When soybean cyst nematode (SCN) first attacked Jim Legvold's soybeans in the 1990s, he quickly thwarted it by planting SCN-resistant soybeans.

End of story? Not quite. Over time, SCN outfoxed the resistant beans.

"Even though I was planting resistant varieties, the SCN wasn't going away," says the Vincent, Iowa, farmer. "It actually became worse, with cyst counts in the soil going up. If you're growing SCN-resistant beans, that isn't what should occur. They should be going down."

Meanwhile, Extension specialists like Terry Niblack, University of Illinois (U of I) Extension nematologist, heard similar complaints.

"I'd get phone calls from people who grew resistant varieties and were not happy with them," she says. "I would go out and dig up roots and find cysts on what was supposed to be a resistant variety."

Further U of I study showed hundreds of SCN-resistant varieties had excessive SCN root feeding.

"Most nematodes will attack resistant varieties at some level," says Niblack. "It's going to be important to pay attention to levels of resistance in (SCN) resistant varieties."

All this hearkens back to the same resistance rap of weed and insect control. Anytime the same control method repeatedly targets a pest, resistance inevitably results.

In this case, the culprit is PI 88788, the source of SCN resistance for over 95% of soybean varieties. Over time, SCN has adapted to this resistance source. In Illinois, overreliance on PI 88788 resistance has keyed an SCN-type switch in just under 15 years. In Illinois, HG Type 2 (to which PI 88788 is not resistant) made up only approximately 30% of SCN types in 1991. By 2005, it made up around two thirds.

That's what keyed resistance on Legvold's farm. SCN soil tests sent to Iowa State University (ISU) found an HG type that wasn't impacted by PI 88788. Legvold then switched to resistant varieties with another source of resistance, Peking. So far, the Peking soybeans have reduced cyst and egg counts and yielded well.