Soybean cyst nematode — SCN — is North America’s most damaging and costly soybean pathogen, according to The SCN Coalition. The coalition, a partnership that includes UW-Madison and soybean checkoff dollars, provides a variety of educational resources to help more farmers understand the importance of soil testing for actively managing soybean cyst nematode.
“A soil test is still the best place to start because knowing your SCN numbers helps determine the appropriate management strategies to implement for your fields,” says Horacio Lopez-Nicora. Lopez-Nicora is a plant pathologist and nematologist with The Ohio State University.
Many farmers who followed their state recommendations to test soil for soybean cyst nematode are reviewing their results.
“The most commonly asked questions we get from farmers when they receive their SCN soil test results are about what the results mean and what to do with that information,” explains Dylan Mangel. Mangel is a plant pathologist with the University of Nebraska–Lincoln.
Look At Egg Counts
First, look at what life stage of the nematode is counted and reported. Most report soybean cyst nematode egg counts, but some labs report cysts or juveniles. Within a cyst may be hundreds of eggs, which hatch into juveniles that move into the soil.
“Egg counts provide a more accurate assessment of SCN pressure in a field, despite the variability in soil test results,” says Lopez-Nicora. “Cysts tend to cluster and contain varying numbers of eggs, leading to significant fluctuations in soil sample outcomes. Thus, these results should be considered an approximate estimation of the actual SCN population in the field.”
If you get results with cyst counts, multiply the number of cysts by 100 to convert to an estimate of eggs. This accounts for cysts of various ages containing a variable number of eggs in the soil sample.
When comparing soil sample results, make sure results are reported in similar soil volumes — most commonly 100 cm3 of soil.
Estimate Yield & Profit Losses
Farmers can use The SCN Coalition’s new SCN Profit Checker calculator to estimate how soybean cyst nematode is impacting yields and profits: https://www.thescncoalition.com/profitchecker/
“Farmers, agronomists or crop advisors can use the new SCN Profit Checker to get an estimate of yield and profit losses from SCN per acre,” says Mangel. “Users input field-specific information, like the SCN egg count and SCN female index on PI 88788, and see an estimate based on expected yield and soybean price.”
Default female indexes on PI 88788 for most states are provided in the tool if that information is unknown. The profit checker also considers a field’s sand content and soil pH, as these impact soybean cyst nematode reproduction and the management strategies that work best for a particular field.
Understand SCN Population Levels
There is no “standard” SCN soil test result, and each lab has its own reporting process. But egg counts are often designated as low, medium or high levels.
“It’s important to keep in mind that what may be considered a high SCN egg count in some states is low or moderate in others, because different environmental factors like sand content and soil pH can impact SCN reproduction,” says Lopez-Nicora.
Regardless of the soybean cyst nematode egg count, once detected, farmers should work with a trusted expert to develop a management plan.
“If left unmanaged, SCN populations can build up very quickly, increasing from hundreds to thousands in one growing season,” Mangel says. “Even if SCN egg counts are zero, it’s possible that SCN is still in that field and was missed during sample collection.”
Build An SCN Management Plan
Whether egg counts are low or extremely high, you can use management practices that help combat the damaging pest. An active management plan should include:
- Rotation of SCN-resistant soybean varieties
- Rotation to nonhost crops (corn, alfalfa, oats, etc.)
- Consideration of nematode-protectant seed treatments
You can compare soil test results red to previous years to see the success of implemented management practices.
“In an ideal world, every farmer would test fields each time they come out of soybeans, but the best place to start is with those problem areas in fields with unexplained yield loss,” says Mangel. “It’s also just as important to know which fields don’t have SCN to avoid introducing it from infested fields.”
For farmers who haven’t tested their soil for soybean cyst nematode yet, there’s still time. You can take soil samples any time of year when the ground isn’t frozen. The easiest time is in the spring before planting or in the fall after harvest.
