There aren’t many polite things to say about a pest that can steal 30% of your yield before plants even show any symptoms. Soybean cyst nematode, or SCN, is the industry’s top yield thief, causing more than $1.2 billion in annual losses in the U.S. Still, SCN earns a grudging respect among the scientists who work with it.

“SCN is the ultimate pathogen,” marvels Greg Tylka, Iowa State University Extension nematologist and plant pathologist. “It causes yield loss directly and indirectly by making other things like soybean sudden death syndrome and soybean brown stem rot worse.

“SCN has a unique biology that makes it very difficult to control,” Tylka adds. Females mate with many different males, lay hundreds of eggs at a time, and cycle through four or five generations in a single growing season, he explains. That’s a recipe for creating exploding populations and remarkable genetic diversity — the reason cultural controls and resistance genes get overwhelmed by ever-changing populations of the nematode.

Subtle yield stealer

Symptoms of SCN damage can be easy to miss, so massive populations can build up in fields for years before farmers even notice there’s a problem. Yields can decline slowly, and stunting may be masked by environmental conditions that make patches of shorter plants less obvious, note Allen Wrather and Melissa Ketchum of the University of Missouri’s Division of Plant Sciences.

As infestations increase and crops encounter greater environmental stress, chlorosis, top die-back, stunting and plant death — including outbreaks of sudden death syndrome, or SDS — can become apparent. The trick is not to mistake them for nutrient deficiency, herbicide damage or drought stress, which SCN symptoms often appear to be. 

To be sure, dig up chlorotic plants — particularly around the edges of areas you suspect are infested, where roots are likely to still be healthy enough to support the nematodes — and look for lemon-shaped white or yellow females on the roots. A soil sample may confirm the presence of SCN, too, though Wrather and Ketchum point out that if there are 2 million SCN eggs in an acre of soil, there is just a 63% chance
of detecting one egg in a pint-sized sample.