The field looked like 80-bushel-per-acre soybeans. Yet when the combine went through, the crop struggled to average 70 bushels, even with dry corners factored out. One large, eroded hill dragged down yield.

“We should sample for soybean cyst nematode, just to be sure,” suggested Steve Gauck, a regional agronomy manager for Beck’s, sponsor of Soybean Watch ’24. “We didn’t see any visible signs of disease or insect issues. Let’s see if something else is going on.”

So, the 50-acre field was sampled soon after harvest — split into four sampling areas, with the hill sampled by itself. The northwest corner produced a bump on the yield monitor, so it was sampled separately too. Samples from the rest of the field were labeled “southwest” and “east.”

Ten to 12 soil cores were pulled from each area. A composite sample from each was shipped to the University of Missouri SCN Diagnostics Lab for testing two days later. Soil samples were pulled in the same areas and sent to A&L Great Lakes Lab in Fort Wayne, Ind.

What results show

Gauck’s hunch paid off. Elevated numbers of SCN eggs were detected in the soil sample from the hill and in the east sample, with moderate but lower numbers in the southwest sample. Lower egg numbers were found in the northwest sample where yields were highest.

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Soil samples confirmed nematodes weren’t the only problem for the hill region, with phosphorus levels at 5 parts per million, according to a Bray P1 test, which is well below the critical level of 15 ppm. A response to phosphate would be expected at P levels below 15 ppm.

However, pH and potassium level were reasonable, and P and K were above critical levels in the rest of the field.

TESTING FOR SCN: This graphic shows the Soybean Watch ’24 field. The arrow indicates direction of rows. The field consists of outwash or terrace soils underlain by gravel, a hill with 10% to 12% slope, and nearly level upland soils. Samples for testing were pulled from areas marked “northwest,” “southwest,” “hill” and “east,” on terrace position soils. (Google Earth/Getty Images/Lisa Lynd)

What specialists say

Mandy Bish, Extension plant pathologist at the University of Missouri, helps interpret results from SCN tests. Darcy Telenko, Extension plant pathologist at Purdue, guides growers with SCN issues in Indiana. Both also rely heavily on advice posted by The SCN Coalition.

Here are questions you might ask if this were your field:

We didn’t see typical visible symptoms of SCN. Does this mean there could be nematodes in the grower’s other fields?

Bish: Yes. This is one challenge with SCN. If you see aboveground symptoms, yield losses are already significant. It is worth recommending that the grower sample other fields — especially fields with similar management practices and/or unknown SCN population densities.

Telenko: SCN is probably found in all fields at varying levels, depending on how you have managed it. Resistance has been great in our soybeans, but now populations are evolving and becoming resistant. The PI88788 SCN resistance source is used in about 95% of soybean varieties. Now SCN can reproduce on these lines.

Egg counts varied from 300 per 100 cubic centimeters to over 5,200. Why would nematode egg count vary that much across the field?

Bish: Nematodes can only move a few inches on their own. So, distribution in the field can be quite uneven.

Telenko: It is the aggregated nature of pathogens. Levels change due to variations in soil type, environmental conditions and whether the population is replicating or not.

Are moderate levels enough to cause yield loss?

Bish: Yes, these levels can cause yield losses.

Telenko: See the yield loss calculator provided by The SCN Coalition. You can use it to estimate yield losses. It is based on data from actual fields in your state.

Part of the field has moderate levels, but part just has levels at the high end of the low range. Should the grower act or just monitor it?

Bish: Acting now is ideal to prevent future problems. Use crop rotation, and where possible, rotate among varieties with different types of SCN resistance. Once you get to high SCN levels, it will be more difficult to reduce those numbers with typical management strategies.

I recommend farmers experiment with a couple of units of Peking seed. If you can work toward a rotation of corn, PI88788 soybean, corn, Peking soybean, corn and PI88788 soybean, it seems reasonable to think you would drive down those numbers.

Telenko: Be aware and think about variety selection and other management options.

Editor’s note: The SCN Coalition’s SCN Profit Checker calculator was used, and results are presented in the table. Note that the highest egg count does not mean the most expected yield loss. Other input factors include soil pH and sand content. The sand content for the east sample was higher, leading to a different result. Watch the website for the second part of this story, which will dive into what to do if you have SCN in your fields.