How to manage the $1.5 billion soybean cyst nematode problem

With a rapid life cycle and estimated annual $1.5 billion yield loss potential, soybean cyst nematode (SCN) has consistently been rated the most damaging soybean pathogen in North America.

"It's widespread, it reproduces very quickly, it can survive like the dickens, and it causes a lot of yield loss," says Greg Tylka, an Iowa State University Extension nematologist.

• Related link: Check roots for SCN this growing season

Since the 1990s, farmers have relied almost completely on the PI88788 source of SCN resistance to protect yields.

"It's hard to breed in genetic resistance for SCN because it's not a single gene," says Sam Markell, a North Dakota State University plant pathologist. "It takes more time and more effort. When PI88788 was found and that resistance could be bred in relatively easily without a yield drag, everybody started using it. The companies got on board."

Today in Iowa, 95% of SCN-resistant soybeans contain the PI88788 form of resistance, and it's starting to lose its edge.

"When I speak to farmer groups, I ask them, 'Can you imagine what would happen if you used a single herbicide active ingredient for 25 years?'" Tylka says. "They start to giggle because they used Roundup [glyphosate] for 25 years, and now there's weeds all over the place that are resistant to Roundup. It's the exact same thing that has happened with SCN resistance."

Peking, an alternative form of SCN resistance, exists but can be difficult to find.

"When SCN was becoming a problem, seed companies documented that the Peking type of resistance made the plants lower yielding," Tylka says. "Today, the yield reduction caused by SCN on PI88788 varieties is much greater than the lower yield potential that Peking used to have, but the industry hasn't acknowledged or shifted to address that."

Tylka says of the 872 soybean varieties suitable to grow in Iowa for 2022, only 34 of those contain the Peking source of resistance.

A proactive approach in choosing soybean varieties can pay off. A southeast Iowa variety trial in sandy soils found Peking varieties yielded 22 bushels an acre more than surrounding PI88788 varieties, Tylka says.

"It's really striking," Tylka says. "Not every field in Iowa is sandy, but the data we get from southeast Iowa is a vision of the future for medium and heavy soils. It's a train wreck in slow motion."

If farmers start asking for more variety in SCN resistances, supply should follow.

"Companies are pretty good about responding to growers' needs," says Markell. "If you don't have a source of resistance that's not PI88788, ask."

Other management steps

Soil sampling is the first step in combating the pest.

"The best time to soil sample is in the fall after harvest because it's easier to zigzag your way through a field after the plants have been removed," Tylka says. "Fall is also a perfect time because when you send that sample to a lab, you might not get results for two to four weeks, but you'll still have time to come up with a plan for the next growing season."

Rotation with non-host crops is an easy way to minimize populations, but with increased input costs, more farmers are considering back-to-back soybean crops.

"There's the fear that we may go away from some of our rotations and have more back-to-back soybean fields," says Carl Bradley, a University of Kentucky Extension plant pathologist. "Those fields are where we're more likely to see population increases of SCN."

Rotating soybean varieties, even within varieties carrying the PI88788 resistance, can also be beneficial. "There can be variability across varieties that are utilizing the same source of resistance, so even if we just have PI88788, it's important for farmers to not go with the exact same variety year after year," Bradley says.

Though PI88788 resistance is worrisome, don't lose hope.

"This change is happening really slowly and there's still good value in PI88788 for the vast majority of growers," says Markell.

Other Management Steps