Now's the Time to Dig Up Roots to Look for the Most Costly Soybean Pest

It might come as a surprise to many farmers but soybean cyst nematode or SCN is the No. 1 yield-robbing pest in soybeans. There are no in-season rescue treatments, but farmers can check fields now for this pest and put together an integrated management plan for next year. 

Greg Tylka, is a professor at Iowa State University and is also a lead for the Soybean Cyst Nematode Coalition.  He says nematodes are microscopic worms that live in the soil.  "When they infect soybean roots, eventually, that female swells into a little lemon-shaped object, but it's tiny. It's about the size of a period at the end of a printed sentence, but we can see them with our naked eye.

He says fields with low levels of infestation might show very few symptoms but become more pronounced as populations climb.  "So there's stunting and there's yellowing, but it won't be uniform. It'll be in patches because the nematode is very patchy or aggregated," he says. "And some indirect symptoms include areas of a field with poor weed control. They might not look sick, but because of poor weed control they are stunted and the plants don't close over as quickly."

So, Tylka says many farmers don’t even realize they have SCN, which can make it a silent yield robber. "We can have up to 30% yield loss without any loss of color of the plants, height of the plants or weight of the leaves. When you get into severe situations where you can see even mild stunting, or mild yellowing, it goes up from there.  Simple math shows with a 30% loss on 70 bu. beans that's 21 bu. With $14 soybeans that’s $294 per acre," he explains. 

Research backs up the losses and proves SCN is the most costly pest in soybeans.  “Soybean cyst nematode, for the last 20 years has been ranked the most damaging soybean pathogen in all of North America, so that's the U.S. and Canada," Tylka says. "Estimates are consistently around $1.5 billion. That's billion with a B annually."

Farmers can check for SCN through fall soil sampling or an in-season root dig.   "This time of year, we really encourage farmers and agronomists to get out in their field. Carry a spade; you don't want to pull the plants, you want to dig the plants. Shake the soil from the roots and look for those females," he says.

Once detected, farmers can use an integrated management plan, which includes varietal selection.  Tylka says 95% of varieties with common resistance have lost effectiveness, but there is one with a unique type of resistance.  “It's called peaking resistance, peaking like the city in China. So farmers will want to grow both. They're going to have to grow the common resistance and search out the uncommon peaking resistance,” he explains.  

Rotation is also part of the three-pronged plan. "Farmers should rotate to corn because corn is a non-host crop. Any year a farmer grows corn in a field, SCN egg numbers will drop; some of the eggs are going to hatch out and the little worms are going to starve. That drop can be as little as 5% or 10% to up to 50%, so corn is our best tool to lower numbers." 

Finally, Tylka recommends using a seed treatment.  “There are eight or nine or 10 different seed treatments, every one with a different active ingredient, and therefore everyone with a different mode of action."

SCN populations are higher in hot, dry years and in soils with higher pH.  The pest is spread mostly by wind-borne soil erosion and on equipment.  Research also shows SCN makes sudden death syndrome occur earlier and become more severe.