Greg Tylka, Iowa State University nematologist, has found that soybean cyst nematode (SCN) resistant soybean varieties are becoming less effective.

"This is an alarming trend and sets the stage for even greater yield loss from SCN in the future," Tylka said.

Resistant varieties have helped farmers manage the nematodes for decades. Almost all SCN-resistant soybean varieties possess the same resistance genes, from a...