A soil sample collected from a field near Agency, Iowa and submitted to the Nematology Laboratory at Iowa State University in December 1992 was infested with soybean cyst nematode (SCN).

This was the first confirmed report of SCN in Wapello County, which brings the total number of Iowa counties known to be infested with SCN to 55 (see map). SCN was present in low densities in this sample, indicating that the nematode probably had been recently introduced into the field. Refer to ISU Extension Publication Pm- 879, Soybean Cyst Nematode, for a complete description of SCN biology and suggested management strategies.

Pre-season SCN soil sampling

Soybean cyst nematode can be detected in soil samples collected in the spring, after the soil has thawed and prior to planting. Each soil sample should represent no more than 20 acres. Collect 15 to 20 soil cores in a systematic pattern throughout the entire sampling area. Soil cores should be combined in a bucket and mixed thoroughly, and two pints of mixed soil should be placed in a plastic or paper soil sample bag. Each soil sample should be in a separate, securely sealed bag, and bags should be labeled with the grower~s name and the field or sample name using a permanent marker. Avoid storing soil samples in direct sunlight. Soil samples should be mailed, along with a completed Plant Nematode Sample Submission Form (ISU Publication PD-32) and a check for the appropriate fee, to the ISU Plant Disease Clinic (see information next page).

Early season SCN identification

Soybeans infected with SCN can be easily identified in the field beginning approximately four to five weeks after planting. Adult females and cysts appear as small white- or cream-colored objects, each approximately the size of a pinhead, on the roots. The females and cysts (figure 1, upper pointer) are lighter in color and much smaller than the nitrogen nodules (figure 1, lower pointer) normally found on soybean roots.

To scout for SCN in the field, one should dig up young soybean plants and carefully shake the soil from the roots. The characteristic adult females and cysts (figure 3) will be easily observable on the roots of infected plants when roots are removed from the soil in this manner, even if only a few nematodes are present.

Once SCN females and cysts begin to appear, they should be evident on infected roots throughout much of the growing season. Do not rely on the appearance of above-ground symptoms of SCN damage to identify infected soybeans. Serious SCN infestations can exist in the absence of any noticeable above-ground symptoms, particularly in years with adequate to excess rainfall during the growing season.

Plant disease clinic information

The ISU Department of Plant Pathology Plant Disease Clinic located at 323 Bessey Hall now processes SCN soil samples and provides management recommendations to growers. In the past, SCN soil samples were sent to the Plant Nematologist in 321 Bessey Hall. The fee for SCN soil sample processing increased from $8 to $15 per sample beginning April 1, 1993. SCN race determination testing, which was previously available for a fee of $40, has been discontinued. Complete nematode analysis for all crops is still available for $15 per sample. All soil samples submitted to the clinic for nematode analysis must be accompanied by a Plant Nematode Sample Submission Form (ISU publication PD-32). The forms can be obtained at county extension offices or from Publications Distribution Center, Printing and Publications Building, Iowa State University, Ames, IA 50011, (515) 294-5247.