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The Search for SCN Resistance

By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) — Imagine a soybean trait that could save many Midwest soybean growers up to $200 per acre — in the very first year it was planted.

Greg Tylka has a surprise for you. It already exists.

The Iowa State nematologist has spent decades tracking soybean cyst nematode’s (SCN) growing resistance to the most common source of genetic resistance in soybeans, PI 88788. So it was no surprise to him when field experiments in Iowa last year showed that soybeans with an alternate type of resistance called Peking out-yielded PI 88788 varieties by an average of 21 bushels per acre — nearly $200 per acre, at $9 soybeans.

So where is this goldmine of a soybean trait?

At the moment, growers have only a handful of soybean varieties with Peking resistance to choose from.

Despite its value as a second mode of action against SCN, Peking has remained stubbornly scarce in the marketplace, even as PI 88788-resistant SCN has steadily spread across the major soybean-growing states of the country.

Each year, for 25 years, Tylka has published a list of SCN-resistant soybean varieties suitable for Iowa growers (ranging from Maturity Group 0 to 3.9), with their SCN resistance source included.

This year, of the 891 varieties described in the list, 38 had Peking resistance. Bruce Potter from the University of Minnesota, scraped up 37 varieties with Peking resistance suitable for Minnesota growers. See the Iowa list here: https://soybeanresearchinfo.com/… and the Minnesota list here: http://www.dtn.com/….

Both of these numbers are a bit deceiving, because the same varieties may be marketed by different companies under different brand names and product numbers. That means the actual number of unique soybean varieties with Peking resistance for Iowa and Minnesota growers is almost certainly lower, both Tylka and Potter noted.

A LONG JOURNEY FOR BREEDERS

So what’s the holdup?

For many years, the time-consuming task of breeding Peking resistance genes into high-yielding, elite soybean germplasm was the primary hurdle, Tylka explained.

Many sources of SCN resistance come from wild-type soybeans with undesirable characteristics, such as black seeds, vine-like growth and small plants. Of all the potential sources of resistance genes, PI 88788 emerged as the least difficult to breed into desirable soybean varieties, with the least yield drag, Tylka recalled.

Moreover, keeping up with the rapidly changing herbicide-tolerant platforms is a challenge for breeders, added Jose Aponte, a soybean breeder working as North American Market Segment Lead for Syngenta.

First, efforts focused on getting SCN-resistant traits into Roundup Ready 2 varieties. Then Syngenta breeders started making the transition to Xtend in 2016. Now, just four years later, they’re racing to move their agronomic packages into next-generation platforms such as Enlist E3 and XtendFlex.

“Over the last four to five years, we’ve been transitioning in and out of these new technologies,” Aponte explained. “And it’s very hard to move all these defensive-type traits, like SCN-resistance, to these new herbicide-tolerant platforms.”

But breeders have ultimately seen success with all these efforts. Today’s Peking varieties show competitive yields, Tylka said, and they now contain a host of other beneficial agronomic traits, such as a range of herbicide-tolerance.

Moreover, many growers don’t realize that their PI 88788 soybean varieties may be experiencing significant yield drag of their own, Tylka stressed.

“SCN is chewing up PI 88788 varieties, so they are experiencing a very different — but very real — kind of yield drag that has nothing to do with genetics of the plants and everything to do with the genetics of the soybean cyst nematode,” he said. “So as human breeders have removed the yield drag of Peking-resistant varieties, Mother Nature has dragged down the yield performance of PI 88788 varieties.”

The results of Tylka’s Iowa study showing Peking far out-yielding PI 88788 varieties are unlikely to be limited to that state, where Tylka estimates more than 95% of SCN populations have elevated reproduction on PI 88788.

Other SCN researchers put that number at 88% in Illinois, 56% in Indiana, 60% in Kentucky, 94% in Michigan, 100% in Missouri, 25% in Nebraska, 54% in Ohio, 93% in Tennessee and 67% in Wisconsin. See more here: https://www.thescncoalition.com/….

See Tylka’s study here: https://crops.extension.iastate.edu/….

FUTURE TRAITS TRICKLING IN

Beyond Peking, other sources of resistance to SCN remain years away from commercialization, with one notable exception from Syngenta. In 2021, the company is releasing introductory volumes of a new soybean variety that contains another source of SCN resistance, known as PI 89772.

The variety is the result of 24 years of research and breeding, Aponte said. Syngenta will sell it under two brand names: Golden Harvest GH 2329X and NK S23-G5X. It is a Maturity Group 2.3 bean, with the Xtend trait and a range of other agronomic traits, such as tolerance to SDS, white mold and Phytophthora, Aponte explained. The company expects a full commercial launch of the variety in 2021.

Melissa Mitchum, a nematologist at the University of Georgia, is in phase two of a multi-state project to start pyramiding and rotating PI 88788 and Peking resistance with another source of resistance from the wild soybean, Glycine soja. Across three states where SCN resistance to PI 88788 is common, the researchers’ lab and initial field studies show that rotating among these three varieties has a promising effect on both SCN numbers and their genetic make-up that will boost farmer yields. However, the scientists have two more years of field trials ahead.

For now, that leaves growers with perhaps two dozen Peking varieties and one PI 89772 variety to battle the slow-moving train-wreck of PI 88788-resistant soybean cyst nematode, Tylka noted.

If a grower can find and plant Peking or PI 89772 varieties, they should not discard PI 88788 varieties, Tylka added.

“You have to rotate back, otherwise we’re going down the exact same path but with a different set of genetics,” he said. “We can never put all our eggs in the same basket again, whether it’s Peking or PI 88788.”

THE OTHER SIDE OF THE SEED

In the meantime, companies have turned a lot of their research and development focus for SCN management toward nematicide seed treatments, which can be applied to any soybean seed, regardless of agronomic packages. That makes them highly “portable” and complementary to SCN-resistant varieties, said Dale Ireland, who oversees corn and seed treatments for Syngenta, which has three such products on the market.

Overall, at least 10 nematicide seed treatments are now marketed by seven companies targeting SCN through a range of active ingredients, including biological parasites, chemical controls and plant-growth-promoting microorganisms.

Years of testing these formulations has left Tylka convinced that genetic resistance in soybean varieties is the more consistent source of SCN management, however.

With soybean checkoff funding from the Iowa Soybean Association, he tests these nematode-targeting seed treatments against each company’s base seed treatments at nine locations across Iowa each year. “And we still don’t see consistent yield [increases] from any of these seed treatments,” he said. “They vary tremendously in their yield effects and none are able to replace good resistant varieties. At best they will help bolster the genetic resistance out there.”

See those products here: https://www.thescncoalition.com/….

Mitchum hopes her research project with pyramiding SCN resistance and rotating genetic sources of resistance will help push industry to devote more resources toward diversifying the genetic resistance in the marketplace. The goal of her research is to allow scientists to return farmers’ SCN soil samples rapidly with identification of the type of resistant (or susceptible) SCN populations they have, and instructions on switching to a specific source of resistance.

But for these tests to help farmers, of course, those sources must exist in the marketplace.

See more on soybean cyst nematode from The SCN Coalition here: https://www.thescncoalition.com/….

Learn how to test for SCN resistance here: https://www.plantpath.iastate.edu/… and how to read the results here: https://www.thescncoalition.com/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com.

Follow her on Twitter @Emily_Unglesbee.

(PSSK)

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