SCN Resistance Habits Are Hard to Break

Habits are hard to break, especially if they involve using a farming practice that works. Greg Tylka has long beat the drum for broader use of Peking soybean varieties to tackle the industry's reliance on PI 88788, the primary source of genetic resistance to control soybean cyst nematode (SCN).

Now, the Iowa State University nematologist and leader of The SCN Coalition is issuing another warning as more Peking varieties finally become more available. If farmers plant Peking continually, they may also lose its effectiveness over SCN. Even more ominous, Tylka says it could happen in less time than it took with PI 88788.

The why behind that statement helps to explain his recommendation to understand what type of genetic resistance is in the variety being planted and to deploy both crop rotation and rotation of the genetic resistance.

HISTORY OF GENETIC RESISTANCE

Most farmers understand how 20-plus years of dependence on glyphosate led to weeds that could resist the herbicide. A release from The SCN Coalition uses that herbicide overuse comparison to describe the complicated road of SCN resistance.

Just as glyphosate was extraordinary, so was PI 88788. Brian Diers, who is retired from the University of Illinois and led groundbreaking research on SCN resistance, gives this simple explanation for the industry's prolonged use of PI 88788: "It's been so darn good." He says the successful and high-yielding germplasm has been tough to beat, despite efforts to find alternatives.

Tylka says there were numerous breeding lines with SCN resistance available for variety development in the late 1980s, but none had the agronomic or maturity characteristics needed for soybean production in the Midwest. Therefore, soybean breeders began crossing SCN resistance breeding lines with varieties possessing desirable agronomic characteristics.

PI 88788 stood out because it was the least difficult to work with, and it outperformed other breeding lines on yield. Seed companies continually strive to create new varieties with increased yields and other improved agronomic traits. Once PI 88788 SCN resistance became available in some soybean varieties and breeding materials, these plants were used to develop additional new soybean lines and varieties.

"It's most efficient to take what's already good in the breeding program with PI 88788 SCN resistance and improve on it," Tylka adds. "That's a main reason why 95% of SCN-resistant soybeans on the market today rely on PI 88788."

And, varieties with SCN resistance from PI 88788 did hold up for a long time. PI 88788 was slow acting on the nematode and "leaky," meaning there were low levels (less than 10%) of SCN reproduction on varieties with this SCN resistance from the beginning, he says.

Still, the ability of SCN populations to reproduce on PI 88788 slowly increased over time, snowballing to the point that many farmers now face a major resistance problem.

ENTER PEKING VARIETIES

The good news is the second-most common source of SCN resistance, Peking, is fast acting and highly effective, Tylka notes. That's good, but he adds that in many cases, the stronger the chemical or pest resistance, the quicker the targeted pest population will overcome it.

"Very few nematodes are able to overcome Peking resistance," Tylka explains. "But, those that do have a high likelihood of passing that ability along to their offspring. With Peking SCN resistance, the very few nematodes that successfully reproduce will likely become a high percentage of SCN populations in fewer years than we saw with PI 88788."

The slow but dramatic buildup of SCN reproduction on soybean varieties with PI 88788 resistance has resulted in varieties with Peking SCN resistance that now outyield those by 20 bushels an acre or more, according to a 2019 study by Tylka (https://crops.extension.iastate.edu/…). Now, he's concerned farmers may be tempted to plant Peking year after year to grab those yield gains.

Instead, Tylka urges farmers to:

1. Start growing varieties with Peking SCN resistance now.

2. Never grow varieties with Peking SCN resistance in consecutive soybean crops.

3. Alternate growing Peking SCN-resistant varieties with varieties containing PI 88788 resistance.

4. Plant nonhost crops, such as corn, in annual rotation with soybeans.

ROTATIONAL HANDICAPS

For 2023, the number of varieties with Peking SCN resistance doubled to 53 for Tylka's SCN-resistant soybean variety trials for maturity Groups 1, 2 and 3. "While PI 88788 remains the dominant resistance source in SCN-resistant soybean varieties available to farmers, the increase in Peking offerings for these maturity groups shows seed companies are increasingly aware of the problem and working to fight SCN -- now and in the future," he says.

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-- To learn more about The SCN Coalition and instructional guides and videos, visit https://www.thescncoalition.com/…

-- Greg Tylka's 2019 study can be found at https://crops.extension.iastate.edu/…

-- Follow the latest from Pamela Smith, Crops Technology Editor, by visiting the Production Blogs at https://www.dtnpf.com/… or following her on X (formerly Twitter) @PamSmithDTN

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