A map available from South Dakota State University offers a detailed look at where soybean cyst nematode has been confirmed over the years in Union, Clay, Turner, and Lincoln counties.

Download the free map online. Or ask at your county Extension office.

The map is in PDF format. Computer users can use free Adobe Acrobat Reader software to zoom in on specific areas of the map to study locations in detail.

Professor and research nematologist Jim Smolik said the map comes from an analysis of data from SDSU’s ongoing program testing and monitoring for soybean cyst nematode, the most damaging pest of soybeans in the United States.

The soybean cyst nematode, or SCN, is a small, plant-parasitic roundworm (Heterodera glycines) that feeds on the roots of soybeans. Most nematodes are too small to be seen with the naked eye, though the adult females and cysts of SCN, at about 1/32 of an inch long, are big enough to be visible.

SCN was first confirmed in South Dakota in 1995 in Union County. It has since spread to 19 counties in the eastern part of the state, but infestations remain the worst in the southeast corner.

“We went back to our records that we have accumulated over the last seven or eight years, primarily based on samples that we’ve received in the nematode testing service,” Smolik said. “The initial county that we found it in was Union. Subsequently we found it in nearby counties — Clay, Turner, and also Lincoln. Those seem to be the counties that currently have the highest percentage of fields that are infested.”

SDSU has assembled a map of those four counties. The map has sections shaded to indicate those sections that have a field or fields that were positive for soybean cyst nematode. Crop consultants from the four-county area assisted in preparing the map, Smolik said.

“Certainly this map is not complete. There are other fields out there that we didn’t test, or that were not identified well enough for us to go ahead and mark them,” Smolik said.

“What we hope to convey with this map is the widespread distribution of soybean cyst nematode, particularly in southeastern South Dakota. If you think you have the nematode, you might want to have your soil tested. Or if you don’t have time to do that, and you have not yet purchased soybean seed, plus it looks like you’re in an area that has high levels of infested fields, you might want to consider planting an SCN-resistant variety.”

Using crop rotations and planting SCN-resistant soybean varieties are among the best ways of managing soybean cyst nematode. Smolik said 11 years of field plot data from SDSU tests plots in Clay, Roberts, Turner and Union counties indicate that SCN-resistant lines yield from 23 percent to 63 percent more than susceptible varieties.

Smolik added that the South Dakota Soybean Research and Promotion Council began supporting the SCN testing program at SDSU in 1995, the same year in which SCN was first discovered in South Dakota in Union County. Since then, with the council’s support, SDSU has tested about 10,500 samples for soybean cyst nematode.

“We’ve been able to test soils for soybean cyst nematode from South Dakota soybean producers at no cost. I think that’s probably helped encourage folks to send in samples and it has enabled us to track its distribution and spread.”

That, in turn, has helped producers do a better job of managing SCN in areas where the pest is found.

SDSU Extension Fact Sheet 902A, Soybean Cyst Nematode has more details about managing SCN. Or ask at your county Extension office.