Because of recent finds in insect traps in California, speakers at a grape symposium warned grape growers to keep their guard up against an insect pest that can quickly spread deadly Pierce’s disease in vineyards.

The insect was found recently in commercial vineyards in west Fresno County and near the San Joaquin River and is gradually spreading into rural areas, said Stephen Vasquez, University of California viticulture advisor for Fresno County, and Fred Rinder, the county’s deputy agriculture commissioner.

Both Rinder and Vasquez said the combination of that disease, which has destroyed tens of thousands of acres of grapes in California dating back more than a century, and the glassy-winged sharpshooter (GWSS), a relative newcomer to the state, could be “devastating.”

Fresno County is the state’s top grape producer with a value for the county’s crop at just under $1 billion.

Vasquez described symptoms for Pierce’s and urged testing of any suspect vines, and Rinder talked of the county’s GWSS program for trapping, surveying, treatment and biological control.

Rinder said the county treats only residential areas, but growers are urged to do their own treatments for GWSS if the insect is found in their vineyards.

“Don’t rely on big brother to do the spraying,” he said.

Rinder recommended they make sure restricted materials permits are accurate because they are used by the county to notify growers of a find.

The insect was found in the Fresno-Clovis area in the late 1990s, and more recent finds – coupled with treatment -- were made in the Kerman area and in southeast Fresno County. Some 4,000 properties in the county have been treated for GWSS. Treatments resume in March and trapping with yellow sticky panels will start in May.

The insect has a wide range of host plants, including ornamentals.

Traps will be placed in nurseries, commercial citrus orchards and towns throughout Fresno County along with the perimeter of the Fresno-Clovis area. Bio control efforts will continue through the release of a tiny wasp that parasitizes the eggs of the GWSS.

The insect overwinters in citrus groves and comes out of the groves and into vineyards in April or May. Rinder said that if a citrus grove is within a half mile of a GWSS find, its grower may be able to get funding through the U.S. Department of Agriculture to pay for voluntary spraying of the grove for the insect, which does not harm citrus. He said about 12,000 acres of citrus in Fresno County has been treated for GWSS.

Hot spots, BMSB, mealybug

Vasquez said Pierce’s disease and the GWSS have taken a back seat in recent years as other invasive pests, including the European grapevine moth, captured headlines and because there has been a low incidence of Pierce’s throughout the Valley. But there are areas in the Valley where Pierce’s has cropped up historically, and Vasquez said remaining vigilant is important in order “to minimize potential ‘hot spots’ from developing should GWSS become established.”

Vines infected by the Pierce’s bacterium display several symptoms in late summer. They include burned or scalded canopies, shriveling of fruit, dropping of leaf blades and irregular wood maturity characterized by green and brown patches.

But some of the symptoms may be trigged by other diseases, by pests, chemicals or cultural practices such as poor irrigation, Vasquez said. He listed laboratories where tests can be done to determine if the plant is infected with Pierce’s.

Vasquez also said growers and work crews should keep an eye out for other exotic pests and should take any suspect insect to an entomologist with the Fresno County Department of Agriculture.

He said growers, for example, need to be watchful for the marmorated stink bug, a pest not yet believed to be in vines in the Valley. Vasquez said that pest could be “especially bad for wine grapes because it definitely taints the wine.”

The San Joaquin Valley Grape Symposium was presented by the University of California Cooperative Extension and Fresno-based Allied Grape Growers. Other speakers included:

• Jeff Bitter, vice president of operations for Allied, a statewide 600-member wine grape and concentrate grape marketing association, who reiterated a bullish stance that he took at an earlier meeting of the San Joaquin Valley Winegrowers Association.

Bitter said a record 3.9 million tons of grapes were crushed into wine in 2012 but there was “no need to panic” over last year’s large crop. He cited declining acreage in Europe and a chart that showed shipments in line with production.

• Andrew Waterhouse, a professor with the Department of Viticulture and Enology at UC Davis, who discussed the effects of winter freezing in avoiding Pierce’s Disease.

Waterhouse talked of what is being termed a “cold curing” effect on vines that makes them more resistant to the disease. He cited studies on resistance to almond leaf scorch caused by the same bacterium linked to Pierce’s and said researches are looking at materials that could be used to enhance development of phenolic compounds that stave off disease.

• Kevin Fort, a post graduate researcher at UC Davis, who talked of the breeding of salt tolerant rootstocks.

Fort pointed out that studies done in a greenhouse setting and field setting often have wildly variable results. He said the exclusion capacity of two rootstocks, 140Ru and St. George, are now firmly established but more work needs to be done to characterize all commercially available rootstocks for chloride exclusion.

• Walt Bentley, an integrated pest management entomologist emeritus with the UC Kearney Agricultural Research and Extension Center, who discussed management of vine mealybug.

The pest produces honeydew that can lead to black sooty mold. It can be difficult to manage because of its movement on the vine. Bentley said two potential candidates for natural control have been imported and released in Riverside, Kern and Fresno counties.

The pest can be spread on contaminated equipment. It can also be spread if infested cluster stems or pomace is piled within the vineyard.