Drift is a chronic grape problem

An example of phenoxy herbicide drift damage to grapes.

Although much progress has been made in reducing vineyard injury from phenoxy, hormone-mimic herbicides, don’t expect the problem to drift away, says Dr. Vince Hebert, Washington State University associate Extension specialist.

“You can expect some chronic level injury,” he said, referring to many areas within the state that continually seem to have drift issues.

Hebert, who is the laboratory research director of WSU’s Food and Environmental Quality Lab at its Tri-Cities branch, is involved in a regional air-monitoring program focusing on off-target herbicide residues and their impact on grape production.

Herbicide drift has plagued Pacific Northwest vineyardists since World War II and the development of herbicides that mimic plant auxin hormones, like 2, 4-D, he said. Hormone-mimic herbicides work at very low concentrations, slowly metabolizing in the plant, he explained.

In recent years, wine grape production in Washington has expanded to areas like Paterson, Alderdale, Walla Walla, and even the north central Cascades. Phenoxy drift problems on grapes have also increased.

“Off-target movement of the hormone-mimic herbicides is still a serious and complex problem in Pacific Northwest wine production,” Hebert said during a recent grape grower meeting.

Symptoms

Severity of drift symptoms depends on timing and level of exposure, with greater exposure disrupting the vine physiology and resulting in stunted growth. Grapevines are most sensitive from mid-April to June, from bud break to cluster initiation. If exposure occurs during this time, damage can include shot berries and reduced yields, he said. However, if exposure occurs later in the season, some vineyards can grow through the setback and still have good yields.

In most cases, white varieties are more susceptible than red varieties.

The off-target movement in the Northwest is usually a regional transport issue, he said, adding that particle drift typically moves from volatilization, but can also move by getting into the air mass caused by atmospheric inversions. Additionally, herbicide residues can move with water, drop as precipitation, or move as airborne dust.

Long-distance movement of herbicides is not usually in a high enough concentration to affect production, but it is not controllable, Hebert stressed. Occasionally, the residues are high enough to cause problems; a Walla Walla vineyard he sampled last year lost 50 percent of production because of drift occurring from rainfall.

“Low pressure conditions, cloud cover, a well defined low-level wind flow, advection, or precipitation can all add up to the perfect storm for drift problems,” Hebert said. “It can travel for miles; therefore, we must take a more regional perspective. We share some appellations with Oregon, and we need better coordination between the states.”

Washington State University has worked with cereal grain growers within the state in developing best management practices for herbicide application. Label requirements specify the type of phenoxy esters that can be used in the state and when, such as not using certain formulations during time of critical grape sensitivity. However, Oregon does not have specific conditions on the labels of potentially damaging herbicides, although Umatilla County has a voluntary program to reduce drift on grapes.

Leaf indexing

In cooperation with the Washington State Department of Agriculture, WSU developed a leaf indexing and severity rating to help grape growers document off-target movement of broadleaf herbicides and characterize symptoms and damage. The leaf indexing form will be used to accompany herbicide exposure complaints to WSDA to provide better information on herbicide-related symptoms and injury.

The form requires that weekly vineyard observations be made, tracking the same shoots from bud break through bloom and vine hardening. Leaf position dating—identifying the date when the leaves unfurl—is important because there is a lag period between exposure and when symptoms appear.

The form asks growers to rate the severity of injury, from 1 to 5, with 5 being heavily affected. Growers are also encouraged to take date-stamped photos to document injury.