Soybean Farmers Struggle with SCN Control

The Lost Bushels of SCN

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
Connect with Emily:
Closeup of soybean cyst nematode (Pamela Smith)

For years, Aimee Bissell has known she's going to lose yield and profit on the soybeans she plants on her Bedford, Iowa, farm, well before they ever emerge.

The problem? The varieties she plants each year have already been outsmarted by a tiny but devastating pest: soybean cyst nematode (SCN). Bissell has long asked her local seed dealers for access to an alternative source of genetic SCN resistance called Peking.

But, a stubborn mix of breeding complexity and industry agronomic priorities have kept this option in short supply in a limited set of maturity groups for decades. Bissell is hoping farmers can use their market sway to help break through the logjam. "If I could just get more people to fight with me," she says. "If we make enough noise, maybe people will pay attention."

BILLION-DOLLAR PROBLEM

For decades now, the most common source of SCN resistance, a genetic trait called PI 88788, has been found in at least 95% of SCN-resistant soybean varieties. In a distant second place, Peking accounts for only 5% of varieties. For now, there is no third-place contender.

Not surprisingly, farmers across the Soybean Belt, such as Bissell, are facing down SCN populations that can successfully reproduce and steal yield on PI 88788 varieties. It is a growing and costly resistance problem for the soybean industry that somehow still flies under many farmers' radars, says Iowa State University nematologist Greg Tylka.

In the meantime, the amount of money farmers hemorrhage to this microscopic root parasite each year is staggering.

Two independent, multiuniversity studies have concluded that SCN steals a devastating $1.5 billion from soybean farmers across the U.S. and Canada each year. On a more personal level, a recent study of Tylka's found that farmers with PI 88788-resistant SCN in their fields can gain up to 22 bushels per acre (bpa) by switching to Peking soybeans.

At $15 soybeans, that's a value of $330 per acre. "That's breathtaking," Tylka says. Even at more average yield responses in his past studies, which range from 5 to 15 bpa, the savings are impressive: $75 to $225 per acre saved by switching to Peking in fields with SCN that have outsmarted PI 88788.

LIMITED OPTIONS

Currently, most of the available Peking varieties sit in the Maturity Group 1 or 2, which puts them out of many farmers' geographic reach, including Bissell, who hasn't been able to source locally the Maturity Group 3 soybeans she needs.

Although seed treatments targeting SCN abound, their effects are far less consistent than varietal resistance traits in university trials, Tylka adds. "Seed treatments are not a replacement for effective SCN resistance, even when the seed treatments are working at their best," he says.

Seed-treatment options turn over with some frequency, as well. Bissell recently lost access to a product called Clariva, a nematicide which she felt worked well on her fields but was recently discontinued. Its owner, Syngenta, has turned its focus to a new fungicidal seed treatment called Saltro, which the company says also helps control SCN.

"I have been beyond frustrated," Bissell admits. "The scientists have given us a beautiful road map on how to take care of this. I really just need to switch up my soybean genetics, but I cannot do it."

The good news is that seed companies such as Syngenta and Corteva predict they will have a broader range of Peking varieties in their portfolios in the next year or two, company representatives say.

The bad news is the same problem that has slowed the market for new SCN-resistant soybeans remains in place: SCN traits outside of PI 88788 can hold a complicated mix of genes that don't move easily from variety to variety via breeding. That's a major handicap in the fast-moving world of soybean breeding, where new traits such as herbicide tolerance drive the market and require companies to trade out their elite germplasm every few years.

SLOW AND STEADY IS LOSING THE RACE

Breeding a package of genes into high-yielding soybean germplasm is a slow process. It takes years of crossing and backcrossing plants trying to bring over only the SCN genes and not any less-desirable traits -- such as lower yield -- along the way, explain Golden Harvest corn product manager Ryan Fuller and Syngenta head of soybean breeding for North America Jose Aponte. In the past decade, seed companies have moved through multiple weed-control platforms such as Roundup Ready 2, Xtend, Enlist E3 and XtendFlex. That's a breakneck speed for soybean breeders.

"As trait technology keeps moving and changing faster and faster, it makes it difficult to get things brought into the new varieties as you transition to new trait platforms every few years," Fuller says.

One such victim of this fast-moving trait world was a novel form of SCN resistance known as PI 89772. In 2020, Syngenta introduced a new variety to the marketplace with this resistance, sold under the Golden Harvest and NK brands. Field tests showed the variety excelled at controlling SCN, including PI 88788-resistant SCN. (See more from DTN at https://www.dtnpf.com/…)

It had only one fatal flaw: It was in a Roundup Ready 2 Xtend variety, a herbicide-tolerant platform that is rapidly vanishing from the marketplace in favor of XtendFlex and Enlist E3.

"Ultimately, resistant weeds are still driving the trait market in soybeans," Fuller says. "As we transition to XtendFlex and E3 germplasm, this doesn't have a home currently."

Syngenta is still working with PI 89772 in its breeding projects, and the source may someday make it back to the market, Aponte explains.

