Photo: Albert Tenuta, OMAFRA
Soybean cyst nematode (SCN) is a microscopic roundworm that parasitizes soybean roots. Different types of nematodes can commonly be found in Manitoba, but not all are harmful to crops. SCN is a harmful type of nematode and one of the most damaging pests to soybean crops in North America. Preventative action, early detection and timely management are key in avoiding significant yield loss from SCN.
Infection of SCN can impact growth and yield by removing plant nutrients, disrupting nutrient and water uptake and impeding root growth. It can also reduce the number of root nodules and therefore inhibit nitrogen fixation. SCN-infected plants may also be more susceptible to the development of root rot and seedling diseases, due to openings in the root created by cysts. Once above-ground symptoms are noticed, up to 30% yield loss can already be expected.
Status in Manitoba
The presence of SCN has been confirmed in Manitoba. This confirmation was made possible due to survey efforts led by Dr. Mario Tenuta and his laboratory (University of Manitoba) that have taken place from 2012 to 2019. SCN was identified by visual and molecular DNA methods in four out of 106 fields and four out of 18 municipalities sampled across all surveys.
Rural municipalities where SCN has been detected are Norfolk-Treherne, Rhineland, Emerson-Franklin and Montcalm (Figure 1). Given the large gap between regions with positive identification, there is a possibility that SCN may be present in fields that were not included in the survey. Further, SCN may have established in a field since sampling has occurred.
Cyst populations found in these four fields are extremely low and consistent with recent establishment of this pest. The arrival of SCN in Manitoba is not surprising, given the northward spread and distribution across North America.
Prevention and Management
Once SCN has arrived, it cannot be completely eradicated and spreads rapidly. Anything that causes soil movement can contribute to the spread of SCN, including field equipment, flood water, birds, footwear and clothing. Cleaning soil from equipment, vehicles, soil sampling equipment and clothing will reduce the risk of introduction or spread.
Eggs can survive for several years in the absence of a soybean crop. However, SCN requires a host plant to reproduce. Management options to minimize the population of SCN include rotating to non-host crops, growing SCN-resistant varieties, growing cover crops, reducing tillage and controlling host weed species.
Scouting and Identification
Scout for SCN from late July through September, during the reproductive stages until maturity, by gently digging up roots and examining them for cysts. Roots should be soaked in water to avoid breaking off any potential cysts. Late-season scouting should be done prior to any tillage operations. Assess the most high-risk areas of a field, including:
- Approaches and entrances
- Low spots
- Shelter belts
- High pH areas
- Any generally low-yielding areas
Unfortunately there are no distinct, above-ground symptoms specific to SCN. So it is possible that we may already be seeing it, but have not identified it. At low levels of infection, plants may still appear healthy. At high levels of infection, plants may appear stunted and chlorotic or necrotic. Symptoms may resemble nutrient deficiencies, drought stress or other abiotic factors. Therefore, digging up the roots is the best way to identify this pest.
Below-ground symptoms are more unique due to the presence of white, lemon-shaped cysts on the roots. These cysts are much smaller than root nodules and may require the use of a magnifying lens. When scouting, it is recommended to carefully dig out the entire root system and soak in water to remove the soil without breaking off any cysts.
If you suspect SCN in your field, contact Manitoba Pulse & Soybean Growers.
Soil Testing for SCN
- Target soil collection from suspect areas.
- Sample after harvest and prior to soil freeze-up. If sampling after soybean harvest, sample directly within harvested rows prior to tillage. Following other crop harvest, sample after fall tillage if this is your common practice.
- Collect 15 cores from every 20 acres. Sample the top 8 inches using a push probe or small diameter soil auger.
- Mix soil cores in a pail and place in a zip-lock bag. Label, refrigerate and submit to Agvise as you would with other samples.
- If results come back positive, contact Dr. Mario Tenuta (Soil Ecology Lab UManitoba).
- Cysts were recovered from 12/30 fields in 2017. Most of these cysts were broken and not filled with eggs, preventing accurate identification. Intact cysts were recovered from 7/12 fields. Only 4/7 fields had cysts in good enough condition for molecular testing.
- Molecular DNA analysis of eggs in cysts was conducted using multiple methods. This included two diagnostic polymerase chain reaction (PCR) tests and sequencing of multiple DNA gene targets.
- Cyst populations from the four fields were extremely low and consistent with recent establishment of the nematode. Populations were 2, 1, 14, and 14 cysts per 5 lbs soil in each of the fields. This is compared to 3,000-4,000 cysts per 5 lbs soil in areas of Ontario and the U.S. Midwest, where the nematode has been present for many decades.
- The four fields with positive cyst identification were revisited and sampled again in May 2019. These four fields again had cysts similar to soybean cyst nematode in appearance. Two of the four fields had cysts in good enough shape to examine their molecular DNA, which matched the DNA of soybean cyst nematode.
- One field with a relatively higher cyst population was revisited and sampled again in August 2019. Roots were randomly examined for cysts and some plants had a few white to yellow lemon-shaped cyst nematodes on their roots. There were no visible above-ground symptoms (e.g., stunting, poor canopy closure, chlorosis).