Look out below: Soybean cyst nematode management tips

COLUMBIA, Mo. — Soybean cyst nematode continues to move into new geographies with the expansion of soybean acres.

“This is a pest that hasn’t stopped expanding in regions. New areas are being detected every year, new counties and new provinces. This is an underground pest that continues to rob yields in the soil,” said Kaitlyn Bissonnette, University of Missouri plant pathologist and director of SCN Diagnostics.

“What really matters about this disease is the fact that for years running it has been the No. 1 disease of soybeans in the U.S. and Canada. This disease can cause hundreds of thousands of dollars in yield losses annually and we’re seeing bushels upon bushels lost as a result of soybean cyst nematode in the soil.

“The other problem is we can’t get rid of this just by rotating from one crop to another. This is a problem that persists in the soil long-term.”

Bissonnette participated in a “Grow in the Know” webinar hosted by BASF and the SCN Coalition. The plant pathologist referenced farmer surveys conducted in 2015 and 2020 that focused on SCN management strategies.

The 2015 survey of 1,000 growers found 22% utilized seed treatments to manage SCN and the number grew to 40% in 2020.

Seventy-one percent of farmers surveyed in 2015 used crop rotation to aid in SCN management, rotating to non-host crops — corn, wheat or other cereals. That increased to 77% in the 2020 survey.

“The first line of defense is utilizing SCN resistant soybean varieties along with these other management strategies. There was a 7% increase from 2015 to 2020 in the utilization of soybean cyst nematode-resistant varieties by growers surveyed. This is a huge jump in understanding how we can better manage soybean cyst nematode by combining all these different management strategies,” Bissonnette noted.

There are nuances to keep in mind with SCN-resistant soybean varieties.

Different types of genetic resistance can be utilized. Growers in that same survey indicated they were rotating the source of genetic resistance they were using, whether it is PI 88788, Peking or some other source of resistance, growing from 39% in 2015 to 49% in 2020.

Those who identified Peking as a source of resistance because it is a different source of resistance than the traditional PI 88788 grew by 10% from 2015 to 2020.

Unseen

“The biggest challenges that we really face in SCN stem from the fact that symptoms of SCN traditionally have been stunted plants, yellowing and yield loss. However, yield loss can occur in the absence of symptoms,” Bissonnette said.

“A field may look fairly uniform. There might be a little bit of tinges of yellow in some areas, but it looks pretty much like a normal soybean field. These yield losses from SCN can be occurring up to 30% in a field that looks just like a regular soybean field. That’s a big challenge that we face with SCN.”

Adding to the challenges is the pest cannot be seen above ground as opposed to sudden death syndrome.

“With SCN living below ground, we have to be looking for those females on the roots below ground, looking for eggs and cysts in the soil below ground and being able to crush those cysts to see the eggs, and the fact these higher populations of eggs in the soil can increase that infection potential for the next season,” Bissonnette said.

“Another challenge we face from a genetic perspective is SCN populations are adapting to the PI 88788 genetic source of resistance which is the No. 1 source of resistance used in about 95% of SCN varieties commercially grown. There’s higher than 10% reproduction of SCN field populations on PI 88788 of SCN in states, indicating the genetic resistance in those soybeans are no longer as resistant as they once were.

“We’ve also found that all SCN-resistant varieties do not perform equally. Just because variety has a high yield with a PI 88788 resistance source does not necessarily mean that equates to lower SCN reproduction. Rotating varieties with different resistance sources is important.”

Other diseases can also be impacted by the presence of SCN, including sudden death syndrome, brown stem rot and soybean aphids. SCN can play a role in increasing disease symptoms and increasing susceptibility to diseases.

“Most notably, SDS and SCN is a complex relationship. This is a relationship that is often discussed because it is the most impactful in the Midwestern U.S. and the bottom line of SDS and SCN and that relationship is the fact that SCN is not required for SDS to occur. However, when SDS and SCN are both present in the same field, SDS symptoms are often more severe and where both pathogens are wide spread, management of both diseases are needed,” Bissonnette added.

Management Tips

Bissonnette provided the following key components to an SCN management plan.

• The first step is taking a soil test. “You can’t really start from anywhere unless you know your numbers. You’re looking for an SCN egg count and ideally you’ll want to split your field into management zones. Look at the different waterways or hillsides or fence lines might be as a way to best quantify what your SCN numbers are. The best time to soil test is in the fall right after harvesting the soybean crop,” Bissonnette said.

• Select SCN-resistant soybean varieties and rotate resistant sources. Also look at rotating varieties within that source of resistance like PI 88788. Not all varieties perform equally.

• Crop rotation is still effective within reason. “Understand how to reduce your egg count from one season to another. Corn, wheat, other cereal crops are non-hosts and will not increase your egg counts from season to season. Think about what types of cover crops might be able to help between seasons because they are still non-hosts. Also, understand how to utilize weed management as there are weed hosts,” Bissonnette said.

• Consider using SCN seed treatments, “especially in cases where SCN populations have gotten out of control or you need to add another management component into the mix,” Bissonnette said.