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Corn+Soybean Digest

Stealthy Thieves

Corn nematodes could be feeding on your corn profits without you realizing it because you can't always tell just by looking at your crop.

Only soil and root samples sent in for analysis can give you a reliable picture. Even when above-ground symptoms occur — wilting, slow growth and off-color leaves — they are sometimes mistakenly chalked up to a variety of other causes with similar symptoms, says Tom Powers, a University of Nebraska-Lincoln (UN-L) nematologist.

A concern among some nematologists is that changes in corn production practices in recent years may open the door to higher levels of corn nematodes.

Continuously planted transgenic corn hybrids with the corn rootworm resistance trait replacing soil insecticides for rootworm control is one factor, suggests UN-L Extension plant pathologist Tamra Jackson. Even when they were applied at lower rates for soil insect control, those products had some activity against nematodes, which are microscopic roundworms.

Pyrethroids, sometimes used in place of the more toxic organophosphates and carbamates, lack the latter's punch against nematodes, according to Jackson.

Kansas State University Nematologist Tim Todd shares Jackson's concerns, saying: “We don't have data that says that, but I think it's kind of a mutual concern (among nematologists).”

Dozens of nematode species populate soils, many of them causing no crop problems and, in many cases, benefiting plants by decomposing organic matter and cycling nutrients. Only a few are damaging to crops either directly by root feeding or indirectly from fungi and bacteria that enter the plant through feeding wounds on the roots. Some nematodes also transmit plant viruses.

“Corn nematodes” is a collective term for several types of nematodes capable of dragging down corn performance. They include: Lesion, needle, sting, lance, dagger and stubby root nematodes. Within each of those types can be a number of species with varying degrees of ability to damage your crops.

Chances are, soils on your farm harbor several types of corn nematodes. The question is, do they exist at crop-damaging levels?

Among those listed above, lesion nematodes are probably the most widespread, occurring in both fine-textured soils (such as silt loams) and sandy soils.

Lesion nematodes are present in many Corn Belt fields, says Greg Tylka, an Iowa State University nematologist. But just because they are there doesn't necessarily mean they are causing economic losses, he adds. And even when they do cause damage, their feeding may not be evident through crop appearance. While they can cause severe yield losses, they may exact a toll that's more chronic than dramatic — maybe a 5- to 10-bu./acre yield reduction, Tylka says.

“I like to point out to growers that problems are more quickly seen on sandy soils,” says Powers. That's because the symptoms are similar to drought — wilting, slow growing plants. “But that doesn't mean the same amount of nutrients aren't being extracted in finer-textured soils.”

Needle can be a “really bad one” in sandy soils, Tylka says. So can sting, another type associated mainly with sandy soils. Damage thresholds for needle or sting nematodes can be as low as one nematode per 100 cc (about ½ cup) of soil, he says. Contrast that with a damage threshold of about 1,000 nematodes per 100 cc for other nematodes.

Todd at Kansas State University agrees, saying that sting and needle are capable of causing a great deal more damage than lesion nematodes. “There's no argument; if you have them, you need to do something.”

Symptoms such as wilting, spindly plants and uneven crop height may be more obvious with those two types than what you might see from lesion nematode feeding. Affected plants may occur in irregular areas within a field, not uniformly throughout the field, Tylka says.

“Some people believe corn nematodes are causing as much loss to corn as cyst nematodes are causing in soybeans,” Tylka says. And the damage may not be limited to just one type of nematode but possibly two or more types combined within an area of a field.

Some types of nematodes, such as lesion and lance, spend most of their lives inside the roots, while others such as needle, sting, dagger and stubby root nematodes live in the soil, outside the root. Because more than one type of nematode may be contributing to crop loss, both soil and root samples should be taken for analysis.

Sampling involves a significant amount of labor. Cost of analysis may range from $20 to $30/sample. That's not a “trivial thing,” particularly if you don't actually see symptoms, Tylka says.

For every 20 acres, 10-20 soil cores from 8-12 in. deep should be pulled, says Tylka. Root samples should also be taken, being careful to include the soil around them to avoid leaving infested roots behind. A similar sample from an unaffected area should be included for comparison. Place samples in a moisture-proof bag and keep them cool, such as in an ice chest, for delivery to the lab. Don't freeze them.

Samples should be taken in the middle of the season. Doing so before planting isn't very useful for most types of corn nematodes, Tylka adds. Environmental conditions can affect populations and the amount of damage, especially in the case of lesion nematodes, he says. You could have damaging levels of them one year and not the next. Needle and sting, on the other hand, are so damaging that they can probably be tracked year to year.

When damaging levels of corn nematodes do occur, they can be quite variable between fields and within fields, with some areas not affected. Once damaging levels are verified, your options are limited to managing for the next crop, Tylka says. You can't wait until you see whether damage is occurring before you treat, because nematicides are not useful in-season rescue treatments once damaging levels have been detected.

“I think the real key is monitoring,” says Todd. Unlike soybean cyst nematodes that require year-to-year management — such as planting soybean cyst nematode-resistant varieties or rotating to non-host crops — predicting corn nematode damage from one year to the next and from one field to the next is difficult, he says.

Whether a preventive nematicide treatment or a switch from corn to a non-host crop is merited may be a judgment call. Exceptions are sting and needle. Their presence merits preventative nematicide application or planting a non-host crop.

Since some corn nematodes have more than one crop host, crop rotation choices will depend on the type of nematode or nematodes you're battling. And because certain weeds can also be hosts to some corn nematodes, good weed control is important to nematode management, nematologists say. They urge you to consult your local Extension service for the best management and treatment options.

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