By the time Manitoba confirmed soybean cyst nematode (SCN) in 2019, alarms in the U.S. were already sounding about resistant genetics no longer passing muster.
In 2021, a field in central Manitoba had developed enough of an infestation to start displaying above-ground symptoms.
The latest information has SCN confirmed in five Manitoba municipalities as of the 2024 crop year, and Manitoba’s pulse and soybean experts are warning producers to stay vigilant and be proactive against the pest.
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“The situation was made worse this particular year because it was a dry year, with more stress on the plant, and those symptoms were showing up even more,” said Manitoba Agriculture pulse specialist Dennis Lange — one of the speakers at Manitoba Ag Days in Brandon in January.
WHY IT MATTERS: Soybean cyst nematode is a known soybean yield eater in places like the U.S. and Ontario.
SCN is a microscopic roundworm that infects soybean roots. It hurts plant growth by depleting nutrients, disrupting water and nutrient uptake and hindering root development. It can also lower root nodule numbers, impairing nitrogen fixation, and can make plants more vulnerable to diseases like root rot and seedling infections due to the cysts on the roots. Once the farmer notices visible symptoms, yield loss can reach up to 30 per cent, resources from the Manitoba Pulse and Soybean Growers warn.
It’s been confirmed in the municipalities of Thompson, Norfolk-Treherne, Rhineland, Emerson-Franklin and Montcalm.
“What we see is a plant or root that has all these little nodules or little bumps … lemon-shaped cysts,” Lange said.
Manitoba’s SCN fight
SCN is counted as one of North America’s most destructive pests for soybean production. Effective management involves prevention, early detection and timely intervention to reduce yield loss, according to information found on the Manitoba Pulse and Soybean Growers website.
With Manitoba’s surge of soybeans, experts knew that groundwork was also being laid for soybean pest and disease issues.
Mario Tenuta and his team at the University of Manitoba embarked on a surveying program for SCN, starting in 2012. In 2019, SCN was finally identified using both visual and molecular DNA methods in four of 106 fields and four of 18 municipalities surveyed that year.
Those first confirmed cyst populations were low, ranging from one to 14 cysts per five pounds of soil.
In 2021, where visible symptoms eventually led to a diagnosis of SCN, the populations were around 32 cysts, or 1,350 eggs, per 100 millilitres of soil. An agronomist investigating a soybean field with poor growth and yellowing found the tiny, lemon-shaped cysts on the roots. Tenuta’s lab later confirmed the presence of SCN.
Prevention
Once SCN establishes itself, it cannot be fully eradicated and can spread rapidly through any movement of soil. Cleaning equipment, vehicles, soil sampling tools, and clothing can help prevent further spread, Ag Days attendees heard.
“Equipment brought in from other areas potentially could have had SCN on it. And if you’re doing that, make sure that equipment is clean before you put it in your field,” Lange said.
SCN eggs can survive for several years without a soybean crop, but the worms do require a host plant to reproduce. To manage SCN populations, practices such as crop rotation with non-host plants, growing resistant soybean varieties, reducing tillage and controlling host weeds are recommended.
“Trying to incorporate those soybean cyst nematode [resistant] varieties into your into rotation will help you set up for success,” Lange said.
But genetic tools against SCN have also become more of a quandary in recent years. Materials from the SCN Coalition —a cross-border, multi-stakeholder group dedicated to management of the pest — have tracked the slide in resistance from the PI 88788 breeding line, for decades the go-to genetic solution against SCN. The organization counts resistance as effective if less than 10 per cent of SCN in a field is able to reproduce. A fact sheet on their website notes that states like Iowa, Missouri, Illinois and Michigan are showing 88-100 per cent of SCN beating those levels against PI 88788.
“On some farms, one out of every two nematodes can reproduce,” it notes.
Scouting
Scouting for SCN should be done from late July to September, during the worms’ reproductive stages, by gently digging up roots and looking for cysts. It is essential to soak the roots in water to prevent breaking off cysts, producers were warned.
The most high-risk areas for scouting include low-yielding zones, field edges, low spots, shelterbelts and high pH areas.
“Typically with SCN, you’re going to find it more in the low areas. You’re going to find it along the fence lines. You’re going to find it in depression to the field and the headlines, those types of things,” Lange said.
One challenge lies in how easily SCN damage can mimic other problems or crop stresses. At low infestation levels, plants may appear healthy, while high levels can result in stunted, chlorotic or necrotic plants, often resembling nutrient deficiencies or drought stress.
In 2024, like in 2021, plants in Manitoba were actually showing visual signs that something was wrong. The best way to identify SCN is by digging up and examining the roots. The tell-tale cysts will be smaller than root nodules and may require magnification.
“Most years, when Mario has been doing his research, he’s been able to find cysts in the soil, but that’s only after digging up the soil, taking it back to the lab,” Lange said.
The best way to identify SCN is by digging up and examining the roots. Below ground, SCN is identifiable by white, lemon-shaped cysts on the roots, which are much smaller than root nodules and may require magnification.
Since SCN can look like iron deficiency chlorosis (IDC) producers should not rule out either issue when they notice symptoms, Lange said.
“If you see something such as what you think is IDC through the season, and it’s not going away, call us in and have us get a look at it,” he said.
If SCN is suspected, producers should contact the Manitoba Pulse and Soybean Growers or Manitoba Agriculture. Soil samples can be submitted to Agvise Laboratories for analysis.
Samples should be collected from suspect areas after harvest and prior to freeze-up. A minimum of 15 cores should be collected from every 20 acres, with samples taken from the top eight inches using a push probe or small-diameter auger. After mixing the cores, place them in a zip-lock bag, label, refrigerate and submit them. If the results are positive, producers should contact Tenuta at the University of Manitoba’s Soil Ecology Lab.
