To gain soybean yield, it’s time to rotate sources of soybean cyst nematode resistance.

“If a farmer has a field that is continually registering about 50 bushels per acre, they're probably going to be like ‘it is what it is’ because maybe they're not seeing any symptoms aboveground,” says Mandy Bish, University of Missouri Extension field crop pathologist and a university scientist who partners with the SCN Coalition, a public checkoff and private partnership that works to increase the number of soybean growers across the country actively managing for SCN.

She says farmers do not need to accept this status quo.

Peking is a variety with SCN resistance. Iowa State University research has shown a 22-bushel-per-acre yield increase in Peking over traditional PI 88788 varieties in fields with high SCN pressure. “Peking shows you what [soybeans] might be able to make, if you would change the SCN resistance," Bish says.

However, Bish realizes that some soybean farmers may be hesitant to make the switch.

“Peking gets a bad rap because in the 1960s, there was a yield drag that came with it. They could not move it into commercial varieties without yield drag,” she explains. “But I would say data shows that has been fixed.”

Peking is not available in all maturity groups. For those with the option, seed should be requested early, as options and availability tend to be limited. Peking fits into SCN management of rotating sources of resistance. However, Bish cautions that Peking should not be viewed as a “silver bullet” for SCN management.

Know your genes

Most soybean growers are familiar with PI 88788 and Peking, the two most widely used sources of resistance to soybean cyst nematode, says Melissa Mitchum, University of Georgia molecular nematologist and another scientist collaborating with the SCN Coalition.

“What might be news to growers is these different sources of SCN resistance have different resistance genes, also known as different modes of action,” she says.

Resistance from the PI 88788 line contains one gene. Resistance from Peking contains two genes and requires both for resistance.

“There are also different flavors — aka alleles — of the Rhg1 genes, which is where the A and B designations come into play,” Mitchum explains. “PI 88788 has Rhg1b, and Peking has Rhg1a plus Rhg4.”

There’s yet another wrinkle when it comes to resistance genes: the number of times that gene is repeated in an SCN-resistant soybean variety.

Keep track of your copies

Not all PI 88788 varieties are the same.

There could be varieties from PI 88788 that have slight differences in copy number, Mitchum says. Growers should rotate different varieties of soybeans with PI 88788 to combat resistance.

While copy numbers are part of the equation, the other aspect is how the genes interact.

For example, Mitchum says Rhg4 from Peking doesn’t function on its own; it also doesn’t talk to Rhg1b from PI 88788. “Combining them doesn’t do anything,” she says. So, researchers are looking for options.

“Pyramiding two resistance genes from wild soybean [Glycine soja] onto Rhg1b does have an effect on combating virulent nematodes,” Mitchum adds. Virulence is the ability of SCN to reproduce on a resistant soybean plant.

“Ideally, we’d like growers to rotate among several different modes of action of SCN resistance,” she says. “That’s what we’re working to provide.”

Mitchum is collaborating with soybean breeders across the country to investigate other sources of resistance. The group has already found new genes and gene combinations that are different from what is currently available and can help farmers fight back against virulent SCN.

The goal is to get more SCN-resistant modes of action on the market for farmers and protect existing SCN resistance sources.