Production Blog

You've still got time to pull soil samples to test for soybean cyst nematode (SCN) if the ground isn't frozen. Missed it or already have the soil probe cleaned up? Spring is an alternative testing period.

Need reasons to test? Drought years support SCN reproduction. SCN is already costing farmers more yield loss than any other soybean pathogen. Physical symptoms of SCN at work beneath the soil surface may not be obvious or evident. SCN can move on equipment and has even been found in wind-blown soil.

"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," said Horacio Lopez-Nicora, plant pathologist and nematologist with The Ohio State University in a recent news release from The SCN Coalition.

Iowa State University provides the following guidelines for collecting SCN soil samples:

-- Use a soil probe, not a spade, to collect soil cores.

-- Soil cores should be about 8 inches deep.

-- Collect 15 to 20 soil cores from every 20 acres.

-- The more soil cores collected and the smaller the sampling area, the more accurate the results will be.

-- Collect soil cores from agronomically similar areas or management zones in the field.

-- Combine all soil cores representing a sampling area in a bucket and mix the soil well, then fill a soil sample bag or plastic bag that has been labeled with permanent marker on the outside of the bag.

-- SCN soil samples need not be refrigerated; storage at room temperature is sufficient.

-- In fall, sample following a soybean crop. If sampling in spring, choose a field that will be planted to soybeans.

-- Many state university plant diagnostic clinics process SCN soil samples for farmers and agronomists. Private soil-testing labs are another place to query. Find a listing of SCN testing labs here: https://www.thescncoalition.com/….

Find more about sampling protocols here: https://crops.extension.iastate.edu/…

https://www.thescncoalition.com/….

GOT RESULTS? NOW WHAT?

Recommendations can differ between states as to tactics to take when reviewing SCN test results. The SCN Coalition provides an easy way to check by state here: https://www.thescncoalition.com/….

"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," said Dylan Mangel, plant pathologist with the University of Nebraska-Lincoln.

Following The SCN Coalition suggestions, look first at what life stage of the nematode is being counted and reported. Most labs report SCN egg counts, but some 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," Lopez-Nicora stated. "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 receive 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 AND PROFIT LOSSES

Armed with SCN egg counts, you can use The SCN Coalition's new SCN Profit Checker online calculator to estimate how SCN is affecting yields and profits. Find it here: https://www.thescncoalition.com/….

"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," Mangel said.

Default female indexes on PI 88788 for most states are provided in the tool if that information is unknown. The SCN Profit Checker also considers a field's sand content and soil pH, as these affect SCN reproduction and the management strategies that work best for a particular field.

Find a DTN article on how to use the tool here: https://www.dtnpf.com/….

UNDERSTAND SCN POPULATIONS

While there is no "standard" SCN soil test result and each lab has its own reporting process, 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," explained Lopez-Nicora.

Regardless of the SCN egg count, once detected, farmers should develop a management plan. Mangel noted that it's important to keep low SCN numbers low, considering SCN population densities can rise in a short amount of time.

"If left unmanaged, SCN populations can build up very quickly, increasing from hundreds to thousands in one growing season," Mangel added. "Even if SCN egg counts are zero, it's possible that SCN is still in that field and was missed during sample collection."

Implementing active SCN management strategies will help limit SCN population growth and new infestations, helping to protect future crops. An active management plan should include:

-- Rotation of SCN-resistant soybean varieties.

-- Rotation to nonhost crops (corn, alfalfa, oats, etc.) Find more on choosing cover crops with SCN in mind here: https://www.thescncoalition.com/….

-- Consideration of nematode-protectant seed treatments. See https://www.thescncoalition.com/….

The SCN resistance source known as PI 88788 dominates the SCN-resistant soybean varieties available today but is losing its effectiveness. Peking varieties have proven to be highly effective against SCN but should not be relied on as the only source of resistance. SCN soil test results can also be compared to previous years and provide insights into 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," Mangel said. "It's also just as important to know which fields don't have SCN to avoid introducing it from infested fields."

For a look at SCN resistant varieties available in Iowa for 2024 go to: https://crops.extension.iastate.edu/….

For more information from The SCN Coalition go to: https://www.thescncoalition.com/….

To learn more about a SCN testing project between DTN and The SCN Coalition go to: https://www.dtnpf.com/….

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on X, formerly known as Twitter, @PamSmithDTN

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