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Corn+Soybean Digest

SCN Symptoms: What You Can't See Hurts

Dave Broghamer had never been satisfied with yields from one of his farms. It's 60 acres of "pretty good ground" he rotates between corn and soybeans.

But last year the Decorah, IA, grower had enough.

"It was easy to see I had a problem; plants were not developing properly," Broghamer says. "They were smaller than normal."

Although parts of fields produced healthy plants, plants a few rows away were struggling.

So he called in his local co-op agronomist, who suggested soil tests.

Soybean cyst nematodes strike again.

Broghamer is just one of many farmers who early on failed to recognize the above-ground symptoms of SCN. That's because there aren't any - until the nematodes are fully entrenched in a field and affecting yield.

"Unfortunately, the message got out fairly early that growers could see above-ground symptoms of soybean cyst nematodes," says University of Missouri extension nematologist Pat Donald.

"And that, if they had chlorotic leaves or dying plants, then they had SCN, and that's not necessarily true."

What is true is that the main early SCN symptom is a "half-empty weigh wagon vs. an overflowing one," says John Ferris, a Purdue University nematologist.

Walker Kirby, a University of Illinois plant pathologist, says the soils in his area are rich enough to support vigorous plant growth, even in the presence of nematodes.

"But the plants tend to set fewer flowers and have fewer pods, so they generally don't yield as well. To growers driving by the field, they look absolutely perfect."

And there's the rub, Donald says.

"Our biggest problem is getting people to get out of the pickups and sample fields," she states. "I recommend, if you have a field going into soybeans, that you sample. Period.

"We have people who say they've been growing soybeans forever and don't have a problem because they don't see any symptoms. When I finally get them to do egg counts, the egg counts come back at a level of 40,000 eggs per cup of soil."

That's enough to cause damage, even on resistant varieties, says Donald.

"The big symptom you don't visually notice is the cyst nematode feeding on the roots," says Kelly Holthaus, Broghamer's agronomist and branch manager of the Winneshiek Co-op, Burr Oak.

"The cysts basically eat off the root system and starve the plant," Holthaus explains. "After they start eating, the roots can't take up the nutrients. Then the visual effects start showing up."

By then it's too late to do something that year for that crop, Donald says.

Growers who've noticed uneven patches in their fields should dig up roots and check their condition. Broghamer's roots "didn't finger out like they should have."

Other than little white cysts clinging to roots, you might find adult females actually feeding on them.

Most experts recommend sampling right ahead of or during harvest. But the best time to dig up and view roots is late June or early July, depending on the soil temperature and when the crop was planted, Donald says.

Symptoms growers do see - when SCN populations are quite high and yields may be reduced by 10 bu/acre or more - include stunted, yellowing plants.

Yellowing is more visible in drought years and in sandy soils, Ferris says.

Plants are also smaller and less vigorous. Dead or uneven patches appear in fields.

"One of the things that I do, when the timing is right, is go out and look at the field with a producer and say, 'Look, this field is very uneven; there are a lot of different heights in the field,' " says Donald.

SCN also prevents good canopy closure, which increases weed pressure, she warns.

"There should be a light bulb going off when they see weeds where they haven't seen weeds before."

And that leads to something else Donald would love to see growers do: track their field histories.

"You need to get a field history so that you know that the management techniques you're using are actually working. If you're doing something that's making the problem worse, and if you don't test repeatedly, then you don't know whether you're helping.

"You may delude yourself into thinking that you're really taking care of something when you are not."

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