For more than 30 years, Greg Tylka has been involved with soybeans and their pests at Iowa State University. And when he starts talking about soybean cyst nematodes, farmers sit up and listen. During the recent Integrated Crop Management Conference, Tylka, a nematologist and director of the Iowa Soybean Center, gave an update on the spread of the yield-robbing pest and how the resistance picture is changing.

Opening with a map of the spread of SCN, which now is thriving in every Iowa county, Tylka offered a look at how the pest has spread into more states and areas, including New York. Then he asked: “Why waste valuable minutes to tell you about soybean cyst nematode’s spread if it’s already everywhere in Iowa? If it can spread to other fields in other states, why would it not spread from field to field in our state as well? When was the last time you checked a particular field for soybean cyst nematode?”

The challenge is that SCN damage may not always be visible. Plants may be stunted in a field area, but show little other damage — yet yields may drop. Knowing you have the problem is the first step in combating the pest.

Tylka points to a simple solution to breaking down nematode population: Rotate to a nonhost crop.

“If you rotate to a year of corn, you can see a 5% to 10% drop in the first year,” Tylka said. “Not as much will happen in the second or third year — almost all of the death will occur in the first year of corn.” After a second year of corn, he explains that eggs may not hatch, going dormant in wait for the return of soybeans, but you can see a healthy decrease in egg counts from the rotation.

How resistance works

There are SCN-resistant soybeans on the market, and a majority rely on PI88788 resistance, which is becoming more problematic as it appears that resistance is breaking down. Tylka shared charts showing the rise in breakthrough impacts of nematodes on fields planted to resistant varieties. Though he noted there’s even variety in how PI88788 works.

PI88788 resistance relies on a gene allele called Rhg1b for “resistance to heterodera glycines 1b” with heterodera glycines being the scientific name of SCN. “Scientists use goofy names for these things,” Tylka said.

That gene — Rhg1b — is even in susceptible plants, but it appears that for PI88788 resistance to work, more of this allele must be present in the plant.

“For resistance to be present, you need seven to 10 copies of that gene, and the more copies you have, the more resistant the plant will be — and that’s the reason why not every variety controls soybean cyst nematode the same way,” Tylka explained.

He added that no one knows how those gene copy numbers relate to losing effectiveness, or how the nematode becomes resistant to PI88788.

There’s another resistance tool on the market, in limited varieties, that shows solid performance against nematodes. Peking resistance relies on two genes: Rhg1a and Rhg4, each occurring as a single copy, but both must be present in the soybean plant for resistance to occur.

“You can’t have just one, and that’s not always guaranteed when making crosses,” Tylka said. “You might have one gene make its way through a cross but not the other. You do need both.”

Loss of resistance

However, in field trials, Peking resistance remains strong, while PI88788 in many varieties is showing more breakthroughs. A look back at 30 years’ of data shows the breakdown of this resistance. While Peking remains effective, it is not widely available.

Tylka showed that in one trial in southeast Iowa, the PI88788 variety yielded 51.2 bushels per acre while Peking on the same field yielded 72.4 bushels per acre.

“We rent from farmers, and we don’t use the whole field, but if the farmer got 50 bushels per acre and lost that 22 bushels, even with beans at $9, he left a lot of money in the field,” Tylka said.

If you’re dealing with soybean cyst nematodes, rotating to a nonhost crop is a solid approach to reduce the population. And consider a look at varieties with Peking resistance. Iowa State has a publication, IPM52, that you can find at the ISU Extension Store. The 2020 report is online, and the 2021 report will be published soon. Go to the site and search “IPM52”; the report is free.