Lloyd Wendel, director of USDA’s General Beale GWSS/PD project in Bakersfield, outlined recent developments for grape growers recently at a meeting there.
Wendel is credited with uniting the grape, citrus, and other industries in a common front against the incurable disease and its vector.
Noting that it will take time to subdue the disease-vector complex, Wendel said the effort is showing successes, thanks in large part to the cooperation of growers.
The thrust of the campaign in vineyards has been to knock down populations of GWSS with Admire in the spring and then make additional treatments later to keep populations low. Imidacloprid, the active ingredient in foliar applied Admire, is not harmful to important GWSS egg parasitoids released in the program.
Wendel said some 40,000 acres of citrus in Tulare County is infested with the insect, which feeds on the host plant’s vascular system. Timing of treatments has been adjusted to the fall to head off movement of the pest on harvested fruit to other areas. Traps, checked weekly, are in place throughout Kern and Tulare counties.
"The project is active in a large area wherever the insect is found. In a cooperative effort, growers treat to fill in any spots that might become reinfested," he said.
Historical and current survey data from traps by county officials and the California Department of Food and Agriculture generates a database for recommendations to growers on selection of materials and timing of applications.
Moves between crops
Management of the insect in locations of adjacent grapes and citrus is particularly important because it moves readily between the two crops.
"If we can prevent it from feeding on grape vines and then moving back into citrus where it feeds during the winter, we will have a significant impact on it," Wendel said.
"First, we want to reduce the number of hosts it has to feed on. Then we can make applications in a smaller area and have more influence in reducing its populations."
The approach is to try to squeeze the insect from the north and from the south to confine it in the center for control applications.
The IPM-based program uses biocontrol agents to avoid disrupting enemies of other pests. He said harsher treatments may have caused some disruption of beneficials for a single year, but where good IPM is practiced, natural enemies are returning with some effect on GWSS.
Four species of tiny wasp Gonatocerus parasitoids that go to GWSS eggs have been reared in Texas for release by CDFA in urban areas of California to avoid use of insecticides in densely populated areas.
Meanwhile, surveys are under way outside the U.S. to find additional natural enemies of GWSS that are more aggressive during the spring.
Wendel said high levels of parasitized GWSS eggs can be found in the General Beale area. However, he added, little reduction in overwintering GWSS has been observed during the past two years.
The project, established in 2001, takes its name from the General Beale Road area of 13,000 acres of citrus, grapes, and other crops east of Bakersfield. Measuring three miles by six miles in size, it is bounded by the Tehachapi Mountain foothills on the east, Towerline Road on the west, Highway 58 on the north, and Bear Mountain Boulevard on the south. Trapping and inspections later expanded into Tulare County.
Participating in the program are the ag commissioner’s offices of Kern and Tulare counties, CDFA, University of California Cooperative Extension, and USDA’s Animal Plant Health Inspection Service and Agricultural Research Service.
Legislation allowing formation of GWSS/PD pest management districts for table-grape growing counties was signed by Gov. Gray Davis in September. Officials are working on petitions necessary before a referendum among growers can be held.
Hits other crops
PD also strikes almonds, alfalfa, stone fruit, ornamental plants and several weeds, including bermudagrass, which harbors it without symptoms.
Caused by the bacterium Xylella fastidiosa and vectored by GWSS and other sharpshooter species, it attacks the vascular system of plants and was responsible for devastating vineyard losses near Temecula.
The disease has been known to be present, although to a relatively mild extent, in certain locations of Fresno and Tulare counties, particularly along rivers, for the past 50 years.
With the appearance in the early 1990s of a much more efficient vector, the GWSS, symptoms were noted in citrus and grapes in southern California and later in Kern County. The insect is thought to have arrived on nursery stock from the southeastern U.S.
Jennifer Hashim, Kern County farm advisor, in detailing at the meeting the monitoring program she leads, said ATV-mounted crews of inspectors in the General Beale area found more than 2,000 samples of infestation in 2002 in 41 vineyard blocks but only 326 samples in 39 blocks in 2003. Samples are sent to CDFA for identification.
The sharp reduction was due mainly to infested vines removed by growers alert to the problem, she said. "Growers are doing a good job of rouging infested vines, as we find fewer infested vines. We can’t say whether the lower number is also partly because of climatic conditions. We’ve consulted experts and the jury is still out."
She reminded growers that the most susceptible grape varieties are Redglobe and Crimson Seedless, although other varieties are vulnerable too.
She also warned that the effort could have a discouraging part: the first year of infestation may be misleading, showing only a few symptoms which turn into a blow-up the second season.
Weed control is being studied as a component in the campaign. Hashim said weeds such as black nightshade, silver nightshade, common sunflower, annual burr sage, hemlock, and cocklebur also host PD, which is likely being transmitted to crops by GWSS.