Incidence of Pierce's Disease (PD) in Kern County vineyards in 2003 was about one-fourth of what it was in 2002 due to fewer glassy-winged sharpshooters (GWSS) and removal of infected vines, says Kern County farm advisor Jennifer Hashim.

Hashim, who has coordinated monitoring for the joint federal, state, and county General Beale GWSS/PD project since 2001, reported that PD was confirmed in the county on 0.68 vines per 1,000 tested in 2002 but on only 0.155 vines per 1,000 tested in 2003. Surveys were made on about 2 million vines on 4,000 acres each year.

She said the project has generated greater awareness among growers and proactive management of the disease and its insect vector.

“This project also demonstrated that monitoring vineyards for PD, testing, removing infected vines, and replanting is very inexpensive, on the order of less than $5 per acre per year, when PD incidence is low.”

Taking its name from the 13,000 acres of tree and vines and other cropland east of Bakersfield along General Beale Road, the project later expanded to nearly 4,000 acres from Arvin to Delano and to the foothills.

Speaking at the recent San Joaquin Valley Table Grape Seminar at Visalia, Hashim said much has been learned from the project in the past three years.

GWSS populations, she said, were sharply reduced by treatment of vineyards with “soft” but effective insecticides.

Treatments were made with Admire, the foliar-applied imidacloprid formulation, during the spring, followed by supplemental treatments with it later to suppress GWSS populations. The material is not harmful to beneficial wasps released to put additional pressure on the pest.

Area-wide treatment

“We learned that a successful management strategy is associated with the reduction of insects — in an area-wide approach because of the massive number of GWSS host plants — and the removal of vines infected with PD,” she said.

Although all grape varieties are vulnerable to PD, data show the table variety Redglobe was most susceptible, showing symptoms more rapidly and with greater vine loss than others, such as Flame Seedless. The reasons for Redglobe's acute sensitivity are not known. Crimson Seedless is another highly-sensitive variety.

Hashim said University of California scientists at Riverside and Berkeley are now analyzing data, gathered by trained, ATV-mounted technicians searching for signs of PD infection in vineyards, to learn why some varieties are more susceptible than others.

“We are also looking for management techniques to further reduce the number of infections we might have in case we lose federal and state monies for control of GWSS.”

Current monitoring, as of early February, has shown some GWSS activity in windbreaks, but, she added, “vineyards are pretty clean.”

Hashim said she would like to see the project continued “for at least one more year, so I could become comfortable that we have brought PD levels down to a very manageable level. One thing I want to hammer home: it is not just the reduction of the GWSS, but also the removal of infected vines, that makes this management strategy work.”

Citrus and grapes

Project officials say management of GWSS in locations of adjacent citrus and grapes is critical since the insect can easily move between the two crops. It typically feeds on grapes during the summer and goes to citrus in winter.

PD is carried by GWSS among other crop and landscaping hosts, including almonds, alfalfa, stonefruit, ornamental plants, and bermudagrass, which harbors it without showing symptoms. Notable weed hosts are black nightshade, common sunflower, annual burr sage, and cocklebur.

Although the disease has long been known to be in California and transmitted by various sharpshooter species, the introduction of a larger and much more efficient vector, the half-inch-long GWSS, alarmed the grape and citrus industries. Vineyards around Temecula were devastated by PD after GWSS was discovered in Southern California in 1994.

Growers and others at the seminar also heard about precautions in controlling black widow spiders in packed table grapes by fumigation with carbon dioxide and sulfur dioxide.

More black widows

Elizabeth Mitcham, postharvest pomologist at UC, Davis, said procedures were developed several years ago to respond to complaints of black widows being found in table grapes by retailers and consumers, yet reports have been on the increase recently.

“These finds sometimes cause adverse publicity and disrupt table grape movement and sales,” she said. “The increase in black widow spiders in vineyards is likely due to the increased use of reduced-risk pesticides. Previously used broad spectrum materials are being replaced by reduced-risk materials, and growers are seeing increased spider activity in vineyards.”

The key to effectively kill the spiders is fumigation at a field temperature of at least 60 degrees with 6 percent carbon dioxide and 1 percent sulfur dioxide for 30 minutes. Forced-air fumigation is required to penetrate into grape boxes and packaging material.

At least 300 cubic feet of fumigation space per pallet of grape packages should be used, although Mitcham's recent experiments indicated that 430 cubic feet per pallet provides a very high spider kill when TKV boxes are used.

TKV boxes, especially when they are moist, absorb a portion of the sulfur dioxide and reduce the effectiveness of the fumigant. Plastic boxes do not absorb the gas.

She said some interest has been shown in fumigating previously-cooled and stored grapes for spider control. “However, this process is not recommended for a number of reasons. It is very difficult to thoroughly warm pallets of grapes to the fumigation temperature of about 60 degrees.”

During warming, she added, condensation forms on the grapes and within packing materials. That moisture is impossible to dry, and it absorbs sulfur dioxide, creating an acid solution.

That solution can cause significant damage to the fruit. In her 2003 trials she found severe bleaching of Thompson Seedless and Redglobe when they were fumigated following cold storage.

The condensation that forms on the boxes also reduces sulfur dioxide concentrations in the fumigation chamber, impairing its lethality to spiders, she said.