Soybean cyst nematode (SCN), a microscopic roundworm, is the most damaging soybean pathogen in Illinois. It is estimated to cause more than $200 million in losses to producers yearly. SCN is known to occur in every county in the state.

The only way to confirm its presence and population is by taking a soil sample. Fall sampling is suggested since in Illinois the overwintering survival of SCN approaches 100 percent, so the number of nematodes in the fall is highly predictive of the number that will be present in the spring.

For detection purposes, one sample should represent no more than 10 acres and be comprised of 20 to 30 subsamples. Each subsample should be taken 8 to 10 inches deep. Mix the subsamples together in a bucket and remove enough soil to fill a one-quart plastic bag. Mail this bag to a qualified laboratory for analysis. Results will be reported in either number of cysts per 100 cc of soil or preferably the number of eggs per 100 cc of soil.

Even though one cannot eliminate SCN once it infests a field, yield losses can be reduced by following the "three R's" of rotation:

• Rotate the field with a nonhost crop, for instance corn and alfalfa.
• Rotate with SCN-resistant varieties. Level of resistance is available at www.vipsoybeans.org
• Rotate resistant varieties. Never grow the same SCN-resistant variety in the same field twice. No variety is completely resistant to SCN.

If SCN numbers appear to be increasing in a field that has been managed by rotation of resistance, it is likely a race shift has occurred. This means that the nematodes in the field have adapted to the resistant varieties that have been grown in the field. The nematodes may be causing a yield loss even though "resistant" varieties have been grown. In this case, the best way to plan a management strategy is to have a "SCN type" test done by the University of Illinois Nematology Lab, Department of Crop Sciences, AW 101 Turner Hall, Urbana, IL 61801.

Further information is available in the Nematode chapter, of the Illinois Agronomy Handbook, authored by Dr. Terry Niblack, Extension nematologist; tniblack@illinois.edu The Handbook can be ordered online at https://pubsplus.uiuc.edu/C1394.html University of Illinois Extension offices can facilitate individuals obtaining a copy.