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Corn+Soybean Digest

A New Twist on Cyst | Corn Cyst Nematode Identified in South

The word “cyst nematode” strikes fear in the hearts of most soybean growers. Now, a new corn cyst nematode has been identified in the South. Although not yet a menace, the corn cyst nematode is a critter corn growers should keep on their list of potential problems. (Don’t confuse the corn cyst nematode with corn nematodes.)

Patricia Donald, USDA-ARS plant pathologist in Jackson, TN, tells Corn & Soybean Digest that the new corn cyst nematode – recently named the “goose-grass cyst nematode” – poses no immediate threat to growers. But it still has the capability to throw researchers of soybean cyst nematode (SCN) off guard.

Tamra Jackson, University of Nebraska plant pathologist and corn nematode specialist, says plant pathologists need to be aware of the new corn cyst nematode, “even if it isn’t an immediate issue in the Corn Belt.”

First detected in northern Tennessee in 2006, the new corn cyst nematode was discovered by a University of Missouri pathology technician. USDA’s Nematology Lab in Maryland confirmed the identification in 2007, says Donald.

“It was originally thought to be a Cactodera, but a borderline member at best,” says Donald, who’s kept her eye on the tiny creature since it was found. “Further morphological and molecular examination has revealed it is a new genus and species.”

Pathologists have looked at soil samples from cropping systems of silage corn and corn-soybean rotations in Tennessee, Kentucky and Mississippi. “These are the systems where we found the nematode in the two fields,” Donald says.

“Both fields had stunted corn of unknown etiology. That’s not that unusual, so it is a poor diagnostic feature for the nematode.”

Greg Tylka, Iowa State University plant pathologist, says the new corn cyst nematode apparently hasn’t spread northward.

Because it’s difficult to distinguish the new nematode from SCN, Donald believes the new corn cyst nematode could escape detection “until a crop consultant, producer or researcher examines the field history of that particular field,” she says.

SCN research confusion may result. Egg counts alone are not sufficient to determine efficacy of soybean resistance, says Donald. It takes a crop bioassay to learn whether the nematode is SCN or another cyst nematode, she says.

“I’ve followed this closely and this nematode is a concern for us,” adds Jackson. “The other corn nematodes we’re working against are not cyst nematodes. This being a cyst type might make it more difficult to diagnose.”

The females are similar in size and shape to SCN. “The way we run SCN assays now doesn’t allow us to differentiate the two,” says Jackson. “If this nematode gets into the Corn Belt, it might be difficult to track it and test for – because we would have to change the way we test.

“Since they are the same size and shape, it would be difficult to separate them in our ‘wet sieve’ (mesh) testing procedure. We may get false positives for SCN.”

Jackson says there’s no evidence it has moved into any other location, “but plant pathologists should regularly check corn roots for the females. You should be able to see them with your naked eye.”

She notes that from the farmer’s perspective, the two new nematicides on the market, such as Avicta Complete and Votivo, haven’t been tested against the new corn cyst nematode. “Each nematode feeds and reproduces in a different way,” says Jackson, “so it would be necessary to test nematicides against that nematode to see if that option worked.”

Donald says a workshop was conducted this summer (2010) for the Society of Nematologists to better illustrate the difference between the SCN and corn cyst variety, she says.

She says management strategies already in place to reduce SCN should work equally well for the new cyst nematode. Those strategies include knowing which fields are infested, working fields with no detectable or low levels of SCN first, cleaning equipment and reducing movement of soil as much as possible.

Donald says host-plant tests have been done with the nematode. “None of the dicot field crops are a host,” she says. “Grass hosts are limited to corn and close relatives.”

It is too early to determine whether the new cyst nematode will be an economic threat to growers. “But it may mirror the Columbia root-knot story where the nematode was described from an isolated area around the Columbia River drainage area in the Pacific Northwest,” she notes. “Years later, its distribution is now worldwide.”

Control of SCN invasions often involves rotation to crops not susceptible to a particular cyst nematode, coupled with planting resistant seed varieties. You may need to take a similar route to control if it becomes a serious threat to corn.

September 2010

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