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It’s Time to Actively Manage SCN

Gone are the days of only planting an SCN-resistant variety.

The No. 1 yield-robbing soybean pest for U.S. farmers has a new target on its back. Analysis of soybean variety trials conducted by Iowa State University (ISU) shows conclusively that soybean cyst nematode (SCN) is adapting and reproducing on the PI 88788 resistance source – used in more than 95% of resistant soybean varieties – and yields continue to decrease.

Reversing this trend requires a comprehensive SCN management plan. The SCN Coalition has stepped in to provide growers with additional information after a 20-year hiatus.

The SCN Coalition

“The nematode has found ways to overcome the resistance and become aggressive,” says George Bird, Michigan State University nematologist.

The coalition’s message is Take the test. Beat the pest. Like its predecessor, the new SCN Coalition is a public/checkoff/private partnership formed to help the agricultural industry speak with one voice about SCN management. The coalition relaunched after 20 years due to financial support from the soybean checkoff and renewed partnerships with industry, says Bird.

The first SCN Coalition formed in 1997, after a survey of 1,325 soybean farmers showed that 65% had never tested fields for SCN.

One of the problems is that SCN can cause yield loss without soybean plants showing visible symptoms. Soybeans can have a 30% to 40% yield loss while appearing healthy, says Greg Tylka, ISU Extension nematologist.

Typically, yield loss with severe symptoms is closer to 50%, he says.

“Twenty years ago, most soybean growers had never tested their fields for SCN,” says Tylka. “So we encouraged growers to test, and if they had it, to plant a variety that’s resistant to SCN.”

The previous coalition ended because it had accomplished the mission of raising awareness.

“We did a great job,” says Tylka. “We educated farmers about the threat. Then funding was shifted to SCN resistance breeding.”

Farmers started to plant SCN-resistant varieties, and everyone moved on from worrying about the parasitic roundworm.

Now, there’s a new problem with the pest. The difference this time is that SCN is adapting and reproducing on SCN-resistant soybean varieties.

Over time, the yield potential of the breeding line PI 88788 beat out the rest of the resistance breeding lines. Now nematodes have become resistant to PI 88788 resistance.

Resistant varieties (vs. a susceptible variety) should allow less than 10% reproduction. Research has shown that there’s been up to a 50% reproduction on a variety with PI 88788.

Managing SCN is becoming more complicated than planting a resistant variety, explains Bird.

Data from 25 years of ISU variety trial experiments show that as SCN reproduction increases on PI 88788, yields of resistant soybean varieties decrease by as much as 14 bushels per acre.

Coalition leaders believe conditions in Iowa are representative of what’s happening throughout most of the soybean-producing areas of the Midwest, where 70% of soybeans are grown.

That’s why the SCN Coalition is back, warning growers about increasingly aggressive SCN populations and encouraging them to actively manage SCN. Each grower’s SCN numbers, situation, and available management options will be different.

Today, 59% of farmers are growing SCN-resistant soybean varieties, but most don’t know the resistance source. The SCN Coalition recommends that soybean farmers work with their advisers to develop a plan for actively managing SCN.

Recommendations

The SCN Coalition recommends that farmers implement the following practices to manage SCN and keep PI 88788 a valid source of resistance.

  • Test your fields to know your SCN population. “You’ll need those numbers to understand the severity of the problem,” Tylka says. “The higher your numbers, the greater your chances of yield loss, and the higher that yield loss will likely be.”
  • Rotate resistant varieties. While PI 88788 is the main source of resistance used today, other sources of resistance are available, says Bird.
  • SCN populations can adapt to individual resistant varieties, as well. Even if it’s still the same source of resistance, rotating resistant varieties may help to slow the buildup of SCN populations, says Tylka.
  • Rotate to nonhost crops. While a corn and soybean rotation is the most dominant rotation in the Midwest, if you can increase your rotation to three years of nonhost crops, you will decrease your SCN risk, says Bird.
  • Consider a seed treatment nematicide. “It’s important for you to understand that you’re never going to get rid of soybean cyst nematode once you find it in your fields,” says Tylka. “But it’s not a death sentence. It’s similar to finding out you have high blood pressure; you learn to manage it as a chronic health problem.”

Similar to other pests in the agriculture world, there’s no quick fix around the corner. There was no extra cost for the seed of resistant varieties, says Tylka.

Since farmers aren’t accustomed to paying for resistant varieties, companies have had no financial incentive to bring other sources of SCN resistance to the market.

That means farmers will have to use integrated pest management and as many management tactics as reasonable for their operations, says Tylka.

By turning up the volume on SCN resistance management, the goal is to increase soybean farmers’ profit potential and realize higher yields.

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