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Corn+Soybean Digest

Stealthy Yield-Stealer

Corn nematodes are pests in every cornfield. They're not major pests and certainly not very noticeable, because they don't usually show visual symptoms.

The damage is hard to quantify in dollars and cents, too, but some experts claim a 5% yield loss is common.

Every cornfield has nematodes, and that alone should be a reason to give them a second look, says Terry Niblack, nematologist at the University of Illinois.

“Corn nematodes should not be at the bottom of your checklist for problems,” she says. “I typically see something that's been diagnosed as herbicide injury and then find a nematode problem.”

Corn nematodes have always been a chronic problem agrees Tim Todd, Kansas State University nematologist. “With many diseases you see acute symptoms you can't overlook,” he says. “But with corn nematodes, that's usually not the case. You frequently have a 5% loss that's hard to see.”

Todd explains that there are actually a dozen or more different types of plant-parasitic corn nematodes.

One of the more common genus, the lesion nematode, has more than six species that are common in U.S. cornfields. Their distribution, partly determined by soil type, places them in most fields. But needle and sting nematodes, two of the most damaging, are limited to sandy soils.

“One of the myths we often hear is that people think they can't have a corn nematode problem because they don't have sandy soils,” says Niblack. “But that's not true. Lesion, lance and dagger nematodes are our biggest and most common problems in heavier soils.”

Needle and sting nematodes, found only in sandy soils, can cause stunted spots across the field, says Tom Powers, nematologist, University of Nebraska.

“One of the reasons people focus on sandy soils is there's a tendency to undergo water stress and they see symptoms in sandier soils quicker,” he says. “But damage does occur in heavier soils from other nematodes.”

The experts all agree that the No. 1 way to keep the population in check is with a corn/soybean rotation.

“A corn/soybean rotation can be an effective management strategy in cases where the soybean is not a host and nematode populations decline rapidly, as is the case with needle,” says Todd. “There are, however, common species, such as lesion, that reproduce well on both crops. In many cases rotation still works because damage thresholds are relatively high.”

For high sting nematode populations in the Midwest, rotation will probably not work very well, Todd says. But, the good news for soybean producers is there is little evidence of nematode damage to soybean for the most common corn nematodes.

What the experts don't agree on is the frequency of sampling. That's not surprising considering different types of nematodes, soils and climates can affect populations, says Jim Smolik, nematologist, South Dakota State University.

Smolik says he rarely encounters high populations of corn nematodes in rotated fields in South Dakota and recommends sampling only to diagnose a problem, but not routinely.

Kansas State's Todd would like to see growers sampling annually or “certainly every couple of years.” Nebraska's Powers agrees. Illinois' Niblack advocates fewer samplings in some cases. “I'd be soil sampling probably once every 6-8 years in a corn/soybean rotation. But in continuous corn, I'd be more diligent about it.”

So how often should you sample? It depends on what you're trying to accomplish. Monitoring is the key to handling your corn nematode population, says Todd.

“You can't just take a one-time sampling,” he says. “Monitor the population as you would monitor soil fertility. Then you have a record of the background population for a field which makes it easier to evaluate a problem.”

But problems aren't distributed evenly. Powers recommends that growers take close looks at known trouble spots in their fields.

“Sometimes growers think a bad spot in a field is just a low spot or problem area for whatever reason. But often you have nematodes in these patchy areas,” he says.

Powers recommends taking samples in perpetual problem areas and comparing them with samples from a good spot from the same field.

Todd says it's critical for farmers to take samples when they see damage. If they wait until the end of the season, they won't get a clear answer.

“Because there are no definitive thresholds for most nematodes, it's extremely critical that a representative sample from the unaffected area be included for comparison if diagnostic sampling is to be effective,” says Todd.

Arguments can be made on when to sample for either season. The key is consistency, says Todd. Sample the same time of year each year so you'll have comparable results.

“Sampling at harvesttime can give you an idea of what's around. But some of the most damaging sandy-soil nematodes migrate down further into the soil and are harder to find at the end of the season,” says Powers, who recommends spring sampling.

Even if nematodes are becoming too big a problem to ignore, experts say that rotation is still your first line of defense. Treating with a nematicide can cost around $18/acre. And there are very few insecticides labeled for corn nematodes. Counter and Mocap are the most common.

New Practices Magnifying Old Problem?

With the increased use of Bt corn, and the move away from broad-spectrum insecticides, Illinois Nematologist Terry Niblack is concerned that farmers might be losing out on the added benefit that some of those insecticides provided — reducing the nematode population. She notes that the carbamate- and organophosphate-based insecticides were effective in that area.

“When you were controlling corn rootworm, you were inadvertently gaining some control on nematodes, too,” says South Dakota Nematologist, Jim Smolik.

With the high cost per acre, it's difficult to make treating with nematicides cost-effective. Spot treating is the most economical way to treat corn nematodes, Kansas Nematologist Tim Todd says, but he realizes it may not be practical if a field has too many spots.

“I know growers in the irrigated sands in Nebraska and Kansas who treat their fields,” says Todd. “So in extreme conditions, in those irrigated sandy soils where corn production may not be viable if they don't do something about their nematode populations, it can be economical. But that's rare.”

Todd notes that on the other end of the spectrum, from the rare problem fields, there is the more common subtle annual loss that doesn't really have a solution.

“Based on yield-loss models for common corn nematodes, like lesion nematodes, I believe there is evidence that a 5% loss is a frequent occurrence,” says Todd. “But, there is no documentation of actual losses on a large scale that I am aware of.”

He notes that corn nematodes are definitely a livable problem, but says there are other diseases that only cause 5% yield reduction that are getting more attention.

“I think there's enough evidence that it would be worth finding some economical management strategies for corn nematodes,” says Todd. Until then, he says rotation is the way to go.

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