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Tuesday, Dec 5, 2023

AGRICULTURE–No Major Local Impact Seen From Vineyard Pest

Agriculture: Grape Crop Not Large Here; Citrus Crop a Possible Target

San Diego County is largely free of the devastating effects of a tiny insect killing off grapevines throughout Southern California, but growers of other crops , including nursery products, avocados and citrus , could still feel some impacts.

The glassy-winged sharpshooter, a parasite that feeds on a number of popular plants, was discovered in 1998 in Temecula and has since threatened a number of crops throughout Southern California.

“We’re losing vines,” said Ben Drake, a grower in Temecula whose 250 acres of vineyards are under siege. He estimated 90 to 95 percent of the vineyards in Temecula have shown signs of infestation.

“(It’s) affected most of the vineyards around here, somewhere around 20 percent. Callaway (Winery) has 750 acres, and they’ve removed about 120.”

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The insect goes from vine to vine, spreading plant bacteria as it feeds. These bacteria cause Pierce’s disease, in which the vines’ xylem system becomes clogged, making it impossible for water and nutrients to get from the stem to the rest of the plant, according to information from the University of California Cooperative Extension office.

Plants wither, and the grapes fail to mature, drying on the vine, according to the UC extension office.

Like A Vineyard Cancer

Most vines in Temecula are doing well , even the ones that may have Pierce’s disease, since the plants aren’t under much stress when the weather is cool, Drake said.

That will change as temperatures warm up. Plants put out more leaves, which require more water. The fruit on the vines are trying to mature, placing a heavier load on the xylem system, he said.

Once infected, the vines must be cut back in the same way a surgeon would remove a cancer. But biologists from UC Davis are working on possible cures, such as injecting the vines with antibiotics or zinc, Drake said.

Drake hopes the treatments work as an alternative to cutting the vines back.

“It’s so costly to put back in a vineyard or replace a vine, because you’re out of production for such a long period of time. So we’d like to try to save the vines that we have, to keep production up,” he said.

San Diego County doesn’t have as much risk for Pierce’s disease, largely because there are so few acres of land here devoted to grapes. Grapes generated only $240,000 worth of sales in San Diego County last year, said Eric Larson, executive director of the San Diego County Farm Bureau.

Other Crops In Danger

Still, other crops could be at risk. In addition to infecting grapevines, the insects can feed on other plants, causing other diseases with names like almond leaf scorch, alfalfa dwarf, oleander leaf scorch, and citrus variegated chlorosis, according to the UC extension office.

Of special note is the fact that glassy-winged sharpshooters tend to live and lay their eggs in avocado and citrus trees , two of the three biggest crops in San Diego, with sales of $130 million and $73 million last year, Larson said.

Avocado trees are not susceptible to sharpshooter-borne diseases, but citrus trees are. So far, however, Larson has not seen any evidence of the disease spreading to citrus, he said.

No orange trees in the area have shown signs of disease wrought by the insects, said Al Stehley, board member of Riverside-based orange packers Corona College Heights. Stehley, whose firm packs oranges from San Diego County and elsewhere, noted sharpshooters here do not carry the particular bacterium that harms citrus trees.

Stehley has much sympathy for the owners of the county’s few vineyards. The owners are all small farmers and likely would not be able to survive having to take out a large percentage of their vines, he said.

Larson noted there were a few isolated cases of Pierce’s disease in San Diego County a few years back. But these probably weren’t caused by sharpshooters. Other insects may have been the cause, or perhaps the plant was inadvertently infected by human hands, he said.

Other Impacts Noted

Even though the county is relatively free of the disease, the sharpshooter still affects the local economy. Nursery products , the largest agricultural crop in the county, with sales of $640 million last year , have felt “some grief” from precautions taken against San Diego plants.

So far, 10 California counties , mostly prominent wine growing areas , have demanded inspections before accepting any plants from San Diego. Inspections are needed at both ends of the shipment, Larson said.

The problem is many of the county’s nurseries are nestled between citrus orchards and avocado growers. So they’re getting it “from both sides,” said Sharon Geraty, deputy directory for pesticide and plant protection with the county’s Department of Agriculture-Weights and Measures.

Geraty’s department is working with the local nurseries on inspections and certification to keep local crops free of the pests. That should keep the problem under control, she said.


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