Less than 50 percent of glassy-winged sharpshooters (GWSS) transmit the bacteria that causes Pierce's disease, University of California, Berkeley, entomologist Alexander Purcell told scientists, growers and others attending the Pierce's Disease Research Symposium held in December in San Diego. That sounds like good news, but scientists have counted more than 4,000 GWSS on a single untreated lemon tree. Millions of insects could still carry the lethal grapevine disease.
Scientists from across the country reported on 64 research projects exploring different approaches to controlling Pierce's disease and one of its major vectors, the glassy-winged sharpshooter. The three-day symposium was hosted by the California Department of Food and Agriculture.
“This conference is important at this early stage in the battle against Pierce's disease because it provided an overview of much of the research that is being done and allowed researchers to discuss new ideas, alternative approaches, and possible collaborations,” said Joseph Morse, the Ag Policy and Pest Management program leader for UC Division of Agriculture and Natural Resources.
The projects, funded by 17 different agencies, addressed short-term goals such as controlling GWSS as well as the long-term goal of defeating Xylella fastidiosa, the bacterium that clogs the water-conducting system in grapevines, often killing them within a couple of years.
The economic impact of Pierce's disease has yet to be quantified, but it will be long term, said Ted Batkin, president of the Citrus Research Board. He explained that its costs reach beyond pest management, crop loss and plant replacement to research, inspections and export quarantine regulations.
Although Pierce's disease is largely seen as a grape problem, the same strain of the bacteria that causes Pierce's disease is also to blame for almond leaf scorch and alfalfa dwarf. Different strains cause oleander leaf scorch and phony peach disease. A related citrus disease, citrus variegated chlorosis, is not present in California. Citrus growers are bearing the added expense of controlling glassy-winged sharpshooters, which overwinter in citrus groves. The insects suck copious amounts of fluid out of the trees and growers are studying this to determine if it may reduce their crop yields, Batkin said.
Scientists working on area-wide management of GWSS in Temecula and Kern County reported on the effectiveness of chemicals, barriers and biocontrol strategies. Experiments have shown some insecticides are very effective in controlling specific life stages of the glassy-winged sharpshooter. Coating grapevines with kaolin clay also deterred the insects from feeding and laying eggs on the plants. Further studies are needed to determine whether kaolin prevents transmission of the bacteria and thus lowers the incidence of the disease.
Beth Grafton-Cardwell, a UC research entomologist and a specialist with the UC Statewide IPM Project, said the best suppression of GWSS in citrus has been with foliar pyrethroid, organophosphate or carbamate sprays in the spring to kill overwintering insects, followed by a systemic Admire treatment to reduce their numbers in the summer. Unfortunately the insecticides that best control GWSS also kill beneficial insects, which eventually leads to outbreaks of other pests. “Because of the seriousness of the GWSS as a vector of disease in grapes, several citrus growers in Kern County have abandoned an excellent IPM program,” Grafton-Cardwell said.
Although growers haven't been able to prevent GWSS from overwintering, they are taking steps to halt the pest's spread. Growers are disinfecting the fruit bins before moving them from harvested citrus from Kern County orchards to Tulare County packinghouses.
UC Riverside entomologist Raymond Hix reported on the status of the Temecula area, estimating that 1,540 acres of vines have been removed since 1996 or will be removed by April 1, 2002, due to Pierce's disease. Based on grower surveys, there were about 2,800 acres of wine grapes in the area in 1996. “The current situation in Temecula is serious, but cautious optimism prevails,” Hix said. “First, glassy-winged sharpshooter populations are currently the lowest in Temecula citrus in three years. Second, 2001 grapevine removal due to PD was the lowest in the past three years.” He noted that controlling the sharpshooter in the area's 300 acres of organic citrus remains a challenge.
Entomologists at UC, CDFA and the U.S. Department of Agriculture have been working together to identify and test egg parasitoids. Parasitoids lay their eggs inside sharpshooter eggs, preventing the sharpshooters from hatching. Gonatocerus ashmeadi is a parasitoid that is plentiful in California, but primarily effective in the summer. Scientists have imported a stingless wasp called Gonatocerus triguttatus from Mexico and Texas and are trying to propagate it to destroy sharpshooter eggs earlier in the season. Last year CDFA released 140,000 parasitoids. More will be needed to keep up with the millions of sharpshooters. From experiments, the entomologists have learned that the parasitoids attack eggs that are one to six days old.
In Kern County's General Beale Road Pilot Project, scientists learned that the glassy-winged sharpshooter has habitat preferences. “In the current project, preferred habitats are citrus, windbreaks, grapes and other ornamental and wild plants,” said farm advisor emeritus Don Luvisi. He also observed that the sharpshooters in Kern County were less likely to survive the winter at elevations below 500 feet because the lingering fog makes it colder than at higher elevations.
Chemicals and biocontrol are short-term fixes. For long-term solutions, scientists are investigating the interaction between X. fastidiosa and grape plants on a molecular level. UC Davis plant pathologists are sequencing the genes of Chardonnay and Cabernet Sauvignon plants, seeking differences between healthy vines and those infected with Pierce's disease. The goal of this research is to learn enough about what makes grape plants resistant or susceptible to Pierce's disease so they can disrupt the disease process.
Brazilian scientists have mapped the X. fastidiosa genome. Using this information, UC scientists are attempting to identify genes that play a role in attaching the bacterium to xylem, which could hold the key to preventing the clogging that kills vines. UC Riverside plant pathologists are studying xanthan gum, a substance produced by the bacteria that clogs up water-conducting tissue and eventually kills the vine. They are exploring the use of endophytic bacteria to degrade this gum.
More than $12 million from federal, state and industry sources has been invested in Pierce's disease research. Patrick Gleeson, executive director of the American Vineyard Foundation, told the symposium attendees that he expects success to made incrementally, first by controlling the spread of glassy-winged sharpshooter and implementing treatment protocols for nurseries. Secondly, Pierce's disease will be managed by disrupting the interaction between host plants, vector and disease, and using environmentally sensitive treatments until the problem is solved. “We've been working with Pierce's disease since the 1800s, the solution will come in years, not months,” Gleeson said.