But, for now, Syngenta soybean breeders are more focused on pushing Peking into more varieties instead. That's largely because the Peking trait is dependent on a smaller package of two genes, which makes moving it into new varieties a comparatively simpler process, though still less straightforward than PI 88788.

"We're trying to improve the frequency of the Peking alleles in our germplasm," Fuller explains. "We're making some progress on Enlist E3, in particular. We're not only increasing the number of varieties, but the diversity among the different maturity groups is starting to widen," he says. In the next couple of years, Syngenta should start rolling out Peking varieties deeper into Maturity Group 2 all the way through to Maturity Group 0.

For now, with 25 individual varieties in the market, Corteva's Pioneer brand remains the leader for Peking availability, and the company expects to expand its maturity group offerings soon, too, Corteva soybean breeder Don Kyle says.

"We're poised to see a pretty large increase in Peking varieties in the coming years," he says. He predicted that within the next two years, growers will be able to find Pioneer Peking varieties in Maturity Group 3 or even 4.

Further in the future lies a promising nematode-resistant soybean (NRS) trait under development by BASF. The Bt protein Cry14Ab-1 is the first of its kind to target SCN. "It shows moderate efficacy for the control of SCN and is expected to be bred into soybean varieties with native SCN resistance, like Peking, to present multiple modes of action," says BASF strategic marketing lead Jonathan Fischer.

BASF, Corteva and MS Technologies recently signed licensing agreements for use of the NRS trait. The three companies anticipate commercialization of Enlist E3 soybean varieties containing the NRS trait in the late 2020s, pending regulatory reviews and completion of field testing.

Likewise, the use of gene editing, which allows scientists to more precisely insert or remove single genes from plant DNA, could speed up the transition of SCN genes into newer varieties, both Syngenta and Corteva representatives say. Kyle says Corteva is in the "early stages" of integrating one such method, Crispr-Cas9, into its breeding program, and SCN resistance is one possible area for its use. Syngenta is also conducting promising "proof of technology" tests on gene editing, Aponte adds, but commercial products are likely a decade away.

MAKING FARMERS CARE

Bissell was able to hunt down some Clariva to fight SCN through the 2022 season, but as the product leaves the market, she will start her hunt for Peking varieties afresh for 2023.

Why more farmers aren't taking Bissell's proactive approach -- testing their fields and banging on the door of seed companies each year for more sources of SCN resistance -- remains a question that has plagued Tylka for years.

He blames the secret weapon of SCN: its invisibility. Soybeans can leach a surprising amount of yield to their root parasites without showing obvious symptoms aboveground, he says.

"Our problem is the plants still don't look sick even though underground, the PI 88788 resistance is no longer working," he says. "Fields that don't have it still look very similar to a field losing 10 or 15 bushels per acre."

Researchers such as Tylka have recently rebooted The SCN Coalition, an industry-university-soybean checkoff collaboration launched in the 1990s, which urged farmers to test their fields, find PI 88788 varieties and rotate to corn.

"That worked wonderfully well," Tylka recalls. "We made a lot of progress. But, I think many farmers heard that message, switched to PI 88788-resistant varieties, rotated with corn and then just forgot about SCN."

But, SCN was far from beat. The wily nematode has now evolved resistance to PI 88788 across the Soybean Belt. For example, university researchers estimate that 100% of SCN populations have elevated reproduction on PI 88788 soybeans in the state of Missouri, 60% in Kentucky, 88% in Illinois, 56% in Indiana, 95% in Iowa, 94% in Michigan, 64% in Minnesota, 25% in Nebraska, 54% in Ohio and 93% in Tennessee.

Another hiccup is the difficulty of sampling for SCN. Populations can vary widely through a field, and because of the nematodes' microscopic size, catching an infested spot with a soil probe isn't easy.

"Hot spots can be the size of a pickup truck, and we have 2,500 acres of soybeans," Bissell explains. "To grid-sample down to the size of a pickup isn't economical. So, it's challenging to figure out how significant your problem is and -- if you find out it is significant -- what will you do anyway?"

In Fruitland, Iowa, Shawna and Dave Dean have had better luck. They are working sandy, Mississippi river-bottom ground owned by Shawna's father, Ron Shepard. This is the same ground where Tylka documented a 22-bushel-per-acre boost from Peking varieties in 2019.

On their most SCN-infested fields, they have been able to source and use Peking soybean varieties in Maturity Group 2.5. "It has been very helpful," Dave says. "Especially on our lighter soil, it really does better [than PI 88788]."

But, for growers who lean exclusively on Peking, the same grim fate lies ahead, Tylka worries.

"Resistance to Peking can and will develop," he says. Rotating between Peking and PI 88788 can help slow that, but ultimately, the industry needs more genetic sources for a true integrated SCN management, he adds.

**

FOR MORE INFORMATION:

-- For current and past market availability of PI 88788 and Peking soybean varieties, visit https://faculty.sites.iastate.edu/… for a guide from Iowa State University.

-- Visit The SCN Coalition's website for information on identifying and managing SCN: https://www.thescncoalition.com/…

[PF_0922]

Emily Unglesbee