The Glassy-Winged Sharpshooter and Pierce's Disease

Don Luvisi

UCCE Farm Advisor, Kern County

Viticulture - Grapes

The Glassy-Winged Sharpshooter and Pierce's Disease

It has now been a year since University of California President Richard C. Atkinson established a task force of experts to address Pierce's disease. Pierce's disease is caused by the bacterium Xylella fastidiosa, which is transmitted by the glassy-winged sharpshooter (GWSS), and threatens California's grape industry. The UC Pierce's Disease and Emergency Response Task Force published its report in April 2000.

In the report, the Task Force identified six high-priority research areas:

Monitoring and Database Management
Control of Glassy-Winged and Blue-Green Sharpshooters
Control of Pierce's Disease

Plant Resistance to Pierce's Disease
Movement, Multiplication and Pathology of X. fastidiosa
Economic Analyses

It should be noted that the Task Force members are convinced that breeding resistance to the X. fastidiosa bacterium into plants will provide the only long-term protection for grapes and other commodities. This will require time, fiscal resources and the application of genetic engineering and other biotechnology techniques. More immediately, a combination of physical and plant barriers, biological controls, chemical compounds, micronutrients, winter pruning, grower education, and intensive monitoring and data collection efforts show promise in reducing the spread of Pierce's disease.

A significant amount of funding has been allocated statewide from a variety of state and federal sources to combat the GWSS and find the long term solution to Pierce's disease. In Kern County, additional funding has been given by the University, local growers and the Kern County Board of Supervisors. Our University of California Cooperative Extension farm advisors are addressing the problem in their various areas of expertise, including viticulture, citrus, almonds, and environmental horticulture.

The University's website has the full copy of the Task Force report, color enlargements of the GWSS, and a list of experts that may be contacted. Some of these experts are listed below.

Darlene Liesch, County Director

Beth Grafton-Cardwell
Entomologist
UC Riverside
(559)646-6591; bethgc@ucdavis.edu
Expertise: Effects of glassy-winged sharpshooter

Don Luvisi,Viticulture Farm Advisor, EmeritusUC Cooperative Extension - Kern County(661)868-6226; daluvisi@ucdavis.eduExpertise: Glassy-winged sharpshooter and
control on citrus IPM Pierce's Disease in the Southern San Joaquin Valley

Phil Phillips
Area IPM Advisor
UC Cooperative Extension - San Luis Obispo County
(805)645-1457; paphillips@ucdavis.edu
Expertise: IPM Strategies

Alexander Purcell, Plant Pathologist,UC Berkeley,(510)642-7285; purcell@nature.berkeley.edu Expertise: Epidemiology, vectors, control

For a full list, go to the website http://danr.ucop.edu

Glassy-Winged
Sharpshooter
Update

The glassy-winged sharpshooter (GWSS) is widespread in some city areas of Bakersfield, east to General Beale Road and following the southern rim of the valley toward Maricopa. To date, finds are limited in the areas south of Arvin to Maricopa, but even one find indicates the continued spread of this pest. Most of the populations are at elevations of 450-700 feet, the frost free area of the valley. This area, which extends all the way to Porterville and Ave 258 from Ducor to its end, is at 525-580 feet elevation. Unfortunately, this pest has implications for homeowners as well as commercial agriculture. Homeowners with heavy infestations of GWSS cannot stay or picnic under trees since it is like a rain or coastal fog as a result of the insects feeding. I have been under eucalyptus trees that feel like you are being rained on and at least one Bakersfield homeowner has reported the same effect to me. If you have not experienced the GWSS rain or seen the population levels for yourself, take a trip to General Beale Road and stand under the eucalyptus trees and look at the citrus trees.

The insect itself is not a major problem on grapes, but the fact that it is a vector for a bacterial disease (Pierce's disease) that can kill a grapevine in two to three years, makes infected GWSS a major threat to the grape industry. At this time, we do not know the implications of this pest on other crops, especially almonds, which have a similar disease called almond leaf scorch. The potential effect on Kern County's 400 million dollar grape industry is real. This is a case of growers planning ahead because, when the problem hits, it will be too late for remedial action. What can we do?

First on the list is to remove known alternate host plants that act as reservoirs for Pierce's disease (PD) such as bermudagrass and Johnson grass. PD has been identified in several vineyards where GWSS is present. To date, only eight to ten vines have been identified and approximately 400 additional tests of surrounding vines have not turned up any additional infected vines. The Kern/Tulare County Task Force is supporting a project which is now underway with Dr. Purcell at UC Berkeley on identifying PD host plants in Kern and Tulare Counties. Alternate host plants should be removed as soon as possible including any vines showing symptoms of PD. Therefore, once a vine is identified, there could already be many other vines in early stages of infection that are not identified but would show symptoms in one or two years. Unfortunately, current tests for PD cannot detect early infections and we will have to wait until the spring for visual verification if new infections are present. Other research projects are developing new techniques for rapid identification of PD. Many vineyards have stands of bermudagrass several years old and it will take several years to eliminate them. In addition, growers should consider eliminating anything green in the vineyards during winter since the
GWSS can feed on a wide range of plants.

