After 30 years of using the same resistance source, SCN is adapting. Learn how Missouri’s research and rotation strategy are helping growers fight back.
Soybean Cyst Nematode (SCN) may be invisible to the naked eye, but its impact on yield is crystal clear. For Missouri farmers, battling this persistent pest means leaning into decades of data, statewide support and a deepening understanding of SCN’s ever-evolving strategies. At the heart of this effort are Mandy Bish, a plant pathologist and extension specialist at the University of Missouri (Mizzou), and Jeff Barizon, a Mizzou research specialist and doctoral candidate specializing in nematology. Together they work to help growers make smarter decisions in fields where SCN is quietly cutting profits.
“I think one of the things that Missouri has specifically contributed to the broader SCN Coalition is we do a lot of the virulence type testing for farmers across the U.S.,” Bish says. “That includes the HG type testing for North Dakota, Nebraska, Indiana and Missouri farmers.”
That testing allows farmers to understand how well SCN populations in their fields can reproduce on different soybean resistance sources — most notably PI 88788 and Peking. Unfortunately, Bish says the picture isn’t great.
“What we have been able to find through some of the Missouri work that helps inform the profit checker and just helps with the messaging of the coalition, is that it’s not surprising that we’re seeing this reproduction on PI 88788,” she explains. “We’ve used the same type of resistance for 30 years. I like to equate it to a farmer who is trying to control waterhemp with glyphosate to help put the problem into a perspective most farmers have experienced.”
Even more concerning? The next-best option — Peking — is showing cracks, too.
“What we’re finding now is that when we do this HG type test or virulence testing, a lot of these SCN populations also can reproduce on Peking soybean,” she says. “So that’s our next seed trait, and it’s already having issues … it’s breaking down.”
That’s why Bish and her team strongly promote a rotational approach, not just between soybean and corn, but between different sources of SCN resistance in soybeans.
“If we can have a rotation that has both PI 88788 and Peking in it, then we’re going to slow down the development of resistance to Peking as well,” she explains. “So I would say, consider putting both types of soybean in a crop rotation is one of the areas where we’re really pushing.”
And for Missouri growers, the most practical and effective strategy remains a corn-soybean rotation that includes both types of SCN-resistant soybeans.
“We don’t have a common third crop in Missouri, so PI 88788–corn–Peking–corn–PI 88788–corn–Peking–corn,” Bish says. “That rotation helps us stretch the effectiveness of both resistance sources.”
Missouri is also taking a proactive approach to field-level surveillance of SCN pressure. Thanks to support from the Missouri Soybean Merchandising Council (MSMC), Bish’s team continues a long-standing tradition of conducting a statewide SCN survey every 10 years.
“The University of Missouri has done a statewide survey every decade since the 1980s… So in the 1980s and then again in the 90s, 2000s — we’re on the fifth iteration of it,” she says.
The results tell a compelling story. Before resistance was widely adopted, SCN was found in around 70% of fields. After the introduction of PI 88788, SCN dropped to just 49% of fields sampled. But then things took a turn.
“In the 2010’s survey, you see this sharp increase to 88%,” she reveals. “You can almost clearly see or speculate where PI 88788 broke in Missouri and presumably in the neighboring states. So we’re seeing similar trends again in our 2020 survey.”
That historical perspective shows exactly why SCN can’t be managed by seed alone.
“We really need an integrated approach, because we can’t just rely on Peking as the silver bullet,” Bish says. “We have some really good results with Peking and yield in high-SCN environments, but then the farmer just wants to run with that part, and it’s like, no, no — it’s got to be integrated.”
To support growers in real time, Missouri also offers SCN sampling through the state’s nematode clinic. The service, supported by MSMC, is free for farmers, and samples can be coordinated through any University of Missouri Extension field agronomist.
“The nematode clinic at Missouri is SCN Diagnostics,” Bish says. “We work with MSMC to provide farmers free sampling through the clinic.”
The best time to sample? That depends on your question.
“At the end of the year is when you’re going to have the highest counts,” she says. “But if somebody is questioning whether their management strategy is working, then having planting counts in addition to that end-of-the-season count can be really helpful.”
And while testing near the end of the season won’t allow for in-season intervention, it still has value.
“It’s knowledge,” she says. “That’s close to when you’re going to be making your seed selection for next year. So that information is valuable. You can do something at that time — not just sit and stew on it.”
Meanwhile, Bish and her team are also keeping an eye on future resistance options. She’s cautiously optimistic about upcoming seed traits but notes that genetic resistance is still the most dependable path forward.
“I think any way that we can get more genetic options out there quickly is going to be the best solution,” she says. “Our seed treatments can help, but they all are environmentally dependent. The more genetic options we have, the better.”
The collaborative effort of the SCN Coalition has been great to generate awareness of this problem, and the bottom line for Bish is clear.
“The bottom line is, how can we help the growers manage this as best we can? We’re already behind on it.”
And as for a world without SCN?
“I try to tell my farmers, once you introduce SCN, or any soilborne pathogen to a field, you can’t unintroduce it,” she says. “You’ll never unintroduce it. I don’t see a world without SCN unless we stop growing soybeans. And we hope that never happens.”
This article is paid for in part by the United Soybean Board.