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Two new grape mites, the rust mite and the bud mite, can reduce yields from stunted shoot growth, as seen here.

Two new grape mites, the rust mite and the bud mite, can reduce yields from stunted shoot growth, as seen here.

New mites showing up in some Washington State vineyards are causing headaches for growers, but their presence indicates that vineyardists have reduced their pesticide inputs, says a university entomologist. Grape rust mites were found in vinifera and juice grape vineyards in the last two years, with bud mite and leaf curl mite found last summer, said Dr. David James, entomologist for Washington State University. “The mites could pose a threat initially but, ultimately, I believe they will be benign and will prove to be a good thing for Washington agriculture, providing more food for predatory mites.”

The three mites are tiny, about one-tenth the size of spider mites, and a handheld lens is needed to see them.

They are visible to the naked eye only when numbers get into the thousands per leaf, he said.

“You see their damage before seeing them.” James explained.

Rust mite

A warning sign of rust mite populations is bronzing of leaves observed from July through September, although no economic damage is occurring then. Bronzing or russeting of grape leaves can be subtle or the leaves almost blackened, as reported in Australia, he said. Populations can number up to 6,000 mites per leaf surface, and though the numbers are alarming, they are not believed to cause serious damage to the vine or affect fruit production or quality. Unlike spider mites, rust mites can be found on the upper sides of the leaves.

“It’s not worth it to spray when you see bronzing because there is no damage at that stage,” James said. But the bronzing does signal that control will be needed the following spring. He noted that the mites can cause other symptoms, including short or stunted shoots and distorted or cupped leaves, which often mimic other physiological disorders, like herbicide or thrips damage.

The rust mite overwinters in the vine wood, emerging in the spring to feed on buds and new growth. If overwintered numbers are large enough, mites will congregate on the limited leaf and bud tissue and cause serious distortion and stunting of leaves, even killing buds,
he said. James has identified 2,000 mites per shoot as the threshold for causing economic damage.

Bud mite

He thinks that bud mites have probably been in Washington for some time, but are now more prevalent as vineyardists use fewer and softer pesticides.

Bud mites are similar to rust mites, though a slightly different color. They, like rust mites, cause short shoots, zigzag growth, and can reduce yields. As the name implies, bud mites spend most of the year inside the bud, overwintering inside buds and emerging after bud burst, only to move into new buds.

“You can’t control them when they are inside the bud,” James said, adding that timing of sprays is critical to target the mite when it has emerged from buds.

2006 experience

The Walla Walla grape district had a major outbreak last year, he said, adding that Cabernet Sauvignon was the most affected variety. Significant damage occurred as certain growers lost up to 80 percent of their crop in some blocks, and in some vineyards, less than one ton of grapes per acre was harvested.

“We were taken by surprise in Walla Walla as there was no obvious leaf bronzing seen in 2005,” James said, adding that growers probably didn’t notice the tracking and scarring on shoots early in the season that were likely there from rust and bud mite damage.

“But it illustrates the point that if you do see bronzing, you need to do something the next spring for control.”

Bronzing is a good indicator of rust mites, but buds must be dissected to learn if bud mites are the problem.

In a sampling of Yakima Valley vineyards last October, James found bud and rust mites in all samples of wine grapes, and for the first time, on Concord grapes. “I have no idea how long they’ve been in Concords, though I’ve not had many complaints from growers of short shoots, except in one block.”

Numbers of mites in the Yakima Valley samples ranged from 26 rust mites congregated in one bud to up to 100 bud mites per bud. He found that the mites were not uniformly distributed in the vineyard, but clustered in different locations.

Control

He believes that predatory mites will eventually provide control of the rust and bud mites. Meanwhile, he recommends a sulfur spray program based on an Australian model until research here shows whether modifications are needed.

Growers should target the mites at the start of their spring migration to the buds and use two, well-timed sulfur sprays in high volume (4 to 5 pounds of sulfur to 100 gallons water).

The timing for rust mite is before bud break, between bud swell and bud burst, he said, adding that good spray coverage is also necessary. Timing for bud mite is slightly later, within seven days after bud burst.

“You need to get to the bud mite before it reproduces its eggs inside the buds,” he said. “Once it gets inside the buds, you can’t reach it, and you won’t get control.”

Sulfur may be toxic to new Concord growth, but he believes that it has potential if used for the earlier applications needed for rust mite.

“You want to avoid using miticides and avoid spraying in the summer,” he said. “Miticides are not as effective and will disrupt predatory mites and cause other problems.”

James is conducting research this year to study population numbers, control, and make sure that bronzing does not have an effect on vine fruitfulness.

“It’s important to treat all bronzed blocks,” he said, adding that he wants to hear from any growers this season who have mite problems. Growers can e-mail him at: david_james@wsu.edu or call him at (509) 786-9280.