There have been many yellow spots in soybean fields throughout Iowa so far this season. In most cases, the yellowing is iron deficiency chlorosis. But since mid-July, additional yellowing of soybean fields has appeared, and it is likely that at least some of the newly appearing chlorosis is being caused by feeding of the soybean cyst nematode (SCN). SCN usually is present in fields for many years before population densities increase to a level that causes obvious stunting or yellowing.
Available at: http://works.bepress.com/gregory-tylka/95/