UC researchers' latest findings on major grape diseases

UC researchers' latest findings on major grape diseases

UC researcher Kent Daane says the vine mealybug is the number one culprit in spreading leafroll viruses in grapes in the San Joaquin Valley. The pest can pick up the virus within one hour and then inoculate another vine within the same amount of time. 

Red leaves are a welcome sight for those who travel in the Eastern U.S. to view autumn colors.

But they are not appreciated in vineyards in California and elsewhere, where they can be a sign of the diseases grapevine red blotch or grapevine leafroll.

“This is not Massachusetts,” said Kent Daane, specialist at the University of California’s Kearney Ag Center in Parlier. Leaves on healthy vines in California simply turn yellow and fall off the vine, he said.

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Daane talked of a major culprit in the spread of leafroll associated viruses, the vine mealybug, and whether spraying insecticides alone can eliminate the threat (Spoiler alert: It can’t.).

Other speakers at a San Joaquin Valley grape symposium in Easton looked at ways to identify diseases in vines, an old nemesis (the glassy-winged sharpshooter which can spread Pierce’s disease) and new regulations governing the use of volatile organic compounds.

40 percent yield decline

Daane explained the leafroll viruses that cause chlorosis and rolling of leaf margins downward and can cut berry yields up to 40 percent. They can delay fruit maturity and impede fruit pigmentation.

He said that in the Valley particularly the vine mealybug is the number one culprit in spreading the virus. It can pick up the virus within one hour and then inoculate another vine within the same amount of time.

In the Valley, there can be five to six generations of the vine mealybug in a year.

Daane and other researchers have found that spraying insecticides to protect against spread of the disease is not effective since various strains can be transmitted from neighboring vines into a clean block.

They recommend an area wide approach which also includes pheromone trapping and mating disruption, along with the removal of infected vines.

No super flu 

Taking a look at the various viruses, researchers also determined that the pathogen has not changed over time.

“It’s not a super flu,” Daane said. “There’s nothing that novel.”

He cited a number of pesticides that can kill mealybug, but pointed out that the vine mealybug is tough to eradicate since overlapping generations can be spread throughout the vine. 

As to the spread of grapevine red blotch, Daane said research has found no instances which indicate that leafhoppers carry the virus from one vine to another.

Incidentally, grapevine red blotch is not a newcomer to the state, said Maher Al Rwahnih, associate project scientist with the Department of Plant Pathology and Foundation Plant Services at UC Davis.

Studies of plants from the foundation’s Herbarium, which has 300,000 dead plants that are decades old, showed the presence of the disease in a vine from Sonoma County dating back to 1940.

Al Rwahnih touted next generation sequencing as the best approach for identifying viral pathogens in vines. But he cautioned that the approach does not prove that a particular pathogen causes a disease.

Other approaches include a biological assay, serological methods, and molecular tests.

Pierce's disease

Just as red blotch dates back to the 1940s, so does Pierce’s Disease, said Allison Ferry-Abee, UC viticulture farm advisor for Tulare and Kings Counties.

And with continuing finds of the glassy-winged sharpshooter, a more efficient vector of the bacterium that causes Pierce’s, growers need to continue to keep their guard up, Ferry-Abee said.

The glassy-winged sharpshooter flies farther than native leafhoppers in California, and it can feed on the wood of two-year old vines, she said.

She said late season signs of infected vines include scalded leaves, dropping of leaf blades leaving petioles attached, irregular wood maturity with green and brown patches, and fruit shriveling.

Ferry-Abee recommends training workers to look out for the symptoms.

“Be vigilant,” she said.

Any infected vines should be removed, she said. “Severe pruning only ‘cures’ vines about 30 percent of the time.”

Vector management

Fred Rinder, deputy agricultural commissioner with the Fresno County Department of Agriculture, talked of the county’s strategy to manage the insect vector.

It centers on a treatment strategy that begins at the perimeter of glassy-winged sharpshooter infestations in the Fresno-Clovis urban core and aims at containment within what he called a “ring of fire” or “horseshoe.”

Rinder has seen sharpshooter numbers decline by 50 percent in the past two years, but he does not know if that’s because of the county’s treatment approach. He said it could be due to the use of bio-control agents, natural predators, or freezing conditions in recent years.

Rinder said the number one host for the glassy-winged sharpshooter is the crepe myrtle. (“We call it the crap myrtle,” he said.) Control is hampered since flowering trees can’t be sprayed and the ground hardens in the summer which makes drenching ineffective.

The Easton meeting opened with a look at new regulations governing the use of chemicals high in volatile organic compounds. Beginning this year, they cannot be used between May 1 and Oct. 31.

Their volatilization helps create smog, said William Griffin, supervising ag standards specialist with the Fresno County Department of Agriculture.

The regulations apply to grapes, alfalfa, almonds, citrus, cotton, pistachios, and walnuts.

One option is to switch to lower VOC products, Griffin said.

He said the use of Telone, a fumigant commonly used on ground where vines were removed to make for almonds, is not allowed during the month of January. He recommended soil testing for nematodes before the costly step of fumigation.

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