Nematode management in the future will require a new approach, says a U.S. Department of Agriculture scientist. As soil fumigation becomes more restrictive, it will take new ways to control a growing problem in Washington State vineyards.
Though most nematodes—microscopic roundworms that live in soil and water—are beneficial, plant-parasitic nematodes are not and cause economic damage by feeding on plant roots. Some nematode species can transmit viruses.
“We keep losing nematode management tools,” said Dr. Inga Zasada, U.S. Department of Agriculture research plant pathologist based in Corvallis, Oregon. “We don’t have the nematode management tools that we had 40 years ago, and because of that, nematode management practices in the future will probably not be as effective as they were in the past.”
The U.S. Environmental Protection Agency initiated a first wave of new soil fumigant labels as part of its reregistration process. The new labels included such things as fumigation management plans and buffer zones.
But the EPA is likely to impose more onerous restrictions when the agency completes its most recent review of soil fumigants, such as chloropicrin, dazomet, 1,3-dichloropropene, methyl bromide, methyl isothiocyanate, and metam sodium/potassium.
Zasada is working to develop sustainable plant-parasitic nematode management systems for small fruits and grapes. Her research encompasses developing production systems that integrate a range of tools to promote root health and suppress nematodes, as well as providing information on management strategies.
Zasada, who spoke during a session at the Washington Association of Wine Grape Growers annual meeting, believes that a better understanding of nematode biology—knowing when they are most active, where they reside in the soil, and what host plants they like—will help growers improve management strategies, especially if soil fumigation becomes more restrictive in the future.
Nematodes in Washington
“We do have nematodes here,” said Bill Riley, senior viticulturist for Ste. Michelle Wine Estates, Washington’s largest wine producer.
Riley said, during a panel discussion, that soil samples taken from several of Ste. Michelle’s older vineyards, like Cold Creek Vineyard, contained plant-parasitic nematodes. Chardonnay and Riesling vineyards had around 100 to 120 nematodes per sample, but one vineyard sample had 7,000 per sample.
“We expect to see nematodes in old vineyards, especially where vines are 30 to 35 years old,” he said, adding that several Ste. Michelle vineyards are scheduled for replanting. “But we’re concerned about going forward,” he said, noting that some of the vineyards they will be replanting are certified under Salmon Safe and LIVE (Low Input Viticulture and Enology), both sustainable programs that prohibit soil fumigation.
Todd Crosby, agronomist for Mercer Canyon Vineyards, Prosser, agrees that nematodes in vineyards are a problem.
“You can manage nematodes in existing vineyards by additional irrigation and nutrients, but in the long term, it’s a growing problem.”
Crosby believes there is potential to develop a nematode degree-day model that could help growers time postplant nematicide applications when juvenile nematodes are most active. Research from the potato industry indicates that this approach might be possible in grapes.
The main four plant-parasitic nematodes of concern in Washington are northern root knot (Meloidogyne hapla Chitwood), dagger (Xiphinema americanum), root lesion (Pratylenchus spp.), and ring (Mesocriconema xenoplax).
A statewide survey conducted several years ago found about 60 percent of the surveyed vineyards had high populations of root knot and dagger nematodes and 45 percent of the vineyards had root lesion nematodes, but at low populations. Ring nematode was found in 14 percent of the vineyards.
Life cycles of plant-parasitic nematodes include the egg, four juvenile stages, and one adult stage, Zasada said. Most nematodes complete their life cycle in 20 to 40 days. For some nematodes, two to three generations are possible per year.
Knowing when nematodes are most active in the soil can help growers collect more accurate samples and better time nematicide applications, she said.
Root knot nematode is a sedentary endoparasite that lives most of its life within plant tissue. Some 300 to 400 eggs are laid on the root surface or in the root. Migration in the soil occurs at the second juvenile stage.
More than 550 host plants are known, including potatoes, alfalfa, dandelion, and lamb’s quarters. In their research, Zasada and a team of scientists found root knot nematodes concentrated in the top 18 inches of soil in clearly defined areas that follow water (drip lines) and roots.
Dagger nematode is a migratory ectoparasite, spending all of its time in the soil living on the exterior of roots. Dagger nematodes can transmit tomato ringspot virus and grape fanleaf virus to grapes. The species that transmits tomato ringspot virus, X. index, has not been found in Washington vineyards.
The list of host plants is extensive and includes broadleaf weeds, grasses, potatoes, and more. Dagger nematodes were found indiscriminately in vineyards at various depths in the soil.
Ring nematode is a migratory ectoparasite and spends its life in the soil. Hosts are mostly woody plants. Ring nematodes were located in similar depths as root knot, and were found in defined areas of water and roots.
Root lesion nematodes are not a major species in the state. Zasada and researchers found little evidence of root lesion nematodes parasitizing vine roots.
The research team has evaluated the susceptibility of commonly grown own-rooted varieties. In microplot trials in the field they found that Chardonnay supported greater root knot nematode populations than Cabernet Sauvignon.
In the greenhouse, the root knot nematode populations at the end of the experiment were three times higher on Chardonnay and Riesling than on several red varieties.
While there are still gaps in the knowledge base in Washington, Zasada said that white varieties appear to be particularly sensitive.
“If you are going into a site with northern root knot nematode, I recommend that white varieties be avoided,” she said.
A long-term project is under way to learn how nematodes impact vine establishment and productivity of a vineyard.
Three white and three red varieties have been planted into areas of a field either fumigated or not.
Population dynamics of plant-parasitic nematodes, including root knot, as well as their impact on vine productivity, will be monitored during the coming years. •