Citrus is a major host for GWSS both overwintering and summer development. This season, vineyards adjacent to citrus had the highest population levels of GWSS. Repeated vineyard sprays did not stop the migration from citrus groves, therefore, cooperation between citrus and grape growers will be an important part of any control strategy, especially in the areas where both GWSS and PD are present. Management programs are under development for suppressing the GWSS in both vineyards and in citrus adjacent to vineyards. Grape growers are well aware of the IPM programs that many citrus growers use and researchers are testing new products that will control GWSS but not upset the IPM program.

This is a short update on the current status of GWSS and some pre-emptive measures that growers can take for the future.

Donald A. Luvisi,
Farm Advisor, Emeritus
Viticulture---Grapes

The Glassy-Winged
Sharpshooter (GWSS)
and Citrus

Citrus appears to be one of the primary over wintering hosts of GWSS and, generally, the highest populations of GWSS in Kern County can be found in infested areas where citrus and grape plantings border one another. GWSS is a strong fiber and GWSS may be found throughout the citrus orchard, vineyards, in surrounding crops, in weeds, and windbreaks.

In infested citrus orchards during the spring, summer and fall, leaves in newly produced flushes become covered with a white exudate that results from GWSS feeding. Glassy-winged sharpshooter egg masses are most easily found from March through October on large leaves of water sprouts arising in the lower interior areas of the tree canopy.
The economic impact of GWSS in citrus is still being defined. Desiccation of young trees and recently budded trees has been reported, and irrigation may have to be increased in heavily infested plantings. Isolated incidences of egg laying in lemon fruit has been reported but large scale sampling of lemon fruit have demonstrated that this is extremely rare. The ability of the packinghouse to remove the exudates in normal washing operations is being examined. The effect on yield of heavy GWSS feeding has not been examined. The glassy-winged sharpshooter is capable of transmitting the strain of Xylella fastidiosa responsible for causing the serious plant disease, citrus variegated chlorosis (CVC), that occurs in South America. However, CVC has not been found in California.

Some growers have been chemically treating GWSS in citrus as a means of reducing the numbers of this pest in their own grove and in their adjacent vineyards or in their neighbors' adjacent vineyards. Some chemicals that have greatly reduced GWSS numbers in citrus include imidacloprid (Admire® under a Section 18 emergency registration, Baythroid (cyfluthrin®), chlorpyrifos (Lorsban®), cygon (Dimethoate®), carbaryl (Sevin®), and formetanate (Carzol®). Read and check label directions of these products carefully to ensure that they are labeled for use on citrus and GWSS when a treatment is planned. Some of these materials when used as post-petal fall citrus thrips sprays in May, significantly reduced GWSS for several months after treatment. Imidaclorprid, while somewhat slower to act, continued to kill emerging GWSS nymphs into September when applied in April or May. Early spring applications of imidacloprid, when used at label rates, will also provide significant control of California red scale and citricola scale, although they were not registered for this purpose in 2000.

Parasitic wasps such as Gonatocerus ashmeadi (Hymenoptera: Mymaridae) have provided partial control of GWSS in some citrus orchards, but parasitism begins in the eggs of the second GWSS generation when populations of this pest are already too high. Recently, an experimental release of a wasp, Gonatocerus triguttatus, collected in northern Mexico has been made. This wasp has been found to parasitize the eggs of the overwintering generation of GWSS and, it is hoped, will eventually prevent the exponential increases in numbers that were observed this year.

Craig Kallsen, Farm Advisor, Citrus, Subtropical Horticulture, Pistachios

The Glassy-Winged Sharpshooter and
Landscape Plants

Kern County has a new insect called the glassy-winged sharpshooter (GWSS) that has been making news as a pest of agriculture, especially of grapes. Aside from economic consequnces, should the grape industry be severely impacted, homeowners should be aware of potential effects of GWSS for the home landscape and weekend barbeques. GWSS has, thus far, been found in scattered locations throughout Bakersfield and in the Arvin-Edison area east and southeast of Bakersfield.

The GWSS is a large leafhopper, approximately ½ inch long, and is dark brown to black with a lighter underside. The upper parts of the head and back are stippled with ivory or yellowish spots; the wings are partly transparent with reddish veins. GWSS adults are shy and will try to keep a leaf or stem between them and an observer.
At last count, over 100 plants have been shown to host this insect, and egg laying has occurred in over 50 species. Some host plants commonly found in the suburban landscape include apple, ash, blackberry, bougainvillea, camellia, Chinese elm, citrus, crape myrtle, eucalyptus, loquat, magnolia, mulberry, oak, oleander, philodendron, pine, privet, most stone fruits, trumpet flower, umbrella tree, willow, wisteria and yucca.

