Late summer is a good time to assess soybean cyst nematode (SCN) in your fields. Investigating areas with poor soybean growth can help detect new infestations and evaluate performance problems with SCN-resistant varieties early, before they become expensive. Scouting and soil sampling for SCN allows profitable soybean variety and rotation decisions.
SCN is one of the most widespread and serious constraints to Minnesota soybean production. The "cyst" and its enclosed eggs allow the SCN to survive for years between soybean crops.
Since it was first detected in a 1978 Faribault County field, SCN has spread through most of the soybean-growing areas of Minnesota. Eventually, infestations throughout Minnesota soybean-producing areas are likely.
Resistant varieties have been effective restoring and maintaining yield levels. All but a few of the current SCN resistant varieties rely on the PI88788 source of resistance. SCN biotypes (Hg types) that can feed and reproduce (are virulent) on resistant varieties occur. In some fields, the exclusive, long-term reliance on varieties using a single source of resistance has selected for these virulent SCN. Then, rotating to another source of SCN resistance or a significant change in crop rotation is needed to maintain yield.
Whether a soybean producer is in an area where SCN is a new pest or where long-term management has created resistance management issues, detecting increases in SCN populations in fields is critical to successful SCN management.
The above-ground symptoms are not unique to SCN and range from barely perceptible to extreme stunting and yellowing. Stunting is most common. Areas of the field with uneven plant heights or poor canopy should be investigated.
SCN infestations interact with soil fertility and trigger nutrient deficiency symptoms. In soils with high pH or otherwise prone, SCN causes or increases iron deficiency chlorosis (IDC) symptoms. Potassium deficiency-like symptoms are more typical in low-potassium fields. Top-die back is a sudden late-season yellowing of the tops of soybean plants. This plant disease is often associated with SCN infestation as is sudden death syndrome (SDS).
Above ground symptoms can help you target scouting efforts. Dig soybean roots and look for the small, lemon-shaped white, yellow or tan SCN females. Heavily infested root systems tend to have small, dark root systems and poor nodulation. Roots can deteriorate to where they can no longer support SCN. Therefore, higher nematode populations are often found at the edges of severely symptomatic areas. The presence of SCN on roots is diagnostic and a qualitative measure of populations but changes as the nematodes go through several generations. A quantitative soil sample can be taken and analyzed for SCN eggs, particularly if an infestation is suspected but females are not observed.
High-yield soybean production is not the result of a single growing season. Early detection of SCN infestations lets you use resistant varieties most effectively. Monitoring the performance of resistant varieties will preserve yield and management options.
For further information see www.extension.umn.edu/go/1121/.