Fig. 1. Female cysts extruding out of soybean plant roots. Soybean plants can be infected with SCN without showing obvious symptoms.
Fig. 1. Female cysts extruding out of soybean plant roots. Soybean plants can be infected with SCN without showing obvious symptoms.
As the soybean harvesting is winding down, questions are being asked as to what may have caused lower than expected yields for some soybean fields. One of the causes of reduced yields could be the soybean cyst nematode (SCN). The SCN can cause yield losses even when no visual symptoms are seen on soybean plants. There is one way to determine if SCN is causing yield losses: sample your field and send soil in for SCN test. Testing for SCN is free for South Dakota producers, thanks to the South Dakota Soybean Research and Promotion Council who sponsors the test.

Testing for SCN serves two purposes: to determine if SCN is present and to determine if management practices are lowering its numbers. Fields that are to be planted into soybean the next growing season (now under corn or other crops) should be tested for SCN in order to determine whether a SCN resistant cultivar will be needed or to change in the SCN management strategy.

If SCN population density is still high (>12,000 eggs per 100 cm3 of soil), it is important consider an extended rotation away from soybeans (at least two years). If SCN population density is medium (between 2,000 and 12,000 eggs per 100 cm3), use a resistant cultivar and also practice crop rotation. Where SCN population density is low (<2,000 eggs per 100 cm3), use a resistant cultivar as well as crop rotation to prevent the population density from increasing. It is easier to keep the SCN population density in the soil low than to bring a high SCN population density down.

There is still some time left to sample soil for SCN before the ground freezes. Soil sampling can be accomplished using a tilling spade or a soil probe. Target depth is the top 6 inches of soil from 20 spots in zig-zag pattern including low spots, field entrance areas, along the fence line, and low yielding areas. Larger fields (>20 acres) should be divided into smaller portions of 15-20 acres and each of these sampled individually.

Producers who farm in counties that have been detected with SCN but do not yet have it in their fields ought to know how it spreads and avoid chances of its introduction into clean fields. This applies also to producers in counties that do not yet have SCN detected but are surrounded by counties that have tested positive. Spreading is through anything that moves SCN-infested soil: farm machinery, water/wind soil erosion, muddy shoes, and wild animals. Soil movement on tillage, planting, or spraying equipment should be avoided and SCN positive fields should be worked on last.

More Information

More information on SCN biology, distribution in South Dakota, and management can be found in the SDSU Extension publication: Soybean Cyst Nematode:The Unseen Yield Robber.