The soybean cyst nematode (SCN) is an extremely damaging and widespread pest of soybeans in Iowa. The nematode infests approximately 70 percent of the fields statewide; however, SCN usually causes no obvious aboveground symptoms for many years after being introduced into a field. Consequently, many SCN-infested fields in Iowa have not been diagnosed. The lack of symptom occurrence and subsequent missed diagnosis are unfortunate because the key to effective management of SCN is early detection, before large nematode population densities develop.
SCN can be detected in soil samples, and the fall is an ideal time to sample fields for this pest. Soil samples can be collected any time throughout the fall until a significant snowfall or a hard freeze occurs. Following are some guidelines for sampling fields for SCN:
- Ideally, fields should be sampled by using a soil probe.
- Soil cores should be 6 to 8 inches in depth.
- Collect soil cores from 15 to 20 places in a sampling area.
- Collect a separate set of soil cores for each 20 acres or so.
- Combine and mix soil cores, and fill a sample bag with one cup or more of soil.
- Label the outside of each sample bag with a permanent marker.
Increasing the number of cores collected and decreasing the area represented by the soil cores will increase the accuracy of the results. It is most logical to sample fields from which corn has just been harvested because these are the fields in which soybeans will be grown next year. Alternatively, samples can be collected from fields in which soybeans were grown in 1998 if unusual plant growth was observed during the season or if unexplained low yields were obtained. One set of soil cores can be collected for both soil fertility and SCN testing, however, the samples may have to be sent to separate laboratories because not all soil fertility laboratories offer SCN testing.
Numerous ISU Extension publications on SCN and SCN soil sample supplies can be obtained free of charge from any county extension office. The free soil sample supplies are provided by the Iowa Soybean Association. Soil sample supplies also can be obtained at no cost by calling the SCN Coalition toll-free hotline (1-877-SCN-TEST). The Iowa State University Plant Disease Clinic offers SCN testing. The mailing address of the Clinic is 323 Bessey Hall, Department of Plant Pathology, Iowa State University, Ames, IA 50011-1020. Current fees for SCN analysis are $15 per sample for 1 to 5 samples, $12 per sample for 6 to 10 samples, and $10 per sample for 11 or more samples.
This article originally appeared on pages 189-190 of the IC-480(24) -- November 9, 1998 issue.