The silent yield killer

Fortunately, tools like resistant varieties can help farmers manage soybean cyst nematode.

High blood pressure and soybean cyst nematode (SCN) seemingly fit as well as a classical violinist at a hip-hop concert. Still, they have more in common than you’d think. 

Apparently healthy people with high blood pressure can quickly die from a stroke or heart attack. “It’s a silent killer, and so is soybean cyst nematode in soybeans,” says Greg Tylka, an Iowa State University (ISU) Extension nematologist. 

The good news is that if a patient is diagnosed with high blood pressure, treatment exists. It’s not a simple one-shot solution, though. 

“There’s not a single pill you take to cure high blood pressure,” says Tylka. “You take medicine, you reduce salt intake, you exercise more, you reduce alcohol intake in order to help manage it. It’s the same thing with soybean cyst nematode. There’s no magic pill.”

Greg Tylka
Photo credit: Gil Gullickson
Soybean varieties steeped in PI 88788 resistance used to be a sound SCN management tool. However, more SCN populations are overcoming this resistance source. This requires that farmers adopt new strategies that include:

  • Crop rotation
  • Alternative resistance sources
  • Seed treatments

“It’s a lot more complicated now to manage SCN,” says Tylka. 

Still, managing SCN is worth doing. “It’s the most damaging soybean pathogen in the United States and Canada,” says Tylka. A rapid reproducer, SCN competes against soybeans for nutrients and water, clipping yields by 40% without showing symptoms, according to ISU research. 

“The kicker is that after you stop growing soybeans for a year or two, you think you’d be in good shape,” says Tylka. “But Mother Nature has given us a cruel twist in that a lot of the eggs that are produced every year are dormant and will not hatch. These survive for 10 years or more in the absence of soybeans. It’s things like these that make SCN the most damaging pathogen year in, year out.”

Where to Start? 

Kaitlyn Bissonette standing in a soybean field
Photo credit: Gil Gullickson
In time, soybeans will exhibit hallmark SCN symptoms of stunted plants and chlorotic plants. Initially, though, soybeans infested with SCN show no symptoms. That’s why sampling for SCN egg numbers following harvest is a good place to start to form a management plan, says Kaitlyn Bissonnette, University of Missouri Extension plant pathologist. 

“Then, farmers can form a management strategy, like planting a resistant variety or other tools like crop rotation or any type of seed treatment they might be adding into the mix,” says Bissonnette. “They can then track levels over time field by field. It’s not just taking one test and being done with it. It’s more like how these tests look over time.”

Solutions 

One SCN management advantage that farmers outside states like Iowa have is crop diversity. 

Sam Markell in a soybean field
Photo credit: Gil Gullickson
“We have some good rotational crops in North Dakota,” says Sam Markell, North Dakota State University Extension plant pathologist. “Some crops we grow like dry edible beans are susceptible to SCN, but we also grow a lot of good rotational crops like wheat, sunflowers, and barley. So, we have an advantage over other states that are predominantly corn and soybeans. We can afford to lengthen the rotation between soybean crops because we have other crops we grow.”

SCN-resistant varieties remain an option, although SCN resistance to the PI 88788 resistance source is increasing. Resistance has occurred in the same way it has with weeds and insects – use of the same control measure over time , says Tylka. PI 88788 SCN resistance is the source of resistance for about 95% of SCN-resistant varieties, says Tylka. 

Alternative resistance sources exist. High-yielding Peking varieties are on the market, says Tylka. In 2021, Syngenta plans a full commercial launch of two varieties with PI 89772 resistance: GH 2329X (Golden Harvest) and NK S23-G5X (NK Seeds). 

There’s a hitch, though. Alternative resistant varieties aren’t available everywhere. 

“In Missouri and farther south, we are limited in the options we have available,” says Bissonnette. “I really encourage farmers to look at what’s available from companies because there will be varieties with new resistance sources eventually coming to market.” 

SCN resistant soybeans
Photo credit: Gil Gullickson

Farmers also need to take care when planting varieties with new resistance sources and not to repeat past continuous planting patterns. 

“As more Peking varieties come to market, we do not want to switch everything to them for the next 10 soybean crops,” says Tylka. “Twenty years from now (if soybeans are annually rotated with corn), we do not want to be in the same situation, talking about why Peking varieties no longer work. The more things you can throw at SCN or weeds or any other pest, the more robust a management program will need to be.”

Biological and chemical seed treatments are another management tool. 

“Oftentimes, that’s how we manage nematodes early in the season,” says Bissonnette. “In Missouri, we can have as many as three to six generations in a season.” Seed treatments can help deter some early-season SCN damage, she says.

Spot SCN female cysts

SCN closeup under magnifying glass
Soybean farmers can assess in-season SCN infestation potential around mid-July by carefully digging up roots, says Sam Markell, North Dakota State University Extension plant pathologist. Be gentle, though: SCN female cysts can easily fall off soybean roots, he advises. 

The cream-color cysts, each containing hundreds of eggs, can be confused with the nitrogen-fixing nodules present on soybean roots. 

“You’re looking for something much smaller, around one-fifth to one-tenth the size of a nodule,” says Markell, “Typically, I will bring a hand lens with me and a flashlight because you can’t see them very well with the naked eye.”

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