Soybean cyst nematode (SCN) is an important soybean pest in Iowa that often goes unnoticed. The only consistent and reliable sign of an SCN infestation in the field is the presence of adult SCN females and cysts (dead females) on the roots of infected soybean plants. SCN females and cysts have been observed during the past two weeks on roots of soybeans planted in early- to mid-May.
||Adult soybean cyst nematode females on soybean roots.
SCN females and cysts appear as small, white- to yellow-colored objects, each approximately the size of the head of a pin. Seeing large numbers of SCN females on roots early in the season is an indication of early development and reproduction of the nematode. It also may indicate that damage from this soybean pest will be severe in 1997, particularly if dry weather persists for much of the growing season.
Once adult SCN females and cysts appear, they can be seen on the roots of infected soybean plants through much of the growing season, until late summer or early fall when the plants begin to mature. To scout for SCN in fields where the nematode has yet to be found, you can target fields that have been planted to soybean over the past several years, or fields where soybean yields have declined over time for no apparent reason. Since SCN is spread by the movement of infested soil, checking roots of plants near the entrance of fields where farm equipment enters, and along fence lines where wind-blown soil accumulates, may increase the likelihood of finding SCN-infected plants. An ISU Extension publication, Scouting for Soybean Cyst Nematode, IPM-47-s, illustrates the proper way to scout for this pest.
Aboveground symptoms of SCN damage to soybean are stunting and yellowing of the plants. Symptoms can occur in a localized area, called a "hot spot," or be more generalized over an entire field. Unfortunately, symptoms often do not appear when SCN population densities are low or in years when the crop is not stressed by some other factor. SCN populations can develop to seriously damaging levels without the appearance of aboveground symptoms. Also, symptoms of SCN damage can be confused with those caused by iron deficiency, potassium deficiency, or other soybean production problems. Severe stunting and yellowing due to SCN usually do not occur early in the season. When severe SCN aboveground symptoms are observed, large numbers of females and cysts almost always are readily observable on the roots of affected plants.
An alternative to digging roots and looking for the presence of adult SCN females and cysts is to collect soil samples from fields suspected of being infested with SCN. Using soil samples to test for the presence of SCN can be done at any time during the growing season. Soil samples should be submitted to private laboratories throughout the state that offer nematode testing as a service, or to the ISU Plant Disease Clinic for extraction and counting of SCN eggs.
Samples sent to the ISU Plant Disease Clinic should be accompanied by a completed Plant Nematode Sample Submission Form (ask for ISU Extension publication PD-32). Currently there is a $15 per sample charge for this analysis. Detailed instructions on how to collect a representative soil sample for detection of SCN can be found on the back of the Plant Nematode Sample Submission Form and on the Scouting for Soybean Cyst Nematode publication mentioned above.
This article originally appeared on pages 115-116 of the IC-478(15) -- June 30, 1997 issue.