Understanding A "SCN Soil Test"?

The weather, late planting, aphids, root rots and soybean cyst nematode (SCN) are just some of the challenges soybean producers had to face this year. The result was another tough year for many. As the more experienced farmers (another way of saying older) will tell you, with all the advances that have gone on recently we are still very dependent on the weather. Although we may not have any control on the weather, we can reduce the impact of some of these other challenges. This is especially true for soybean cyst nematode and it begins with soil testing. Taking the sample and understanding the results of a SCN soil test is critical to managing this devastating disease.

Negative Test Results

So what do the results mean? If they came back as negative, this may not necessarily mean you do not have SCN. Since the laboratory processing procedures are not 100% efficient, not all cysts will be extracted from the soil. This is especially true for fields with low levels of SCN that may be below detectable levels. Another possible reason for a negative soil test is the natural variability within the SCN field population. The samples may have been collected from parts of the field that were not infested with SCN. On their own, soybean cyst nematode moves very little (2 - 4 cm per year) and although tillage will quicken their dispersal, SCN populations tend to build up (aggregate) in certain areas of the field. To minimize SCN field variability and thus increase the accuracy of the sample, increase the number of soil cores and reduce the area (acres) represented per sample.

Either way, the good news is that if SCN is present it most likely is at low levels. A follow-up test in three or four years would still be recommended. Scouting for cysts on the roots and for SCN field symptoms such as yellowing, stunting, death, etc is also recommended whenever a soybean crop is planted.

Positive Test Results

If SCN cysts and eggs are detected, at any level, than a rotation scheme that incorporates non-host crops and resistant varieties needs to be followed. If nothing is done yields of susceptible varieties will be reduced and SCN populations will increase rapidly. The year following detection it is best advised to plant a non-host crop that will reduce the population. For example, when a preplant SCN egg count exceeds 50 eggs per 100 grams of coarse textured (sandy soils) or 100 eggs per 100 grams of fine textured silt (clay soils) a resistant variety should be planted. A simpler approach would be to use SCN resistant varieties in all fields that the nematode has been detected.

The following table rates your SCN risk and potential yield loss based on SCN population levels.

(Source: Welacky, Anderson and Tenuta)

Remember that resistant varieties are not immune and their yield can be reduced by nematode root damage and lack of nitrogen fixing nodules when SCN egg counts exceed 10,000 eggs per 100 g of soil. Although it is most likely that a resistant variety will still out yield a susceptible variety the affect is enhanced when the crop is under stress. To minimize economic impact it would be best if a non-host crop could be planted.

Management of SCN begins with identification. A "SCN soil test" is an effective tool to identify those fields that are infested with this very destructive pest. Knowing the SCN population levels, will assist in determining which management practices should be used. The soil test also provides a benchmark for future comparison. The effectiveness of your management strategies is best evaluated when compared over time to the original field levels (benchmark). If numbers have dropped substantially, this would indicate that your strategy is working. Remember though, a proper evaluation should begin with a SCN soil test.

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