During the past few weeks, areas of many soybean fields throughout Iowa have turned yellow. In several of these fields, 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. When yellowing occurs, it generally appears in late July or early August. The yellowing often fades after rainfall.
|Yellow spots in field infested with soybean cyst nematode.|
There is nothing that can be done to manage an SCN infestation if it is discovered in the middle of the season. But it is important that fields exhibiting these yellowing symptoms be diagnosed properly this year so that management strategies can be implemented if SCN is discovered. The specific management recommendations for infested fields depend on the SCN population density in the field, but include some combination of 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; roots can be inspected throughout August. Alternatively, soil samples can be collected from suspect areas of the field and sent to a qualified laboratory for testing for SCN cysts or eggs. These soil samples can be collected anytime between now and the end of the growing season. The Iowa State University Plant Disease Clinic offers testing of soil samples for the presence of SCN, as do several private soil-testing laboratories throughout Iowa.
|Soybean cyst nematode females on roots of infected soybean plant.|
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 http://www.scnfacts.org  on the Web.
This article originally appeared on page 166 of the IC-486(21) -- August 20, 2001 issue.