SCN Gears Up to Steal Yield

Soybean Cyst Nematode ( Sam Markell )

Lurking beneath the soil, hungry nematodes eagerly await planting season. Soybean cyst nematodes (SCN) are a particularly hungry pest, leading to an estimated $1 billion in yield loss each year.

As planting season nears, it’s important to take a close look at nematode populations in your fields. While  you’ve likely selected seed varieties and treatments, you can still be mindful of what those decisions mean for nematode population growth.

“If you didn’t sample for nematodes in the fall, consider doing spring sampling to see what your SCN number looks like,” suggests Sam Markell, nematologist at North Dakota State University. “Say you have 5,000 eggs, after you put your soybeans in the ground they’ll have a season to grow so you should sample again in the fall to see if your population goes up, down or stays the same.”

If populations spike, that tells you it might be time to consider adding, or changing, your SCN management. On the other hand, if the population is stable it means your management practices are working.

When you’re out scouting for weeds, diseases and other pests, take time to dig soybean roots and look for infestation.

“Think about what a soybean nodule looks like and the SCN will be much smaller—about 1/5 to even 1/10 the size,” Markell says. “They’re white to cream colored, attached to the roots and under a magnifying lens look like tiny little lemons.”
You’re more likely to find SCN in areas that commonly have standing water and near waterways and field entrances.

“Areas with higher pH have higher SCN reproduction, the higher the pH, the more likely you are to have higher populations,” Markell adds. “pH is more important than soil texture, but sandy soils can show more symptoms of nematode damage if the plant is water stressed.”

Farmers in the “I” states and other areas with known SCN populations should actively manage the pest because it’s developing resistance to certain genetics. However, states where SCN is a new problem face challenges, too.

“We’re taking nasty yield hits because not all varieties have resistance,” Markell says. “When growers plant a susceptible variety in fields with SCN we often see 30% to 40% yield loss in at least parts of the field.”

Fringe soybean states that don’t have a history of SCN are at risk of higher yield losses because soybeans with genetic resistance could be less available. Test for nematodes now and in the fall and check roots throughout the season.

The Future of SCN Resistance Is Here

Researchers, in partnership with the SCN Coalition, have found two forms of resistance that can be combined with other genetic resistance. They’ve also identified how to make PI 88788 more effective.

The new genetic forms of resistance have been released to anyone who wants to use them. They feature resistance found in wild soybeans (Glycine soja), stacked with PI 88788 resistance. Researchers say they’re easy to backcross into current soybean varieties.

Nematologists have identified the two most important genes in soybeans to confer resistance. When the Rhg1 and Rhg4 genes are repeated in the plant it’s resistant, and the greater number of copies of the genes means greater resistance.

“That’s why some SCN-resistance varieties with PI 88788 resistance are more effective than others,” explains Melissa Mitchum, University of Missouri nematologist. Including more copies and stacking additional resistance mechanisms in soybeans can help protect PI 88788, which is used in 95% of SCN resistant varieties.