In annual surveys of scientists working with soybean throughout the country, soybean cyst nematode was ranked as the top yield-limiting disease in the US. The latest estimate indicated that soybean cyst nematode robbed over 134,000 ton of soybean yield per year in South Dakota.
Soybean cyst nematode (SCN), Heterodera glycines, is a tiny roundworm that attacks the roots of soybean and several other leguminous crops, such as cowpea and crimson clover. Above-ground symptoms of SCN infection are not always visible. A soybean crop may lose as much as 30% of yield due to SCN without showing above-ground symptoms. This leads to cases where soybean yield losses occur for a couple of years before the presence of SCN was suspected. Below-expectation yield may indicate the presence of SCN in the field.
When the symptoms are present, it manifests as a slight variation in the height of otherwise healthy-looking dark-green plants. High levels of SCN may induce leaf yellowing, plant stunting and even plant death. These symptoms are similar to, and thus often confused with, the symptoms of nitrogen deficiency, iron chlorosis, herbicide injury, soil compaction, and other plant diseases.
Effective management of SCN include making decisions whether to plant crops that cannot be utilized by the nematodes or to plant soybean cultivars with resistance against the nematodes. In making these decisions, it is important to ascertain whether a field is infested with SCN. Since above-ground symptoms of SCN infection are not always apparent, field testing for the presence of SCN in areas with SCN history is crucial.
To test a field, soil core samples need to be collected. Divide the field into blocks of 10-acre-sections and sample each section separately. Within a section, walk a zig-zag or an M pattern path and collect 10-20 soil core samples (6-8 inches or 15-20 cm deep). Use a cylindrical soil probe to collect the soil core samples. Bulk the core samples from a section and mix it thoroughly in a bucket. Place 1 pint (0.55 L) of mixed soil in a plastic bag or soil sample bag, label the bag with the field information and date of collection, and keep it in a cool and dark environment until shipping.
Send the soil samples to SDSU plant diagnostic clinic (Box 2108, SPSB 153, Plant Science Building, Brookings, SD 57249) for SCN confirmation. Currently, SCN testing at SDSU plant diagnostic clinic is provided free of charge to residents of South Dakota. This free service is made possible by a grant provided by the South Dakota Soybean Research and Promotion Council. Soil samples can be collected at any time, but the optimal time to sample for SCN is as close to soybean harvest as possible since this is the peak period for SCN populations.
This article was written with contributions from Connie Tande and Kelli Tilmon.