May 2001

pest management By Beth Meneghini

A pest no bigger than a dime is threatening two of California’s most important industries, wine and horticulture.

The culprit? The glassy-winged sharpshooter (GWSS). This long distance leafhopper has infested almost all of Southern California, plaguing greenhouses in Kern, Ventura, Los Angeles, San Bernadino, Riverside, Orange and San Diego counties.

On its own, GWSS is a nuisance that damages the aesthetics of the plants it feeds on. But paired with the incurable Pierce’s Disease, which can devastate wine crops, GWSS is an enemy California cannot afford to harbor.

Currently, the most threatening problem posed by GWSS is not its potential to spread Pierce’s Disease to Northern California’s coveted wine region, but rather the increased time, effort and money southern California growers are spending to ensure that the pest doesn’t leave their own greenhouses.

Dissecting the Disease

Pierce’s Disease is caused by a bacterium, Xylella fastidiosa. This bacterium blocks the xylem, or water- and nutrient-conducting vessels of plants. Typically, when a plant is infected with the bacterium, its leaves begin to dry out or “scorch.” Infected vines can die in as little as one to two years.

While Pierce’s Disease is typically associated with the decimation of grapevines, X. fastidiosa also causes several other diseases.

“Pierce’s Disease is part of a family of diseases that affect stone fruits such as almonds and peaches, ornamental plants like oleander, citrus, alfalfa and even coffee,” said Jay Van Rein, information officer for the California Department of Food and Agriculture (CDFA). “If the bacteria is present, even at low levels, the introduction of a vector like GWSS could have very serious consequences.”

Pierce’s Disease has appeared on and off in the United States since the late 19th Century, when it destroyed 40,000 acres of grapes in the Anaheim, Calif. area. The disease’s spread, however, has been limited. In the past, the principal carrier of Pierce’s Disease was the blue-green sharpshooter, a small, rather weak insect not able to fly much further than three feet. But now that the glassy-winged sharpshooter has taken over for its cousin, the disease is spreading farther and faster than it ever has before.

“The difference with GWSS is that it is a much heartier insect than prior carriers of the disease,” said Van Rein. “GWSS breeds more often, flies farther and has the strength and ‘equipment’ necessary to pierce the larger woody parts of vines, allowing it to transmit the disease to main trunks instead of to branches that can be pruned before the entire plant is damaged.”

GWSS is not a picky eater; it will feed on any number of host plants and appears to change from one crop to another to meet its nutritional needs. It feeds on several times its own body weight in fluid each day. Most plants show no symptoms of disease, other than perhaps decreased height or yield, but serve as hosts for the bacteria, which can then be picked up by other GWSS and spread.

The Cost of Battle

To its credit, the state of California acted quickly once GWSS and Pierce’s Disease were first discovered.

“After we realized it was a problem, the state acted quickly and responsibly,” said Farzad Moin, a member of the GWSS nursery committee task force in California. “Preliminary research was completed on the disease and its carrier before any projects were put into place.”

More than $8 million has been channeled to researchers to study the GWSS problem and a statewide program helped jumpstart 50 projects this year.

To stop the immediate spread of the disease, the CDFA has implemented stringent shipping regulations. All nurseries in infested areas (see counties listed in second paragraph) must be registered with the state and adhere to specific shipping protocols before shipping any plant material to a non-infested area of California.

County inspectors visit registered nurseries on a weekly or bi-weekly basis to conduct visual inspections and trappings. Based on the findings, growers are categorized into “types.” If a greenhouse is found to contain more than five GWSS in five days, the facility is considered infested. If less than five GWSS are found in five days, the operation is considered non-infested. If county inspectors find no GWSS, the greenhouse operation is considered “free-from.”

Growers that are considered free-from or non-infested are able to ship plant material to Northern California under the Compliance Agreement of the Pierce’s Disease control program. Under this agreement, a nursery can “self-execute” inspection and spraying of host plant material.

“We continue to inspect and trap at these locations regularly,” said Dawn Nielson, deputy agricultural commissioner for San Diego County, “but not every shipment is looked at by a county inspector.”

Growers that fall into the “infested” category don’t have such luck. If a facility is considered infested, the grower must inspect 100 percent of their plant material before bringing it to a staging area free of GWSS. Here, county inspectors oversee the chemical spray of the material as well as verify that the material is free of GWSS.

“The program can be overwhelming for Á growers,” said Nielson. “We had a hard time implementing things initially, but the program is now firmly in place. We’re doing everything we can to stop this disease from spreading.”

As are growers. Whether a greenhouse operation is considered free-from, non-infested or infested, there is a tremendous outlay of cash and manpower to comply with state protocols.

“There have been immediate impacts on the nursery trade,” said Van Rein. “These companies are spending thousands of dollars each month, mostly on labor for inspections, to adhere to the regulations. Some simply can’t ship north because their facilities, or their entire regions, are infested to such an extent that they cannot exclude GWSS from their shipments.”

Moin echoes this sentiment. “Growers have done all that has been asked of them and more. Some smaller nurseries can’t endure the tremendous expense of compliance much longer. Growers need support, financially and scientifically,” he said.

Moin, who is also assistant manager of production for Altman Plants in Vista, Calif., feels that the grape and citrus industries should help in combating the GWSS problem.

“GWSS favors citrus trees for egg-laying. Only a small corner of our operation is infested with GWSS–the corner closest to our neighboring citrus grower. Our plants get infested through them, yet we put up all the costs. There should be protocols in place for all types of host plants, not just ornamentals.”

Research for

the Future

Despite some shortcomings, California has put together a system of containment and short and long-term research that will hopefully reduce the risks of GWSS and Pierce’s Disease, as well as answer questions about how and why this pest became so volatile.

“One thing researchers are very keen to know is why GWSS and the disease have not spread widely from Southern California since it was first introduced in the early ’90s,” said Van Rein. “The insects certainly could have moved more rapidly and widely to new areas, but apparently have not done so. Altitude, climate, feeding and reproductive preferences may hold clues, but the answers are not yet known.”

Although researchers are working on identifying “softer” chemicals or biocontrol options, several pesticides have proven effective against GWSS. Carbaryl, Marathon and Tame, in both sprays and soil drenches, have been used effectively by growers.

A stingless wasp from Mexico has also shown promise in combating GWSS. Thousands of these wasps are being reared in research labs for timed releases throughout the year.

Although GWSS and the Pierce’s Disease problem seem to be limited to California because of the state’s wine industry, other regions are beginning to take notice of the problem.

“Oregon and Washington State have their ears perked up,” said Nielson. “They definitely do not want to inherit the problems we have here with GWSS.”

States like Florida are also at risk. GWSS is quite prevalent in the citrus-growing regions of this southern state, and while those GWSS currently do not carry the strain of bacteria that causes Pierce’s Disease, they certainly could pick it up.

“The research and regulation now underway in California could teach all of us in agriculture some important lessons about combating infestations and diseases, as well as avoiding problems with good inspection protocols and restrictions on plant movement.”

For more information about GWSS, please visit the CDFA Web site at http://plant.cdfa.ca.gov/gwss/