Fall is Prime Time to Sample Fields for SCN

It is estimated that the soybean cyst nematode (SCN) is the most damaging pathogen of soybeans in Iowa and the entire US, costing producers $1.5 billion annually. The pathogen warrants more attention and effort to detect and manage than often is given. The weeks immediately after corn and soybean harvest is an optimum time to collect soil samples from fields to determine if SCN is present and, if so, at what numbers.

Why sample fields for SCN if no symptoms are observed?

Fields infested with SCN may not show stunting and foliar yellowing when nematode numbers are low or moderate. In fact, the only “symptom” a farmer often observes prior to confirming that a field is infested with SCN is unexplained, lower-than-expected soybean yields.

Independent random surveys for SCN were conducted by Iowa State University in cooperation with USDA National Agricultural Statistics Service personnel based in Iowa in the 1990s, the 2000s, and the 2010s. The surveys were funded by the soybean checkoff through the Iowa Soybean Association and the North Central Soybean Research Program. In all three surveys, SCN was detected in approximately 70% of sampled fields. Given the fact that the fields that were sampled were randomly selected, there’s a 70% likelihood that any soybean field in Iowa may be infested with SCN.

Also, SCN spreads in windblown soil. We know this because we have grown soybeans in soil collected from snow drifts and observed adult SCN females on roots (Figure 1). So even if fields were sampled in the past and SCN was not detected, sampling again is warranted.

How to collect soil samples for SCN?

Below are guidelines for collecting a soil sample for SCN:

• 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. See figure 2.
• 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.
• The ISU Plant and Insect Diagnostic Clinic processes SCN soil samples for farmers and agronomists. Samples can be delivered or mailed to room 2445 Advanced Teaching and Research Building, 2213 Pammel Drive, Ames, IA 50011. The clinic can be contacted by calling 515-294-0581 or by emailing pidc@iastate.edu. More information is available online here and the sample submission form can be downloaded here.
• Many private soil-testing labs also can process samples for SCN.

What are next steps for managing SCN in fields where it is detected?

Managing SCN requires an active, multiple-prong approach. In short, farmers should grow SCN-resistant soybean varieties with PI 88788 and with Peking resistance in alternating soybean crops and in annual rotation with corn, which is an SCN nonhost. Also, there are nematode-protectant soybean seed treatments that may provide added yield protection.

More detailed information about managing SCN will be discussed in additional ICM News articles this fall.