Preplant soil fumigation has long been an effective way to control nematodes, but fumigation in the future may be limited, says Dr. Inga Zasada, U.S. Department of Agriculture scientist.

She offers the following suggestions to help ­vineyardists manage nematodes in the event fumigants are unavailable.

1. Planting material—Use certified plant material to avoid bringing nematodes into your field. Choose a grape variety based on the kinds of nematodes you have. “If you sample and have northern root knot and can’t fumigate, you may want to stay away from white varieties,” she said.

Consider using nematode-­resistant rootstock, matching rootstock resistance to the nematode species in your field (read “Nematode-­resistant rootstocks available”).

2. Fumigation options—If you can fumigate, remember that fumigants are not as effective if plant residue remains in the field so it’s important to remove as much root material as possible. Sedentary nematodes, like root knot nematode, reside in root material.

Nematodes inside nondecomposed plant material will not be controlled by fumigation. By applying knowledge about nematode biology, she sees an opportunity for spot treatment instead of fumigating the entire field. For example, depending on the species, fumigation may only be needed under the vine row that follows the old drip irrigation line.

3. Cover crops—Mustards, Sudan grass, and arugula have been grown as a cover crop in fallow fields to reduce nematode densities. However, growing cover crops in an existing vineyard in the row alleyway may not be very effective if roots and nematodes don’t reside there. Additionally, it may be difficult to find a cover crop that is not a host of the nematode. For ­example, mustards are hosts of root knot nematodes.

4. Postplant grape nematicides—Seventeen postplant nematicides are registered in Washington for grapes, Zasada said, but there is little efficacy data from Washington vineyards. Also, the mode of action on some nematicides is not well understood. “We don’t yet have a good handle on the economic impact of population densities or a lot of information on the efficacy of products.”

Economic threshold numbers (population numbers which cause economic impact) used in California and Australia aren’t applicable in Washington, but she noted that a sample of 100 root knot nematodes per 250 grams of soil is a level that warrants action.

Data has shown that active nematode developmental stages, such as juvenile stages, are more susceptible than adult resting stages. In a trial with Cordon (1,3-dichloropropene) applied to an existing vineyard through drip irrigation, fall applications reduced the nematode populations better than applications made in spring when most of the population is in the egg stage.

“So if you apply in spring when there aren’t that many active juveniles, the chemical won’t be as effective as fall applications when populations are peaking.”