GWSS is capable of carrying a bacterial pathogen that can kill some backyard shrubs and trees including oleander and almond. The major impact on the landscape from this pest, however, is its copious excretion of watery plant fluids in a steady stream of small droplets. On most plants, GWSS feeds on stems rather than leaves, drawing its sustenance from the water-conducting tissues (xylem) of plants, unlike aphids, which tap into the sap conducting tissue (phloem). Since the xylem conducts mostly water, the glassy-winged sharpshooter must pass a lot of fluid from the plant, through its body, more than 1/3 of an ounce every day, to glean what nutrition is available. Hundreds of these insects will produce a veritable rain from infested landscape plants, leaving a white residue on the plant and anything below. This rain of watery excrement will probably be the first indication for many homeowners that this insect is present on their property.

Natural predation and parasitism have been important in limiting the spread of this pest in agricultural fields and should be equally important in landscapes. GWSS can be killed with insecticides appropriately labeled for leafhoppers and the plant under attack, but the degree of control is still unknown. The first generation matures as adults in late May through August, laying eggs which hatch into an overwintering generation. Because the egg laying and hatching period occur over an extended period of time, and since these insects are highly mobile, more than one insecticide application may be required. A systemic insecticide may be necessary for landscape managers wanting to control this pest on tall trees. If you find insects suspected to be the glassy-winged sharpshooter, you may bring them for identification to the UC Cooperative Extension Office and/or the Agricultural Commissioner's Office on South Mt. Vernon Avenue in Bakersfield. Your help is appreciated as the spread of this pest is being monitored throughout the county and the state of California.

Craig Kallsen, Farm Advisor and
John Karlik, Advisor,
Environmental Horticulture/
Environmental Science

Almond Leaf Scorch
Present in Kern County

Almond leaf scorch (ALS) is a disease in almonds caused by the bacterium Xylella fastidiosa. This is the same organism responsible for Pierce's disease in grapes and alfalfa dwarf disease in alfalfa. ALS has been present in many almond growing areas of California. In the early 1950's, it appeared in the Lancaster area (Los Angeles County) where many orchards were infested, and some people believe ALS contributed to the disappearance of the almond industry in that area. ALS was reported more recently in Contra Costa, San Joaquin, Yolo and Stanislaus Counties. In 1994, Dr. Alex Purcell found it in Tulare County, and in 1996 and 2000, we found it in the Delano-McFarland area. The varieties susceptible to infection are Nonpareil, Sonora, Mission (Texas), NePlus Ultra, and Peerless.

Trees affected with ALS lack terminal growth. The shoots have very short internodes with few spurs and few nuts. The most characteristic symptom of this disease appears in June and is leaf scorch, where tips and margins of leaves turn yellow and then brown. In some varieties a golden yellow band may form between the green and brown areas of the leaf. The disease symptom is due to water stress which occurs when the xylem vessels are blocked at the nodes and petioles by diseased tissue. Generally only a portion of the tree, usually the terminal branches, initially shows symptoms, but within three to eight years the entire tree may become affected. Diseased trees are stunted, unproductive, and show no ability to recover. Symptoms may be confused with sodium toxicity; however, a leaf tissue analysis can indicate whether sodium is present at a toxic level.

ALS can be spread very effectively by the glassy-winged sharpshooter, which is now present in Kern County. The bacterium Xylella fastidiosa not only is found in grapes, alfalfa and almonds, but also infects bermudagrass, blackberry, cocklebur, elderberry, fescue grasses, nettle and rye, all of which can serve as a reservoir of the bacterium. The glassy-winged sharpshooter feeds in the xylem tissue, and if it feeds on an infected plant, it can become a carrier of the bacterium, transmitting ALS to an almond tree during subsequent feeding. It has been observed that the glassy-winged sharpshooter feeds on actively growing shoots, which means the potential for infection in almonds occurs in the spring when shoots grow rapidly. ALS can also be spread by budding and grafting, but it is unlikely to be spread by the latter since diseased trees show such obvious symptoms that grafters rarely select budwood from affected trees.

At this time, control is limited to cultural methods. Weed hosts and unproductive alfalfa hay fields should be eliminated. Once almond trees are infected they become unproductive within two or three years, so these trees should be removed and replaced. There are no resistant varieties. Research on the control and spread of the glassy-winged sharpshooter is underway. Unfortunately, there is currently no pesticide spray that can be applied in almond orchards to protect trees from ALS.

Mario Viveros, Farm Advisor
Deciduous Tree Fruits and Nuts