Most farmers and the people who advise them are aware that the soybean cyst nematode is a yield-reducing pest of soybeans in the Midwest. The tiny worm stays underground, feeding on soybean roots. What farmers and others might not know or remember:
- An estimated 70% of Iowa fields are infested with SCN.
- Roughly 30% or more yield loss can occur with no aboveground symptoms.
- Dormant SCN eggs can survive in soil for 10 years or more without soybeans.
- Most resistant soybean varieties no longer control SCN reproduction well.
Greg Tylka, Iowa State University Extension nematologist, says weather can have a tremendous impact on SCN reproduction. “We have data verifying that SCN is worse in hot, dry years,” he says. “It’s not just because plants are stressed from drought but also because the nematode is reproducing much quicker, raising population densities.” He offers these suggestions:
Sample for SCN this fall. Drought conditions that occurred throughout large parts of Iowa in 2020 may have resulted in high levels of SCN reproduction in soybean fields, Tylka says, allowing SCN numbers to increase to very damaging levels. The link between high SCN reproduction and hot, dry soils was discovered in data from more than 25,000 research plots in ISU experiments conducted over 15 years.
Unfortunately, the life cycle of SCN speeds up during drought-like conditions. “We don’t know the mechanism of it, but reproduction happens much more quickly, creating more generations in a single growing season, which ultimately leads to higher egg counts during fall soil sampling,” Tylka says. “In areas impacted by drought with a traditional corn-soybean rotation, I would expect that in 2022 many of the fields that grew soybeans this year would have increased levels of SCN and increased yield loss.”
Sample fields where soybeans grew in 2020. If fields have never been checked for SCN or haven’t been sampled before or after the last three soybean crops, the results of soil samples collected this fall after soybeans are harvested may be real eye-openers, Tylka says. The lack of aboveground symptoms, loss of effectiveness of resistance and high reproduction of SCN in dry soils could have resulted in high end-of-season population densities of the nematode this year.
It’s useful to know what the SCN population densities are in fields this fall even if soybeans will not be grown in these fields in 2021, he notes. Numbers may be so high that planning for multiple years of a nonhost crop in that field might be warranted in the crop rotation.
ZONE SAMPLING: Collect separate multiple-core soil samples from different management zones within a field to account for possible differences among the areas. (Map courtesy of ISU)
Sample fields where soybeans will go in 2021. Although SCN population densities would not have increased in fields where corn was grown in 2020, it is important to know what SCN population densities are present in the soil if soybeans will be grown in these fields in 2021.
Depending on the test results of the soil samples, farmers and agronomists may want to search for effective resistant soybean varieties and nematode-protectant seed treatments. Data on yields and SCN control of hundreds of SCN-resistant soybean varieties in ISU’s SCN-resistant soybean variety trials are available here.
If soil sample results reveal that SCN population densities are greater than 12,000 eggs per 100 cubic centimeters of soil, it would be advisable to grow another year of corn (a nonhost crop) in that field in 2021, Tylka says.
Follow these soil sample guidelines. Tylka recommends that you:
- 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 samples from various management zones in the field, if possible.
- Combine and mix soil cores, and put in soil sample bag.
Numerous laboratories in Iowa process soil samples to provide SCN population densities. Most private soil testing labs offer this service. Also, the ISU Plant and Insect Diagnostic Clinic processes samples for SCN. Click on Iowa on this map for a list of labs in Iowa that test soil for SCN.