Dr. Michelle Moyer, Washington State University

Michelle Moyer

While many growers in California have turned to rootstocks to provide resistance to nematode damage, the pests haven’t been documented causing losses for Washington growers.

But with industrywide replanting, nematode pressure may become a bigger issue, Washington State University Extension viticulturist Michelle Moyer says.

“In a 30-year-old vineyard you could have a high density of nematodes, and the plants might be fine because the massively established root system can withstand the feeding,” Moyer said in January in a presentation to the Washington State Wine Commission, which has funded her research into the issue. “But you rip that out and put in new plants and tiny roots, well they are going to be massively impacted by those nematodes.”

In 2014, Moyer teamed up with U.S. Department of Agriculture plant pathologist Inga Zasada, who specializes in nematode parasites of fruit crops, to plant a test vineyard to evaluate nematode management through fumigation, use of rootstocks and novel pesticides.

Their initial finding was that impact of pre-planting fumigation only lasts about 12 months, Moyer said. If you fumigate in the fall and plant in the spring, as they did, that means nematode populations are already building back up about six months after planting, she said.

Specifically, they were surveying for northern root knot nematode, a tiny parasite that lives most of its life nestled inside root tips, siphoning nutrients from its host plant.

A close relative, the southern root knot nematode, is a significant problem in California vineyards.

They damage their hosts by reducing nutrients and stunting root growth, Zasada said. In greenhouse studies, she’s seen damage rates threefold higher in white varietials, perhaps because those vines tend to have more of the fine roots that the nematodes target.

The damage often goes undetected because the symptoms — reduced growth and yield — could be attributable to many factors. “Growers will say, ‘We don’t see any impacts from nematodes,’ and I say,

‘How do you know that?’” Zasada said. “For a pest that you can’t see, being able to parse that out is difficult, if not impossible.”

And right now, there’s still no good assessment of the impact root knot nematode is having on Washington vineyards. It’s difficult to assess because the damage accumulates slowly over time through lost root productivity.

“Nematode damage in perennial crops is a history lesson. We can’t just throw a vine in a pot with a bunch of nematodes because the response isn’t seen until a few years down the road.” Zasada said. “But I think we’re on the road to success.”

Zasada and Moyer expect their experimental vineyard to offer some insight in the next few years as they compare pruning weights and yields from nematode resistant and susceptible vines.

Rootstock potential

The experimental vineyard includes vines grown on several rootstocks that showed northern root knot nematode resistance in Zasada’s earlier greenhouse studies, along with own-rooted and self-grafted vines.

Part of the vineyard was fumigated prior to planting, part was not, and in some areas the researchers even added additional nematodes.

Data from the first year look promising for the rootstocks. “The rootstocks did what they were supposed to do, even when we inoculated the soils with really high nematode pressure,” Moyer said.

“Rootstocks, I think, are really the future,” Zasada said. “We’re going to be losing fumigants over the next decade or two (due to environmental regulations), so if growers do have a nematode problem, their back is going to be against the wall. That’s why rootstocks are appealing as a nonchemical, very efficient way to address the nematode problems.”

Another aspect of their study looks at which commercial nematicides may be effective for controlling a nematode population after the vineyard is planted.

Two products, which currently are not labelled for use in wine grapes, appear quite promising for controlling the root knot nematode, Moyer said. But the pesticide only works when the nematode is actively moving through the soil during its second juvenile phase, Moyer said. Those juveniles then infect new root tips.

She’s also developing a model to track how the nematode phenology responds to soil temperature and root flushes so that growers can target treatments effectively.

Over the long-term field trial, the researchers also want to look at how viticultural practices such as nutrient and irrigation management can help vines outgrow susceptibility to nematode pressure. They are seeking additional funds to continue the study.

Eventually, the research should be able to help growers understand more about managing for nematode pressure, such as what population density warrants action — whether that’s by applying nematicides in existing vineyards or by considering rootstocks when replanting. •

– by Kate Prengaman