Growers should take proactive measures to manage costly soybean cyst nematodes, which may have developed resistance to soybean varieties commonly used to control them.
By Darcy Maulsby / Photography by Mark Kegans
Call it the Rodney Dangerfield of U.S. soybean production. Soybean cyst nematode (SCN) gets no respect. But agronomists agree that nothing robs more yield—or poses a bigger threat—than SCN.
“We’re sitting on a powder keg,” says Greg Tylka, Ph.D., a professor of plant pathology and nematologist at Iowa State University. “Buildup on resistant varieties is reducing yield and could lead to disaster.”
Though not easily detected above ground, soybean cyst nematodes pose a major threat to soybeans. Healthy roots are key to producing high yields.
Even though SCN is widespread, it’s easy to overlook. These microscopic parasitic worms can feed off soybean plant roots for some time before anyone notices above-ground crop damage. By then, the SCN population has grown more numerous and much stronger, becoming difficult to control and developing into a huge economic threat to soybean farmers.
Studies have shown that SCN can contribute to yield losses of 30 percent to 40 percent with no visual symptoms above ground. “I think the biggest problem with SCN is that producers don’t recognize the symptoms, so they don’t realize they have a problem,” says Kevin Spencer, a grower and Golden Harvest® Seed Advisor™ from Ottawa, Kansas.
Some farmers question yield-loss figures from SCN. “I ask growers, ‘Is SCN so bad that it wipes out yield in your entire county?’” says Dale Ireland, Ph.D., a technical product lead for Syngenta Seedcare. “No, but the truth is that SCN takes out small amounts of yield from millions of acres every year. It’s a very stealthy pest.”
It’s also true that resistance is becoming a major challenge, posing a serious risk to soybean yields. “SCN is an adaptable pathogen, and it’s becoming more aggressive,” says Jason Bond, Ph.D., a professor and plant pathologist at Southern Illinois University. “In the past 10 to 15 years, apathy about this problem has been one of the most devastating things for managing SCN.”
Standing on the Edge of a Cliff
This apathy makes sense on one level. “Most growers think they are doing enough by planting an SCN-resistant variety,” Tylka says. “But this protection is slipping away fast.”
“Most growers think they are doing enough by planting an SCN-resistant variety. But this protection is slipping away fast.”
For more than 20 years, genetic resistance has come almost exclusively from a single source (PI88788), which is now found in more than 90 percent of SCN-resistant soybean varieties. It has been a vital tool to control SCN, which can reproduce three to six times in a growing season, depending on the geography.
As with herbicide resistance, however, reliance on a single SCN-management tool has reduced effectiveness. “When we deploy SCN-resistant varieties, a very small number of individuals within the population will be able to reproduce,” Bond says. “If we continually use the same source of resistance, these individuals will adapt.”
SCN resistance isn’t a new problem, but it’s reaching a tipping point. “Even though it has been subtle and slow, resistance will impact soybean production if nothing is done,” notes Terry Niblack, Ph.D., interim associate dean of the College of Food, Agricultural and Environmental Sciences at The Ohio State University.
"As resistance grows, additional proactive solutions are needed to manage SCN."
If you’ve been planting an SCN-resistant variety for any length of time, there’s a strong chance you’ll have an SCN population that can overcome PI88788, notes Palle Pedersen, Ph.D., the head of Seedcare product marketing at Syngenta. “We’re standing at the edge of a cliff and can fall fast if we don’t start dealing with this challenge.”
Fighting Back with Clariva Complete Beans
Once SCN infects a field, it’s impossible to fully eliminate it. However, there are ways to manage and reduce SCN populations, including:
SCN testing. While soil sampling for SCN is site-specific, the numbers can offer a general sense of whether SCN populations are going up or down. Sample for SCN before every third soybean crop and monitor the trends, Tylka says.
Nonhost crop rotation. Consider introducing wheat or another nonhost crop, such as corn or alfalfa, into your rotation, Ireland says. “But watch out for some cover crops. They can be alternative hosts for SCN. Consult your local extension service about whether your cover crop may host SCN.” Also, try rotating SCN control sources in the varieties you plant. Niblack says, “While varieties other than PI88788 got a reputation early on for yield drag, I’m not sure this is justified anymore.”
Weed control. A number of weeds, including henbit and purple deadnettle, can serve as hosts for SCN. Make it a priority to control weeds, including winter annuals, with an effective herbicide, Ireland says. Flexstar® GT 3.5 herbicide is one solution.
Seed treatment. Seed treatments that offer effective protection against SCN can be another valuable tool, Niblack says. Clariva® Complete Beans, a combination of separately registered products, reduces SCN reproduction and selection pressure and manages damage from sudden death syndrome (SDS) and other SCN-related diseases. Syngenta added a nematicide to CruiserMaxx® Beans with Vibrance® seed treatment to develop Clariva Complete Beans. Compared to an insecticide/fungicide seed treatment alone, Clariva Complete Beans also provides an average yield increase of 2.7 bushels per acre on top of CruiserMaxx Beans with Vibrance.
In some fields, the seed treatment has helped boost yields by more than 10 bushels per acre. Michael East, a grower from Clarkedale, Arkansas, has struggled with SCN and SDS. “After we treated our soybeans with Clariva Complete Beans, we saw great benefits in the yield and protection from diseases,” he says. “Our yield was 20 to 30 bushels per acre higher in some cases.” Clariva is also a longtime investment, Pedersen says. “By using Clariva every time you plant soybeans, you help suppress SCN and reduce populations over time.”
Driving down SCN populations is one of the keys to managing the underground battle zone. “With proper management, SCN is not a death sentence to soybean production,” Tylka says. “While it’s hard to drive high SCN numbers down, it’s fairly easy to keep low numbers low and protect soybean yield potential.”
