feeding species of sharpshooters visit grape so rarely, grapes are not considered a major source for vectors to pick up the bacterium (Xylella fastidiosa) that causes PD. Thus alternate plant hosts of Xylella fastidiosa are must serve as reservoirs of the bacterium. Alfalfa is a good host of Xylella fastidiosa and alfalfa chronically infected with the bacterium develops a disease caused alfalfa dwarf. Alfalfa alone has not been observed to support sharpshooter breeding; the grass weeds in alfalfa, particularly Bermuda grass, are needed to trigger and sustain sharpshooter breeding in alfalfa fields. In permanent pastures, perennial ryegrass, fescue grasses, and Bermudagrass commonly support sharpshooter breeding.
The most critical time for vector infection of grapes is from bud break through early June, although this may be influenced by year to year changes in climate. It is thought that warm winters with few or no freezing temperatures allow the bacteria to survive better than during colder winters. Warmer than average spring weather may allow the bacteria to multiply and move within grapevines more rapidly and thus have a better chance of surviving the following winter dormant period. Otherwise, most late season infections do not appear to survive the winter.
MONITORING
Direct sampling of vegetation is the best way to evaluate a cover crop or weedy habitat as a source of sharpshooters near a vineyard. You must first be sure that you can accurately identify the green sharpshooter and red-headed sharpshooter. Yellow sticky traps are not effective for monitoring either the green or red-headed sharpshooter, probably because their flight during daylight hours is rare. Sticky traps capture both of these sharpshooters if they are abundant, but traps do not attract them.
Adults can be sampled efficiently by a standard insect sweep net. Take 50 sweeps of grasses and low-lying vegetation while walking forward slowly for each sample. Sweep vigorously and as close to the ground as possible. The sweep net is very inefficient in collecting nymphs because they are small and prefer to feed close to the ground. Dumping the net contents into a plastic bag helps facilitate identifying and counting sharpshooter adults and nymphs. With experience, counts can be made directly from the net. Sampling should especially concentrate on identifying breeding habitats in the late summer and fall and again in the spring before bud break in grapes. As a rough guideline, consider low populations to be anything less than one sharpshooter adult per 200 sweeps. This low level indicates that the area should be resampled at a later date to see if sharpshooters are present but at a low point in their population development or if they are adults that are searching for suitable feeding plants. In hay fields, be sure to concentrate on areas with the highest densities of grasses. A minimum of 400 sweeps or the entire habitat (for example, roadside weeds, etc.) should be made before concluding that a habitat does not support sharpshooters.
VECTOR CONTROL
Cultivation or other weed control methods should be applied to eliminate breeding conditions for sharpshooters by eliminating the grasses on which they prefer to feed and reproduce. Obviously this is not an option for permanent pastures, where succulent growth of grasses is the objective of the pasture operator. Nearby sources of sharpshooters that cannot be controlled directly by the vineyard operator present a difficult problem if Pierce’s disease is coming from these areas. To be effective in preventing the crucial early growing season infections of Pierce’s disease, habitats for sharpshooter breeding that are near vineyards should be eliminated before grapes begin to leaf out in the spring.
Insecticides generally are not satisfactory for controlling the grass-feeding sharpshooters to prevent Pierce’s disease in nearby vineyards because sharpshooter eggs that have been deposited within plants survive so that nymphs emerge some weeks later. Irrigated pastures or hay fields are sensitive as to which insecticides can be applied without violating State or Federal regulations. If growers choose to treat weedy premises or other habitats to kill sharpshooters prior to eliminating weeds, they should insure that the materials and methods of application that they use follow the proper regulations and guidelines.
GUIDELINES FOR CENTRAL VALLEY VINEYARDS WITH GWSS
Experience to date in Kern county vineyards indicate that GWSS builds up high populations in citrus crops. The most effective control method is to keep GWSS from building up in citrus. This means that grape and citrus growers must cooperate on a regional basis to control GWSS. Because of this, recent legislation is moving towards the formation of GWSS-PD pest management districts, much as Mosquito Abatement districts function to prevent the spread of diseases caused by mosquito-borne viruses. If approved, the new districts could be in effect by spring of 2004
Contact your county’s Agricultural Commissioner or UC Cooperative Extension office for the latest information on what you can do to prevent damage by GWSS as a vector for Pierce’s disease.
Recent surveys for PD in Kern and Tulare counties revealed that PD occurs at very low levels in grape-growing districts formerly considered as free of PD. Vines with PD can occur widely scattered throughout large vineyards. Wherever GWSS can be expected to occur in vineyards, even at low levels, identifying and promptly removing vines with PD is a worthwhile investment. If you re-establish replacement vines by layering from an adjacent vine, cut the connection to the mother vine after 2 years to prevent a single infection from spreading from among the connected vines.
General guidelines to reduce GWSS in the vineyard may require a combination of soil and foliar applied pesticides and/or repellant products. Infestations may warrant the use of soil applied imidacloprid (Admire™ 2F) at 16 to 32 oz./acre in one or two drip applications. The higher rate is recommended in warm regions where vigorous vine growth is expected and two applications at a 21-45 day interval are recommended on sandy soils.
A non-toxic, kaolin clay product (SurroundR WP) developed to reduce plant transpiration losses has been shown to repel GWSS in field trials where vines are “white-washed” with SurroundR sprays. These can be applied up to fruit formation for table grapes and then may be resumed post-harvest. Application of SurroundR beyond 5mm in berry diameter may leave unsightly deposits on harvested bunches.
Read and check the label directions of products carefully to ensure that they are labeled for use on grape and the guidelines for rates, special local needs, re-entry intervals and pre-harvest intervals are met.