Scouting for nematodes

When your soybean yields are dropping, or the plants are yellow and stunted, it’s easy to blame a dryer growing season.

These above ground symptoms are often confused with damage from compaction, nutrient deficiencies, drought stress, low-lying wet areas, herbicide injury and other plant diseases.

But before you investigate any of those factors, you might want to consider that there may be another culprit to blame.

The soybean cyst nematode (SCN) is a plant-parasitic roundworm that can make its home in your field and stick around for years. It’s an insidious little parasite and can initially cause significant yield loss with no noticeable above ground symptoms.

This is why many producers are unaware of the pest and are not actively managing for SCN until the infestation is beyond control, all the while spreading the injury with their tillage equipment.

Awareness and early discovery are the key to managing this parasite, says Laura Schmidt, production specialist with Manitoba Pulse and Soybean Growers (MPSG). She holds a M.Sc. in Plant Science from the University of Manitoba and has worked as part of MPSG’s research and production team since 2017.

She spends her time scouting soybeans, peas, dry beans and fava beans for emerging pest and production issues, identifying research priorities and providing agronomic advice to farmers.

Schmidt says the SCN is a relatively new pest in Manitoba, but its damage is already taking root in many local crops.

“It directly robs plant nutrients and damages plant roots, limiting water uptake and creating wounds for root rots to infect,” she says, describing the three main stages to the life cycle of the SCM.

The cycle starts in the spring when temperature and moisture levels are adequate for egg hatch to release the juvenile nematode. Once a juvenile penetrates a soybean root, it moves through the root to the vascular tissue. In the vascular tissue the nematode establishes a feeding site.

Eggs are produced mostly inside the female’s body with some of the eggs on the outside. The eggs on the outside of the body hatch and juvenile nematodes re-infect soybean roots. The egg-filled body of the dead female is what is referred to as the cyst. Each cyst can contain up to 400 eggs. There can be three to four generations of SCN in a single growing season, which is why it is currently the most yield limiting disease of soybean in Canada and the U.S.

The pathogen has been on the radar of ag experts for years.

Dr. Mario Tenuta and his lab have been surveying for SCN since 2012. They were first confirmed in Manitoba in 2019 at low levels (1-14cysts/5 lbs soil) consistent with recent establishment.

“There were no visible above ground symptoms, and populations were so low a commercial soil test likely would have missed them,” Schmidt says.

Higher cyst levels found in 2021 with visible above ground symptoms.

Scouting for nematodes

Schmidt says, strangely, the most common above ground symptom is a healthy-looking soybean crop. “At low levels of infection, plants will appear healthy above ground.”

That’s why it’s important to do a bit of scouting.

“Scout from late July through August, gently digging up roots to look for cysts. In clay soils, roots likely need to be soaked to remove soil. Cysts will be small, lemon shaped and whitish beige.”

Above ground symptoms with higher SCN populations may have areas of stunted plants with poor canopy or chlorotic growth. Commonly confused with drowned out areas or patches, SNC is more likely to produce visible symptoms on sandier soils in drier years.

SCN will only move an inch or so on its own. Even the tillage distribution in a field will be patchy in fields with more than three years of soybean or dry bean history. “When scouting, dig up plants from poor producing areas of the field.: headlands, approaches, previously flooded areas, low spots, high pH zones, along fence lines, near buildings, and storage areas.”

Prevention and detection

Since SCN is just getting established here, prevention and early detection are key tools. Schmidt advises, when possible, limit movement of soil. Clean soil off equipment, vehicles, and soil samplers between farms. Ensure any new-to-you equipment is clean before it enters your field.

Make digging up soybean roots a routine scouting activity in late summer.

Observation of adult females and cysts on the roots is one way to confirm SCN infestations in a field. Adult females appear as extremely tiny lemon-shaped bodies on the roots and are initially cream-colored. They later turn yellow and finally tan to brown as they mature to form the cyst. These are seen with the unaided eye and are much smaller than nitrogen nodules.

However, the absence of cysts on the roots does not mean a field is free of SCN. In fields with a low population, very few cysts may be found on the roots, and they may be easy to miss by visual observation.

Managing SCN

“Once established in a field, it can’t be eradicated, only managed. The whole goal is to limit egg and cyst numbers so that they’re low enough during your soybean year, that you can still produce a profitable crop,” Schmidt says.

She advises you to get a soil sample test to determine egg numbers and risk before growing soybeans. “Rotate to non-host crops and control host weed species. Determine your numbers and your risks and monitor them over time. Rotating to non-host crops in the odd years.”

She also advises that when soybeans are grown, you use SCN-resistant varieties and rotate them, and reduce tillage to limit spread throughout the field and among other fields.

“Distribution will stay pretty patchy. It’s just one of the ways that you’re moving soil and moving cysts and nematodes around.”

There is some variability between varieties, Schmidt says “There is certainly a lot more research needed in terms of varieties and how SCNs impact each of these.”

Nematode protected seed treatments do provide some protection, but they do need further testing. “We’ll be testing them in the next year or two with Dr. Tenuda as the next round of research funding takes off.”

Along with SCN, you typically get Sudden Death Syndrome (SDS), which is a root and stem disease. “They’re partners in crime. The symptoms show after flowering. This hasn’t been confirmed in Manitoba yet, but we do think we’ve seen a few cases of it already.”

With SCN being the number one disease threat, SDS falls into the fifth or six in terms of impact on yield.

“It’s here, working in fields undetected. We need to be digging. Early prevention and detection are key. We do have resistant varieties for now, but this won’t be a solution in the long term.”

The only way to know a field is free of SCN is with a soil test.

If cysts are found, contact MPSG, Manitoba Agriculture or Dr. Mario Tenuta

Soil test numbers will vary. SCN distribution will be patchy and aggregated. Soil tests may be taken ahead of the soybean crop. In-season in soybean root zones or post-harvest targeting soybean rows.

There is some variability between varieties, Schmidt says “There is certainly a lot more research needed in terms of varieties and how SCNs impact each of these.”