Gaining on mites

Short and stunted shoots are signs of damage from two new mites found in Washington State, grape leaf rust mite and bud mite.

Washington State grape growers experienced minor damage from two relatively new mite species last season, but vineyardists shouldn’t let their guard down yet. Populations could rebound in 2008, warns a Washington State University entomologist.

Dr. David James said that in 2006, crop losses of up to 80 percent were caused by the mites in some Walla Walla vineyards. "But 2007 was a good year for growers in regards to the mites."

The rust and bud mites, first detected in Washington State in 2002, are very different from spider mites, he said. Both are tiny, about a tenth of the size of spider mites, and their shapes are described as worm- or ­torpedo-like. "You need a very good hand lens to see them, and even then, you may only see movement."

Dduring the annual Washington State Grape Society meeting last fall in Grandview, James said he believes that the two mites have always been present but were kept in check by broad-spectrum insecticides. The higher populations now found are a consequence of the industry’s efforts to reduce chemical sprays, he added.

Widespread

In 2004, growers observed bronzing of the leaves in the vineyard in late summer, particularly in the Mattawa area, he said. By 2005, juice and wine grape growers reported widespread leaf bronzing throughout the Yakima Valley.

The first major economic damage caused by mites occurred in Walla Walla Cabernet Sauvignon vineyards in 2006. Last year, the mites tended to disappear. "From all the reports we’ve received, the numbers were very low and didn’t cause any economic problems," James said. "The populations just never took off in 2007."

While he is not sure why there were such low numbers of mites in 2007, he believes several things may be ­working in favor of growers:

• There was very good widespread adoption of the ­sulfur spray strategy.

• Populations of predatory mites are beginning to develop.

• The overwintering survival of rust mites was poor.

Warning sign

The rust mite spends most of the growing season on the leaves, which is manifested by bronzing of the leaves. In severe cases, as James saw in Australia, the leaves can become blackened and fall off, although he hasn’t seen damage at that level in Washington.

"But there is no evidence that bronzing causes economic damage," he stressed. "Bronzing of leaves is not by itself a trigger to spray at that time," he emphasized. "But it is a trigger to spray in the next year."

Bronzing is considered a warning sign of a developing population of rust mites, James explained. It takes high numbers of mites to cause bronzing—from 1,000 to 6,000 mites per leaf. The best opportunity to see rust mites is to examine bronzed leaves in full sun with a hand lens, he said, adding that they like the hot sun.

If growers miss the bronzing, they will notice restricted spring growth and short shoot growth in early spring. Rust mites overwinter on the vine wood, emerging in the spring to feed on buds and new growth. If overwintered numbers are high enough, mites will congregate on the limited leaf and bud tissue, causing distortion and stunting of leaves, bud damage, and reduced yields. James noted that populations of about 2,000 mites per shoot are required before shoot distortion and damage occurs, data from Australia ­indicate.

"The Walla Walla growers were not aware that they had a problem until it was too late," he said. Walla Walla growers had particular problems with short shoot growth. Cabernet was the most affected variety, with some vineyards yielding less than one ton per acre in 2006.

Rust-mite damage of distorted, cupped leaves is sometimes confused with thrips and herbicide damage. James thinks that some of what growers thought was damage from thrips in the past might have been damage from rust mites.

Bud mites

Bud mites are similar to rust mites, though a slightly different color. Rust mites can also be found occasionally inside grape buds. Bud mites are not as free living as rust mites, he noted, as they spend 11 months of the year living within buds. They emerge after bud burst and move into new buds. Like rust mites, they can cause short shoots, and zigzag growth. In sufficient numbers, bud mites kill and damage buds.

"If you see a lot of bud damage and dead buds, you could have bud mite," James said. "But the only way to know for sure is to cut buds in the winter to ­monitor for mite."

He noted that they have found bud mites in several different vineyards and locations, but in low numbers. Ten mites per bud are probably not enough to cause problems, but 50 or more mites per bud will result in some damage.

Predators

James and his research team are tracking predatory mites associated with bud and rust mites that they have found in vineyards, especially in juice-grape vineyards where the predator numbers seem to be increasing. Higher rust mite numbers have been found in juice grapes than in wine grapes in 2006 and 2007, he said, adding that Washington is the first region to report this species of rust mite on juice grapes.

One of the preditory mites, also found in apple orchards, was found in significant numbers of 140 mites per leaf in a juice-grape vineyard, James reported.

Research suggests that the predator mites might help suppress bud and rust mites in late autumn, late winter, and early spring, he said.

Growers should be prepared for damaging spring populations in 2008 if they had some leaf bronzing last year, he warns. If growers are unsure if they had bronzing and didn’t check buds during winter, he suggests they follow a spring sulfur-spray strategy. Avoid using a broad-spectrum pesticide, to allow predatory mites and other predators to move in. 

Growers who see leaf bronzing, stunted, or distorted shoot growth are encouraged to contact James at david_james@wsu.edu.