Management Options When SCN Infests Fields

After the Test

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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Get a jump-start on SCN detection by digging soybean roots this summer. Look for small, white, adult SCN females (the cysts). Follow with fall soil testing. (Pamela Smith)

So, you've decided to test for soybean cyst nematode (SCN). Most farmers start with a common soil test to detect the presence of eggs in cysts (dead females) in the soil.

Sam Markell, North Dakota State University plant pathologist, says to count yourself lucky if the test result is "zero" or "not detected." But, don't get too comfortable with that diagnosis. "That's the result we want, but it doesn't necessarily mean the problem is not in the field or on the farm. SCN can be very patchy in the field," he says.

Continuing to monitor and test is important. "We know SCN has now spread to almost every soybean-growing area. In many areas, SCN is overcoming the varietal resistance we've depended on for years. Continuing to test is the only way to detect, monitor and know how management tactics are working," he adds.

"The first motivation to test is simple: SCN is costing you money," Markell says. The SCN Coalition has a calculator called "SCN Profit Checker" to estimate that profit penalty based on egg count and soil conditions ( Free soil testing for SCN is available in several states.


-- Rotate Crops. Crop rotation will reduce egg numbers. Research shows that a single year away from soybeans to a nonhost crop such as corn or wheat can reduce SCN egg counts by up to 50%. "Don't grow soybeans on soybeans," Markell says. "Dry edible beans and some winter annual weeds such as henbit, purple deadnettle and pennycress are the other good hosts for SCN," Markell says.

-- Manage Resistance. Genetic resistance has been and remains an important tool for soybean farmers, but it needs to be managed. Most soybean varieties contain a source of resistance called PI 88788. "In many cases, PI 88788 is still reasonably effective against SCN, but it's not what it used to be because nematodes are adapting to it," Markell explains.

PI 88788 resistance is complicated, and effectiveness can vary among varieties. Soybean cultivars differ in how much they reduce SCN reproduction, and a system of rotating varieties, even those that contain PI 88788, is now considered a management strategy.

An alternative source of resistance, called Peking, has become more available in commercial soybean varieties. Yield data from trials by Greg Tylka, Iowa State University nematologist and member of The SCN Coalition, have found Peking varieties outyielding those containing PI 88788 resistance by as much as 20 bushels per acre in some areas of his state. That's good because it shows Peking is working. But, Tylka worries about the temptation to overuse Peking as PI 88788 effectiveness declines.

"Very few nematodes are able to overcome Peking resistance now," Tylka explains. "But, those that do have a high likelihood of passing that ability along to their offspring. With Peking SCN resistance, the very few nematodes that successfully reproduce initially will likely become a high percentage in the SCN populations in fewer years than we saw with PI 88788."

The message is don't grow the same SCN resistance over and over, Markell says. "Grow a Peking variety if available, then rotate to a nonhost crop, and then rotate to a different PI 88788 variety than you grew before."

-- Treat Seeds. Treating seed can be important, particularly in areas that struggle with sudden death syndrome (SDS) -- the two problems often go together. However, nematode-protectant seed treatments should supplement, not replace, SCN management strategies, advises Carl Bradley, University of Kentucky plant pathologist.

The growing list of treatments available makes the market a moving target for farmers. Nematicides, biologicals and biological parasites differ widely in how they work and how well they work. Toss in the wide range of soil and field conditions, and it can be hard to predict how they will perform.

"The data on SCN seed treatments in our plots has been variable, and efficacy can be inconsistent," Bradley says. "Unfortunately, there's no silver bullet with this pest."

Markell reminds growers that seed treatments only last so long, as well. "In general, seed treatments are really designed to get that plant out of the ground and protect it early in the season and not throughout the season," he says.

-- Keep It Clean. Nematodes move in soil. Equipment moving between fields, used equipment and custom applications are all potential transport mechanisms for the pest.

"In North Dakota, we still worry about moving SCN into fields where it doesn't exist. In regions where SCN is more established, the concern is the possibility that you could be moving a population that is more highly adapted to PI 88788 into your field," Markell says. "Washing equipment off before it moves is a good precaution."

-- Think Positive. Help is on the way, Markell assures. "It will take some time, but the soybean checkoff is funding research; companies and universities are working on tools and strategies. In the future, different sources of genetic resistance, new and improved seed treatments and more are likely."

BASF is currently developing a Bt trait, a Cry14Ab protein, as the first GM (genetically modified) solution for nematode control. The nematode-resistant soybean (NRS) will be stacked with PI 88788 initially. The NRS trait is scheduled for commercialization at the end of the decade, pending regulatory reviews.

"Because of how this pest reproduces, there's not going to be a silver bullet, and we need to keep all tools viable," Markell says. "Continuing to manage to prevent profit losses is important. That starts with testing."


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