AMES, Iowa -- During the past few weeks, areas in many Iowa soybean fields have turned yellow. In many of these cases, the yellowing is due to feeding by the soybean cyst nematode (SCN).

"SCN usually is present in fields for many years before population densities increase to a level that causes visible stunting or yellowing," said Greg Tylka, Iowa State University extension nematologist. "When yellowing occurs, it usually appears in late July or early August. The yellowing often will fade following rainfall."

Although there is nothing that can be done to manage a SCN infestation if it is discovered in the middle of the season, Tylka said it is important that fields exhibiting these yellowing symptoms be diagnosed properly so that management strategies can begin to be implemented. Specific recommendations given by Iowa State for SCN-infested vary depending on the level of the nematode present. Management strategies include growing nonhost crops, such as alfalfa and corn, and SCN-resistant soybean varieties.

"Diagnosing SCN infestations is done by digging roots and looking for the small, white or yellow SCN females on the roots," Tylka said. "This can be done through the month of August. Alternatively, soil samples can be collected from the suspect areas of the field and sent to a qualified laboratory for testing for SCN cysts or eggs. Soil samples can be collected anytime between now and the end of the growing season."

For more information about SCN and how to diagnose infestations, contact your county extension office for printed publications on SCN biology, scouting, management and SCN-resistant soybean varieties or visit the Internet, www.scnfacts.org.