Why soybean producers should care about SCN.
Soybean cyst nematode (SCN) is a microscopic worm, the most yield limiting pathogen of soybean in the north central US and capable of causing up to 30% yield loss without conspicuous above-ground symptoms (Figure; Niblack and Riggs, 2015; Wang et al., 2003). Of additional worry in the Red River Valley, research in Wisconsin and Iowa has shown that in alkaline soils SCN population densities tend to both get higher and be slower to fall with management (Pedersen et al., 2010). |
| Figure. SCN cysts (white arrows). |
A sub-par field of soybean.
You may recall that during the 2018 growing season the Minnesota Soybean Research and Promotion Council (MSRPC) sponsored an SCN sampling and education program. Half of a couple that farm near the western edge of Red Lake County attended one of these MSRPC-sponsored SCN educational events that summer and brought home two sample bags. She was determined to find out whether SCN could be a cause of the yellowed, stunted, poorly growing soybeans growing in a particular field that summer. That fall she carefully labeled a sample bag, collected from that field multiple 8-inch soil cores from the soybean root zone, gently mixed soil from the soil cores together and sent about 2 cups of that Red Lake County soil to a lab for analysis.SCN population density influences crop selection.
The laboratory extracted cysts, or dead female nematodes from 100 cubic centimeters (a little bit less than ½ cup) of the sample’s soil, crushed the cysts to release SCN eggs and counted the eggs under a microscope to provide an estimate of the soil’s SCN population density. As a general rule, the more soil cores collected from the smaller the field area, the better the estimate of the density of the SCN population. The SCN population density in the Red Lake County sample in 2018 was 25,450 eggs/100 cc.Planting SCN-resistant soybean varieties or rotating to a crop that is a poor host of SCN are the two primary means of managing SCN. University of Minnesota Extension’s SCN management guidelines indicate that when population densities are between 2,000 and 10,000 eggs/100 cc one should expect some yield loss to occur even when planting an SCN resistant soybean variety. With densities greater than 10,000 eggs/100 cc yield loss would be such that planting soybean (or dry edible beans) is not recommended (Chen, 2011).
Following these guidelines, in 2019 the Red Lake County couple grew spring wheat, a crop on which SCN is unable to complete its lifecycle. When a poor host crop is grown, some SCN eggs will hatch only to encounter an inhospitable root. These nematodes will die without mating and (if female) producing more eggs. Some eggs will also perish without hatching. Over time SCN population densities decline when one plants crops that are poor SCN hosts.
Spring 2020 SCN population density.
With the understanding that their alkaline soil made it unlikely, the couple in Red Lake County set out to determine whether the SCN population density had fallen enough in 2019 (but with the understanding that their alkaline soil made it unlikely) to warrant the option of planting an SCN-resistant soybean variety in 2020. However the weather and field conditions did not support many fall 2019 field operations (including harvest, tillage and spreading fertilizer) before soil throughout northwest Minnesota was untrafficable, frozen or buried in snow. So the couple collected a soil sample this spring. The result: 21,500 eggs/100 cc.These results are not meant to communicate a hopeless situation, but rather to remind soybean producers to today set a calendar reminder for mid-September to collect SCN soil samples. The earlier that you know a field is infested, the faster you can begin to actively manage SCN and protect a field’s suitability for soybean and dry edible bean production!
How to collect an SCN soil sample.
Like many commercial soil testing labs, the nematology lab at the University of Minnesota’s Southern Research and Outreach Center in Waseca processes SCN soil samples. Here is a two page document that on one page contains a sample submission form and on the other instructions for sample collection.References
- Chen, S. (ed.) 2011. Soybean cyst nematode management guide. University of Minnesota Extension. Online.
- Niblack, T.L., and Riggs, R.D. 2015. Soybean Cyst Nematode. Pages 100-104. In Compendium of Soybean Diseases and Pests. Hartman, G. L., Rupe, J. C., Sikora, E. F., Domier, L.L., Davis, J. A. and Steffey, K. L. eds. American Phytopathological Society, St. Paul, MN.
- Pedersen, P., Tylka, G.L., Mallarino, A., Macguidwin, A.E., Koval, N.C. and Grau, C.R. 2010. Correlation between soil pH, Heterodera glycines population densities, and soybean yield. Crop Sci. 50: 1458-1464.
- Wang, J., Niblack, T.L., Tremain, J.A., Wiebold, W.J., Tylka, G.L., Marett, C.C. Noel, G.R., Myers, O. and Schmidt, M.E. 2003. Soybean cyst nematode reduces soybean yield without causing obvious aboveground symptoms. Plant Dis. 87: 623-628.
