Editor’s note: The following was written by Melissa Bartels, Tamra Jackson-Ziems and Kyle Broderick for the University of Nebraska Crop Watch website Sept. 28.
It is a busy time of year with harvest, but this is the perfect time to collect soil samples for soybean cyst nematode while waiting in the field in a grain cart or truck as the combine fills.
Soybean cyst nematode (SCN) causes the most yield-limiting disease of soybeans in North America. Research has shown that SCN can cause over 40% yield loss in soybeans, including 30% yield loss that can occur with no other visible symptoms, making it an invisible yield threat.
SCN reduces yields but typically doesn’t display above-ground visible symptoms in the field during the growing season unless the SCN population is very high, then stunting and yellowing in soybeans may develop.
By the time you see symptoms caused by SCN, population densities may be very high and very difficult to reduce, so we recommend regular monitoring by collecting and submitting soil samples for SCN analyses. You can collect a good sample for SCN in any crop, any time of the year you can get a soil probe in the ground. Since SCN lives in the upper 8 inches of soil, collecting a sample is easy.
Collect SCN samples with a 1-inch diameter soil sampling probe. Some soil probes have a foot-peg on the side for easier sampling.
Collect at least 15 to 20 soil cores from a zigzag pattern across the field. Samples should be collected from a depth of about 8 inches across about 10 to 20 acres. Take 15 to 20 soil cores, breaking them up and mixing them well in a bucket. Place at least 2 cups of the composite soil sample in a bag and submit for SCN testing. A sealable plastic bag works great to prevent samples from drying.
SCN sample bag information to include:
- Email address
- Phone number
- Field name or ID for your reference
- Number of acres the sample represents
- Crop history of the field
- This year’s crop
Submit the sample to a state lab for testing.
Remember, anything that moves soil may also move SCN, including water, wind and farming equipment. These create higher-risk areas of fields where SCN may first be introduced and could be your best target areas for sampling.
Consider sampling these high-risk areas:
- Areas of the field where soybean crops yielded less than expected.
- Areas of the field where soybean plants appeared stunted, yellow, and/or defoliated earlier than the rest of the field.
- Low spots in fields.
- Previously flooded areas of fields.
- Just inside field entryways.
- Along field borders
- Areas where sudden death syndrome (SDS) or brown stem rot (BSR) developed
Results: What do they mean?
Understanding your SCN analysis results can be tricky. The highest densities occur in random patches in the field, which is why we may see small, round, stunted, yellow areas of plants in the field where very high population densities are present. If your results report “0” SCN detected in your sample, keep in mind that low levels of SCN may be difficult to detect in the field and during sample processing.
Detection limits can also vary by lab and may be as high as 1,000-2,000 eggs per 100 cc of soil.
The patchy nature of the nematodes make it difficult to get accurate samples and results. And, multiple samples from a field may provide different results, so it’s important to collect soil cores from as large of an area as is practical.
We recommend continuing to monitor SCN population densities regularly in your fields, especially if soybean yields are less than expected. Sampling every two to three years may be needed to document changes in populations and explain yield loss, especially when other possible causes have been ruled out.