Save SCN Resistance

Conquer Your Soybean Costs - Save SCN Resistance

Looking for SCN females, which appear as tiny white dots on soybean roots from June through early August, is a good first step in evaluating SCN-resistant soybeans. (Progressive Farmer image by Pamela Smith)

Luke Wilhite was born into soybean cyst nematode (SCN) management.

By the time he entered the world in 1991, his father, Pete, had recently identified and gone to battle with an invisible enemy lurking under their soybean fields in central Iowa.

"He had struggled growing decent soybeans for years," Wilhite recalls. "He was only getting 20 to 30 bushels per acre and couldn't understand why." Then, in 1989, his father tried a new AgriPro soybean variety that boasted a newfangled weapon: PI 88788, the first available genetic source of SCN resistance.

PI 88788 expressed a group of genes that greatly reduced the reproduction of SCN: the small, parasitic roundworms that attack soybean roots and rob nutrients.

"It was a complete game changer," Wilhite says. "From there, it immediately doubled our yields."

Fast-forward 30 years, and PI 88788 is an old industry workhorse, staggering under the load of SCN populations that have grown accustomed to it. Scientists in 10 states report the majority of SCN populations there can reproduce on PI 88788 soybean roots at least moderately well. The only other commercially available source of resistance, Peking, is still effective, but those soybean varieties are in short supply on the market.

"Every year, we see greater and greater reproduction on PI 88788," says Iowa State University nematologist Greg Tylka. "And, under the right environmental conditions, we see greater and greater yield loss."

That's why, like his father before him, Wilhite jumped at the chance to try a new type of genetic SCN resistance this summer. Known as PI 89772, this type of resistance is available only in two Syngenta varieties this year, Golden Harvest GH 2329X and NK S23-G5X.

It is the first unique source of genetic resistance to SCN to be commercialized by a major seed company in 25 years, Tylka notes. But, perhaps not for long.

This summer, BASF received the go-ahead from the EPA to start using a novel form of SCN resistance: a Bt soybean trait that provides moderate suppression of the pest. BASF hopes to bring the trait, known as GMB151, to market in the latter half of the decade.

"These are really the first two new things to come down the pike for SCN resistance in decades, so it's pretty important," Tylka says.


Syngenta's PI 89772 varieties were only planted on a limited number of fields this past summer, but a larger rollout is planned in 2021, the company says. For now, the trait is embedded in the Xtend Roundup Ready 2 genetic platform, but the company is working to make alternate SCN sources like Peking and PI 89772 available in both Enlist E3 and XtendFlex soybeans in the years to come, says Travis Kriegshauser, strategic marketing manager for soybeans for Syngenta.

The 2020 season gave the new PI 89772 varieties a robust challenge, says Jeff Anderson, a Syngenta seed adviser and farmer from the central-Iowa county of Hamilton. His state experienced an unusually dry summer, which stresses plants and can favor increased nematode reproduction.

"We dug into a pilot trial with 24 soybean varieties, and every single one was just loaded with cysts, even the PI 88788 varieties," Anderson says. "Then, we started digging on the [PI 89722] variety, and there were very little cysts on the roots. It was very clearly working."

But, for how long? That worries Anderson.

"Mother Nature has a funny way of wanting to survive," he says. "Just like with our Bt traits, if you don't switch them out with other modes of action, they have a tendency to stop working as well."

That concern keeps Tylka up at night, too. He has spent most of his career urging growers to seek out and grow alternate, non-PI 88788 SCN-resistant varieties, such as Peking or now Syngenta's new PI 89772.

Now, he wants to pivot and remind them that they must still rotate with PI 88788 periodically if they want to avoid SCN resistance building up on the new types of resistance.

"Stewardship is more important than ever," he says.

Both Wilhite and Anderson also use seed treatments marketed for SCN management. There are a dozen such products on the market from seven different companies.


Although it remains several years from the field, BASF's GMB151 soybean would be doubly unique, as both the first Bt trait to target nematodes and the first Bt soybean to be commercialized in the U.S.

It contains a Bt protein, Cry14Ab-1, which damages the gut of soybean cyst nematodes that ingest it.

"BASF intends to breed the GMB151 SCN trait into soybean varieties already possessing native resistance traits, such as PI 88788 or Peking, to further bolster the overall nematode-management plans for soybean growers," BASF spokesperson Odessa Hines says.

That strategy may also help preserve yields, Tylka adds.

"In this case, where at least one of the modes of action -- PI 88788 -- has already lost some of its effectiveness, it makes a lot of sense from a yield-protection standpoint," he says.

BASF won't be required to submit a resistance-management plan for GMB151 until it gets a full commercial registration from the EPA. As with the Syngenta varieties, future stewardship will be key, Tylka explains.

"Our experience with Bt corn teaches us that this type of technology is not immune to being overcome by pest populations, so we need to be cautious with how we use it," he continues. "We would be foolish to think Bt resistance alone would be the cure for SCN."


> The SCN Coalition:

> Commercial Seed Treatments for SCN:


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