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The hidden predator in corn
Nov. 13, 2007

A predator that has been lurking in Midwestern soils for years is likely to become an increasing threat to corn production in the near future.

Nematodes that attack corn plants have been in the soil forever. These nematodes have been a problem most corn growers have ignored, and few of today's farmers have ever looked for corn nematode damage.

As farmers have switched to no-till and reduced tillage, used less soil-applied insecticide/nematicides in lieu of transgenic insect resistant corn, and changed the classes of insecticides used from organophosphates and carbamates to pyrethroids that have less effect on nematodes and planted continuous corn, corn nematodes have begun to build up in the soil, thereby presenting new challenges corn growers will have to face in the future.

"Are you old enough to remember the days when corn nematodes created some of the most difficult problems in corn production?" asks Dr. Terry Niblack, professor, Department of Crop Sciences, University of Illinois. "Because of the changes currently taking place in integrated pest management practices in corn production, we can expect corn nematode problems to increase. Search your memories for the things you used to see and look for, because you're likely to see those same things again. Nematodes are the most frequently overlooked cause of corn disease."

Corn nematodes can cause a significant amount of economic damage, says Ray Knake, Bayer CropScience Midwest product development manager. "In the mid 1990s, the University of Illinois did a study and found that corn nematodes caused a $90-million loss in corn. If prices were near $1 per bushel in the 70s and 80s, think how much more economic damage could be out there now that corn prices are above $3."

Niblack says that the University of Illinois has not done a study since the 90s. "The state corn checkoff decided corn nematodes weren't an issue they wanted to fund research on," she says. "And getting federal grants to research it is next to impossible. So we don't know for sure what kind of economic damage is being done."

MULTIPLE CHALLENGES
Corn nematodes pose several challenges to farmers today. Unlike the soybean cyst nematode being the main soybean problem, there are many different types of nematodes that can economically injure corn, not just one single nematode.

"Nematodes are complicated," says Dr. Tamra Jackson, University of Nebraska Extension plant pathologist. "Technically it's an umbrella word because there are many different genera and species of nematodes that attack corn. It's not just one nematode."

These nematodes are known by names such as lesion, sting, lance, stubby-root, needle and dagger, just to name a few. To compound the problem, not all nematodes react the same in the soil either. Some of the corn nematodes are ectoparasites, which mainly live in the soil on the outside of the root penetrating the root to feed, and some are endoparasites, which completely penetrate into a root and spend most of their life there.

Farming practices are partly to blame for a rise in corn nematode problems. Although switching to no-till and reduced tillage has been a boon for the environment by reducing soil erosion, it has allowed some nematodes to remain undisturbed in their soil habitat.

"In addition, modern agricultural production practices have changed from using toxic insecticides like organophosphates and carbamates to less toxic pyrethroids," Jackson says. "The problem is that even though these chemicals are safer for us to use, they are safer on nematodes as well.

"Farmers also have switched from using nematicides to relying on Bt hybrids. However, the protein molecule is too big for microscopic nematodes to ingest. So it's not really effective on nematodes."

The increased demand for ethanol and higher prices for corn are driving more farmers to plant continuous corn instead of rotating among other crops. This encourages corn nematodes because they a have an ideal habitat and food.

Corn nematodes are in all soil types, therefore, most corn production can be affected.

"This past summer, Syngenta Seed Care initiated the first corn nematode survey in the northern corn states," says Cliff Watrin, technical manager, Syngenta Seed Care. "The team surveyed three random samples from each county with at least 25,000 acres of corn. They used six labs: one private lab and five university labs. They found that plant parasitic nematodes were practically everywhere they sampled. The results are still being calculated and should be available this winter. The highest populations of corn nematodes were found in areas of high-yield corn."

EXPLODING SOME MYTHS
But despite the fact that these nematodes are in nearly all soils and they are expected to become more of a problem, research funding has been nearly nonexistent. Soybean cyst nematode has gotten the media's attention and recognition. Most people only think of a nematode as something that attacks soybeans.

However, corn nematodes resemble soybean cyst nematodes in one area. Jackson says researchers believe, but have not scientifically proven, that corn nematodes do interact with fungi and bacteria. It is highly suspected that higher numbers of corn nematodes increase fungal infections because their feeding on the roots leave holes that fungi can use to enter the plant.

In a background article, Niblack explains that there are three myths about corn nematodes. The first is that corn nematodes only occur in sandy soils. "Damage in sandy soils is more likely to be spectacular-dead or dying plants-but damage does occur in heavier soils."

