By John Wilson and Loren Giesler

Thirty years ago last fall, a microscopic pest was identified in a Richardson County soybean field near Falls City. The next spring, a comprehensive sampling program identified the same pest in six counties bordering the Missouri River, as well as Pawnee County. Levels of this pest in the soil indicated it had been here much longer, but had gone undetected.

Today, this pest — the soybean cyst nematode (SCN) — is causing more yield losses for soybean growers in Nebraska and across the U.S. than all other soybean diseases combined. Last year SCN cost Nebraska farmers an estimated $40 million in lost yields; nationally, that loss is estimated at $1.5 billion.

Soybean Board supports SCN sampling
Without a concentrated effort to sample fields for SCN, it was identified in 27 counties over the next 19 years. Then in 2005, the Nebraska Soybean Board started an extremely successful program that continues today. It provides Nebraska soybean farmers with free soil analysis for SCN by the UNL Department of Plant Pathology. It started slowly, but over the years it grew and has now processed 8,230 samples, almost a third of which have been positive for SCN.

Nebraska counties where SCN had been confirmed as of Jan. 1, 2017.

This program had an immediate impact. The first year of the program SCN was identified in seven new counties, and in the first seven years, the number of counties where SCN had been identified doubled the number found in the previous 19 years. This illustrates the importance of sampling. In most fields, the grower had no idea SCN was present, but he or she knew their soybeans weren't yielding as well as they expected, or as well as their neighbor's fields. Farmers can have 20% to 30% yield losses with no visible symptoms on the plant—the plant looks dark green and healthy.

Support from the Nebraska Soybean Board covers the cost of analyzing the soil samples, normally $20 per sample.

SCN indicators
Often the first sign of an SCN infestation is when soybean yields hit a plateau or start to drop off, while corn yields in that field continue to increase.

SCN infestations are rarely uniform across a field. Other signs include field areas you visually observe or that show up on your yield maps with lower-than-expected yields not explained by differences in soil type, weed pressure, herbicide injury, insect infestations, flooded or compacted areas, or other factors.

Another possible sign of SCN infestations are pockets of sudden death syndrome or brown stem rot. Both of these diseases enter the plant through the roots. Because SCN attacks the roots of a plant, they create microscopic wounds that make it easier for these diseases to infect a plant. It is possible to have SDS or BSR without SCN in a field, or you may have SCN in a field without SDS or BSR. However, in areas where SCN populations are highest, you are more likely to have either of these diseases.

If you observe any of these signs of SCN, collect and analyze a soil sample from the lower-yielding area and one from the higher-yielding area, and compare the SCN egg counts.

Identifying the problem in individual fields is important, because once you know you have SCN in fields, you can start managing this devastating pest.

Next steps
Although SCN often goes undetected, it is here and reducing profitability for Nebraska soybean producers. To learn more about SCN or to pick up bags to submit soil samples from your fields, contact your local Nebraska Extension office.

Wilson is a Nebraska Extension educator and Giesler is an Extension plant pathologist. This report comes from UNL CropWatch.