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How to manage corn nematodes

Gil Gullickson 11/01/2011 @ 10:53am Crops Technology Editor for Successful Farming magazine/Agriculture.com

So what's wrong with this field? Compaction? Herbicide damage? Early-season cutworms? Could be all three. In this case, though, the culprits were nematodes that feed on corn.

You may think corn nematodes are a new pest, but they aren't. Most are native to the U.S., and they've long resided in soils before European settlers arrived.

These roundworms, of which there are dozens of different species, are survivors. “The last things to die off this planet will be cockroaches and roundworms,” says Glen Dappen, a retired biologist and nematode specialist for Nebraska Wesleyan University.

Nematodes infest about anything. Many are benign. In corn, though, some species inflict heavy yield damage.

“In corn, nematode damage is frequently misdiagnosed,” says Dappen. Besides stunted plants like the ones pictured above, afflicted plants often show small ears and kernels later in the season.

“The ears will not fill out,” he says. “It's just like the corn plant ran out of gas.”

Why the rise?

The seeming sudden rise in nematodes in corn is mainly tied to three factors:

1. More conservation tillage.

2. Few carbamate and organophosphate insecticides.

3. More continuous corn.

Besides corn rootworm, carbamate and organophosphate insecticides kill nematodes. Meanwhile, less tillage and continuous corn mimic undisturbed prairie in which nematodes thrive.

Most species do not damage corn until they reach certain damage threshold, says Greg Tylka, Iowa State University Extension nematologist.

Damage thresholds vary between species. For example, lance nematodes have damage thresholds from 300 to 400 per 100 cc of soil. Others species, like needle nematodes, have a damage threshold of 1 per 100 cc of soil.

Knowing what type of nematodes you're dealing with is key to dealing with corn nematode. Sampling midseason is recommended because that is when most corn nematode populations increase.

Spring sampling isn't recommended.If numbers are low, you can't predict if numbers will increase to damaging levels if you sample early in spring.

In mid-season, sample 12 inches deep. This will garner the nematode species that are more numerous deeper in the soil and those that occur in high numbers at shallower depths. Collect up to 20 cores from a field and include root samples, too, says Tylka.

Needle and string nematodes are exceptions to the mid-season sampling time, he says. These are the largest and most damaging of all nematodes that feed on corn. They're found only in soils with more than 70% sand. Sampling should occur in spring and fall, rather than midseason. That's because they migrate deep in the soil during the growing season.

What to do?

Suppose your samples come back showing you have nematodes above damage thresholds. Unlike soybeans resistant to SCN, no corn hybrids resistant to nematodes exist, says Tylka. Counter, an organophosphate insecticide aimed at corn rootworm, is also labeled for nematodes in corn.

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