The second myth is that corn nematode injury is rare. It isn't. The third myth is that "corn nematode" means one particular kind of nematode. "There are at least four major types of corn-parasitic nematodes that can seriously damage corn in Illinois," she says.

Those four nematodes are lesion, which can be found in clay soils; needle, which is found in sandy soils only; lance, and dagger, which kills the growing point of the root. Niblack says other common nematodes in Illinois are ring, spiral, stunt, pin, sting, needle and stubby root.

To complicate matters further, a new species of corn nematode was found this past summer in Tennessee. In an article published on July 29, university nematologists from Iowa, Missouri, Nebraska and Tennessee and USDA nematologists announced that a new cyst nematode species that attack corn was discovered. University of Missouri nematologists made the discovery in 2006 in Tennessee.

"The nematode reproduced well on many different corn hybrids but poorly on other monocots," says Greg Tylka, Department of Plant Pathology, Iowa State University. "No dicots were found to be hosts. The cysts of this nematode look different than those of the corn cyst nematode found in 1981, and genetic analyses confirmed that the nematode was not the corn cyst nematode. The nematode appears identical to a cyst nematode discovered on goosegrass, a weed, in Lauderdale County, Tenn., in 1978."

This new nematode could seriously hinder field soil sampling for soybean cyst nematodes. "Currently, no other cyst nematode species commonly exist in Iowa corn and soybean fields, and eggs recovered from cysts extracted from Iowa soils are assumed to be SCN eggs," Tylka says. "But eggs of SCN and other cyst nematodes look similar and cannot be distinguished by appearance.

Determining SCN egg population densities for management or research purposes would be impossible using current techniques if other cyst nematodes were present. A stain that is specific for SCN eggs currently is not available, and developing such a stain likely would take years."

ECONOMIC THRESHOLDS AND SAMPLING
Compounding the corn nematode problems, separate from the new cyst nematode, economic thresholds vary by nematode and often by state, University of Nebraska's Jackson says.
For soil sampling, Jackson recommends taking a soil sample and a root sample. She and Syngenta's Watrin recommend sampling early in the growing season with six plants.

"Syngenta Seed Care recommends sampling for corn nematodes four to six weeks after planting," Watrin says. "That's when the plants will show symptoms if they ever will show them. Corn sets much of its yield potential in the early four- to six-leaf stage. So if you can help the plant have a healthy start, you'll be rewarded with more yield at the end of the season."

Sampling later in the season is more complicated because the plants are bigger and more expensive to ship to a lab. Also, nematode populations vary throughout the year.

However, Jackson says some nematodes go deeper into the soil at the end of the season. They don't really know why. It could be because of drought, temperature or some other unknown factor yet to be discovered.

Because soil labs vary, Jackson recommends retailers, crop consultants and farmers contact the lab they plan to send their sample to, to make sure they accept samples and will also extract nematodes from root samples. It is also necessary that they have permits if samples will be sent from out of the state. Turnaround time for results will vary as well.

MANAGING CORN NEMATODES
Management options are still mainly to rotate crops. However, this does not work for all of the nematodes that attack corn because some of them also attack other plants that host them from one year to the next.

There are still a few nematicides on the market, but they are toxic and require restricted-use handling. They also can be expensive to use. Syngenta Seed Care and Bayer CropScience have products in development. At this time there is no true seed genetic resistance bred into corn.
Syngenta Seed Care's Watrin says, "Technically there is no product labeled for corn nematodes right now. Syngenta does have Avicta registered as a seed treatment in cotton that is effective on nematodes. It is believed cotton nematodes are similar to corn nematodes. Some species do affect both cotton and corn."

Bayer CropScience is experimenting with some compounds that have activity against corn nematodes. "Two are in testing and were tested in 2007," says Bayer's Knake. "Two are seed-applied products that act on nematodes. The first is a hard chemistry because it is a synthesized chemical. So far it's showing a broad activity on nematodes, affecting many species. The second is a biological compound. It is being tested to see if it will be as broad spectrum as the first or if it will have activity on certain specific species."

These products would be seed-applied, not soil applied. They would be commercially applied on corn.

"It is believed that if the plant starts off healthy from the very beginning by reducing early season nematode attack, yield loss from nematodes would be lessened. The plants will actually be able to outgrow some nematode damage if they are healthy early in the season," Knake says.

Despite the fact that there aren't a lot of management options available currently, some seed manufacturers are convinced corn nematodes could be a large problem in the future.

"At the seed company level, every seed company that we asked about nematodes report they could be a big problem in some fields," Knake says. "Researchers are still trying to determine how much of an economic problem corn nematodes are. But there is definitely a big buy-in from several seed companies to devote research into developing solutions to corn nematodes."

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