Managing Plant-Parasitic Nematodes in Pacific Northwest Potatoes

Root‐Knot Nematodes

Several plant‐parasitic nematode species are found in Pacific Northwest (PNW) potato fields, and they can potentially cause agronomic damage alone or, in some cases, in conjunction with diseases.

Root‐knot nematodes are considered the Number 1 nematode pest of PNW potato production, Hafez says. Although they rarely cause aboveground symptoms in potatoes, very low populations can cause significant tuber damage.

Of the species found in the region, Columbia root‐knot nematode (Meloidogyne chitwoodi) tends to be more common and causes more severe tuber damage than northern root‐knot nematode (M. hapla).

Both are sedentary endoparasites. Once females establish permanent feeding sites within plant roots, they remain permanently at those locations although they may protrude from roots as they grow.

Columbia root‐knot nematode also has an extensive host range, surviving on nearly all crops grown in the region. This reduces the usefulness of rotations as management tools, Hafez says.

On the other hand, rotating to non‐host crops, such as cereals or corn, may help suppress or reduce northern root‐knot nematode populations.

Regardless of the nematode species, weed management is critical because many weeds are suitable nematode hosts.

Root‐Lesion Nematode and Potato Early Dying

Of the more than 15 root‐lesion nematode species found in Idaho, Pratylenchus neglectus is the most common and is likely found in about 95% of the state’s potato fields, Hafez says. It also is the most common lesion nematode in the Columbia Basin and Eastern Oregon potato‐growing regions. The related P. penetrans is generally found in potato fields that were previously planted to mint or fruit trees, and it is becoming more common.

Both are migratory endoparasites that tunnel inside and feed on roots, but they also can move back into soil and into new roots. Pratylenchus neglectus doesn’t necessarily cause tuber damage, but it can reduce overall plant vigor and yields. Of greater concern, Hafez says, is its interaction with Verticillium dahliae, the fungal pathogen responsible for potato early dying complex.

Marked by premature vine senescence, potato early dying can reduce tuber yield by up to 50% under severe conditions. Even in the absence of lesion nematode, Verticillium dahliae can cause potato early dying. But co‐infection with high populations of P. neglectus typically increases the incidence and severity of the disease.

Unfortunately, Hafez says, V. dahliae has a microsclerotia stage that can remain dormant in the soil for prolonged periods, surviving high and low temperatures and dry conditions. When moisture and host plant roots are present, the microsclerotia produce hyphae, or vegetative growth structures, that infect the roots. Root‐lesion nematodes also have a wide host range that includes both broadleaf plants and grasses.

These characteristics greatly reduce rotational options although planting biofumigant brassica cover crops has proven to significantly reduce root‐lesion nematodes as well as Verticillium populations.

Stubby‐Root Nematode and Tobacco Rattle Virus

While stubby‐root nematodes don’t necessarily cause economic tuber damage by themselves, they can vector tobacco rattle virus (TRV), the causal agent of corky ringspot disease.

“And that’s a major problem,” Hafez says.

As its name implies, corky ringspot produces internal dark brown corky necrotic areas considered quality defects within potato tubers. Depending on the size of the corky defect, tubers may be downgraded to culls. Under severe infections, up to 50% of the crop may be rejected as unmarketable.

Of the three stubby‐root species reported in the PNW, Paratrichodorus allius is the most common and is present in about 30% of soil samples, according to a February 2019 Northwest Potato Research Consortium report. Based on 2018 surveys, it is the third most common plant‐parasitic nematode, following root‐knot and root‐lesion nematodes.

In Idaho, stubby‐root nematodes have been found on about 60,000 acres of lighter soils in the Burley and Twin Falls regions although they are starting to move westward, Hafez says. Stubby‐root nematodes are ectoparasites, living entirely in the soil and feeding on root tips.

Complicating management of the disease complex is that TRV has one of the broadest host ranges of any virus. It can remain dormant in weeds for years, reappearing when potatoes are planted again. Among symptomless weed hosts are hairy nightshade, redroot pigweed, green foxtail, broadleaf plaintain, wild lettuce, sunflower, and purslane.

Field trials have shown growing alfalfa in TRV‐infested fields can reduce virus levels over time as long as host weeds are carefully managed.

Resistant or tolerant potato varieties are another management tool. After being grown in a heavily TRV‐infested field as part of a trial, Castle Russet from the Tri‐State Potato Breeding Program had no internal symptoms and tested negative for the virus, according to the February 2019 Northwest Potato Research Consortium report. Susceptible cultivars in the same trial had severely necrotic tubers.

Pale Cyst Nematode

The pale cyst nematode, sometimes called the white cyst nematode, is considered a major potato pest worldwide. It also belongs to the Globodera genus, commonly known as potato cyst nematodes. Left uncontrolled, potato cyst nematodes can cause up to 80% yield loss, according to USDA figures.

In addition, potato cyst nematodes can potentially disrupt export markets as trading partners may cut off U.S. imports if eradication efforts are discontinued, says University of Idaho’s Dandurand.

Pale cyst nematodes are sedentary endoparasites. Like other cyst nematodes, they can survive for prolonged periods in the soil as eggs in dead bodies of females. Even in the absence of potatoes, viable cysts may remain in soil for years.

During a routine survey in 2006, pale cyst nematode was discovered in tare dirt at a Blackfoot, ID, processing facility. Additional sampling led to confirmation of the pest in a 45‐acre field in northern Bingham County, Idaho. Because of its potential to disrupt domestic as well as international trade, the USDA quickly enacted a quarantine that regulated host plant material and other agricultural practices that could potentially spread the pest.

Currently, 6,353 acres in northern Bingham County and southern Bonneville County are regulated, according to a February USDA report. Of those, 3,542 acres are infested fields and 2,993 acres are associated fields.

As part of the quarantine, the USDA—with input from university experts—developed best management practices. Among those are sanitizing equipment, using only certified seed, rotating to non‐host crops, preventing reuse of bags and bins, minimizing human foot traffic, and planting cover crops to reduce wind‐and rain‐driven nematode spread.