Soybean Cyst Nematode Becoming More Prevalent

Soybean Cyst Nematode (SCN) causes the greatest yield loss to soybeans of any single pathogen in North America. This has been a year with severe SCN damage in many Ontario fields. Even when symptoms are not evident, yields can be reduced by as much as 30%.

What’s new this year is that SCN has now been found in counties where it hasn’t been previously identified. Fields in Bruce County have been identified this year with cyst numbers as high as any soybean fields in southwestern Ontario. Brant County has also been added to the list of infected Ontario counties this year. Known infected counties include Essex, Kent, Lambton, Elgin, Middlesex, Oxford, Huron, Perth, Bruce, Brant, Haldimand, Norfolk and Peel.

If you have soybeans this year that struggled more than can be explained by obvious causes test your fields for SCN. Visible plant symptoms such as stunting and yellowing are the most obvious visual symptoms but are not always present. Even when no above ground symptoms are present up 30% yield losses are possible. The surest way to identify this pest is to take a soil sample and get an SCN cyst and egg count.

Typical Field Symptoms:

Yellow, stunted soybeans in patches (circles). Refer to Photo #1. Symptoms may look similar to nutrient deficiencies, compaction or drought damage. In severe cases drought stressed plants may actually die.

Small white or yellow cysts on the roots of soybeans. Refer to Photo #2. SCN is visible with the naked eye but close examination of the roots is necessary.

What is Soybean Cyst Nematode?

SCN is a microscopic roundworm related to the animal- parasitic roundworms that infect pets. The juvenile nematode that hatches from the egg infects the soybean roots. If the juvenile becomes a male it leaves the soybean root after feeding for only a few days and moves through the soil no longer contributing to plant damage. If however the juvenile becomes a female she swells into a cyst and becomes embedded within the root. The damage caused to the roots is primarily due to the feeding by the females.

Plant nutrients are removed, water uptake is slowed and root growth is hindered. Infected roots cannot utilize nutrients and water efficiently so visual symptoms are more evident during dry years compared to ideal growing seasons. These damaged roots are more susceptible to other diseases such as Pythium, Rhizoctonia, Phytophthora, Fusarium, etc.

SCN moves very slowly through the soil so infection of new fields is caused by soil movement which moves the SCN. This can occur through dirty farm equipment, flooding, etc. Once the pest is established in a field it is impossible to eradicate.

When to Soil Sample for SCN?

Although soil samples can be taken any time for SCN it’s ideal to collect soil as close to soybean harvest as possible. SCN numbers are generally the highest when soybeans are almost mature and just after harvest. SCN samples should be kept out of direct sunlight and a plastic bag is preferable. Collect to a depth of 6 – 8 inches. Sample an area that is likely to have the problem such as near field entrances and areas where yields are expected to be lower due to yellow stunted plants. Also collect from an unaffected area for comparison.

Management Strategies Prior to Infection:

1. Plant clean seed that is free of soil peds
2. Wash soil from farm equipment that has been in infected fields
3. Use good soil conservation practices to reduce soil movement from field to field

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Management After Infection:

1. A good rotation which uses SCN-resistant varieties and non-host crops such as corn, wheat, alfalfa, clover, etc will keep cyst numbers low and yield impact minimal.
2. Edible beans should not be used to substitute for soybeans in the rotation since they are also a host for SCN.
3. Soil sample every 3 years to monitor levels. Ask for both a cyst and egg count.

Remember that SCN is manageable. Excellent soybean yields are still possible if the pest is kept in check with a good rotation and SCN resistant soybean varieties are used.