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Fall is the perfect time to soil sample for soybean cyst nematode

Sampling can help farmers build a strategy to beat this pest.

With harvest wrapping up or wrapped up in some areas of the Corn Belt, now is a good time to sample for soybean cyst nematode (SCN).

The pest is widespread throughout Iowa and much of the Midwest, says Greg Tylka, Iowa State University Extension nematologist. He adds that most of the resistant soybean varieties grown for decades to maintain profitable yields and keep SCN numbers in check (PI88788-based resistance) have lost much of their effectiveness. Also, SCN reproduction likely was much greater than normal in fields where soybeans were grown in 2021 due to the hot and dry growing season, making the situation worse, he says.

SCN quickly began spreading through Missouri in the 1970s and gained a foothold in most of the state’s soybean-growing counties by the 1990s. SCN is easily transported in soil; cysts and eggs spread via equipment, water, or wind. Testing soil before planting soybean is vital, says University of Missouri (MU) Extension plant pathologist Kaitlyn Bissonnette. SCN is the No. 1 soybean disease in the United States and Canada, with yield losses up to 30% per infected acre. Symptoms include stunted growth and yellowing, but yield loss can happen even when there are no visible symptoms, Bissonnette says. She advises sampling for SCN every three to five years.

Sampling Guidelines

Samples collected from fields where soybeans were grown in 2021 will reveal whether SCN is present and at what level, says Tylka. 

Sampling fields in the fall that are slotted for soybean production in 2022 will provide information about the yield potential of soybeans in those fields next year. Soil samples can be collected anytime after harvest until the soil freezes or is covered with snow.

Tylka advises farmers to:

  • Use a soil probe, not a spade, to collect soil cores.
  • Collect 15 to 20 soil cores 8 inches deep from every 20 acres.
  • Collect multiple-core soil samples from management zones in the field, if possible.
  • Combine the 15 to 20 cores from a sample area in a bucket, break up and mix the cores well, and then fill a soil sample bag with the mixed soil.

Where to Send Samples

In Iowa, the Iowa State University Plant and Insect Diagnostic Clinic processes soil samples for SCN egg counts. The cost is $20 per sample for in-state samples and $25 per sample for out-of-state samples. The Nematode Sample Submission Form is available through the Extension Store and provides details about where to send the samples.

There also are numerous private soil testing laboratories in Iowa and surrounding states that process soil samples for SCN eggs. The SCN Coalition has a map of the names and locations of these labs online.

Missouri farmers can submit two free SCN tests by contacting their county University of Missouri Extension Center or the SCN Diagnostics Laboratory, says Bissonnette.

United Soybean Board checkoff dollars fund up to two free tests for Missouri farmers as part of SCN Coalition efforts to raise awareness of SCN. The SCN Coalition is a public-private partnership of university researchers, Extension specialists and industry representatives.

For more information, Missouri farmers can contact their local MU Extension center or the SCN Diagnostics lab at scndiagnostics@missouri.edu or 573-884-9118. Sample submission forms are on the SCN Diagnostics website. 

Managing SCN

Once a field is known to be infested with SCN, an integrated approach to manage the nematode is recommended, says Tylka. Farmers should grow SCN-resistant soybean varieties in rotation with nonhost crops, such as corn, and consider using nematode-protectant seed treatments on soybeans. Soybean varieties with PI88788 resistance have lost much of their effectiveness because SCN populations in Iowa and surrounding states have become resistant to PI88788 resistance, Tylka says. Farmers should look for and grow resistant soybean varieties with the uncommon Peking resistance, which is very effective against SCN populations in most Iowa fields, providing high yields and suppressing nematode reproduction.

There are not enough varieties with Peking SCN resistance for everyone to use, so farmers also should look for varieties with PI88788 resistance that are known to provide high yields and good SCN control, says Tylka. Such information is available at Iowa State’s SCN-resistant Soybean Variety Trial Program website.

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