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Corn+Soybean Digest

Test Soil For Corn Nematodes

Even light infestations can steal yields Just when you thought you had a handle on soybean cyst nematodes, along come corn nematodes - pests potentially as costly and even less likely to be detected.

More than 10 nematode species are linked to economic damage in corn. Nematodes are found throughout the Corn Belt, but the species and potential damage vary with soil type and geographic location.

"Certain nematodes can do extensive damage at very low levels," says Pat Donald, research plant pathologist at the University of Missouri. "Others must be present at levels of hundreds or thousands before they cause any damage.

"We aren't finding many economic problems that growers are blissfully unaware of," she adds. "They may not know why, but most corn growers with corn nematodes in the field sense something is wrong. With lighter infestations, corn plants may look just a little puny. With heavier nematode loads, plants may wilt and look off-colored."

Corn nematode damage often mimics herbicide injury or nutrient deficiency. American Cyanamid (now part of BASF) notes that yield losses of up to 30 bu/acre have been documented. But, since nematodes attack roots, considerable yield loss can occur with few above-ground symptoms.

When symptoms do show up, they may include: - Poor or uneven stands.

- Stunted corn plants.

- Patchy, irregular shapes in the field with yellowed, unthrifty corn plants.

- Unexplained yield drops in parts of the field.

- Small or poorly filled ears.

"Weather definitely has an effect, too, especially moisture," says Donald. "Nematodes need oxygen - they drown in waterlogged soils. However, once corn nematodes invade roots, they are not so much affected by moisture."

Corn nematodes have been found in virtually all soil types. But, like most nematodes, they prefer lighter soils. They move better, have more oxygen and reproduce at a higher rate in sandy soils than in heavier soil types.

The best way to detect the tiny pests is to collect soil samples and have them tested. Donald analyzes several hundred samples for nematodes each year.

"Sting nematodes cause stunting within the first few weeks after corn is planted on sandy soils," she notes. "If you suspect sting nematodes, pull samples within the first month after planting. And take the entire plant - roots and all - not just soil."

For other corn nematodes (dagger, lance, lesion, needle, spiral), sample later in the season, but while corn plants are actively growing.

"Take several soil samples adjacent to corn plants," she suggests. "Probe 8-10" deep, and get close enough to plants to get some root pieces in the sample. If you're in a corn-soybean rotation, this is also the best time to sample for cyst nematodes, before you come back with beans."

Take subsamples from several areas of a suspected field, then mix them in a clean bucket. Put 1-2 pints of mixed soil in a plastic bag and label the bag with the field name or number and the date. Avoid exposing samples to freezing or extreme heat, since they contain living organisms. Be sure to designate which nematodes you're testing for, since the extraction techniques are different for soybean cyst nematodes and corn nematodes.

Donald tests for both. She first separates the different organisms by screening them through different-sized meshes after floating the nematodes out of the soil. Then she pulverizes the soybean cyst nematode "carcasses" to release the eggs when testing for cyst nematodes. Corn nematodes are separated from the soil by centrifugation.

"The number of eggs is what we use to gauge the level of infestation for cyst nematodes, but with other nematodes we count all the life stages present in the soil," she says.

If nematodes have invaded your cornfields to damaging levels, there isn't much in the way of genetic resistance to fall back on. Crop rotation is a good first line of defense against all nematodes, and rotating to soybeans helps control most corn nematode species.

If corn nematodes are stealing 7-8 bu/acre from yields, that may be enough to pay for control with a nematicide. Counter, Furadan and Mocap are nematicides and soil insecticides controlling corn nematodes as well as rootworms, wireworms and white grubs.

Mad Cow Disease Could Spike Soybean Prices Three European countries, France, Italy and Greece, have banned meat and bone meal in all livestock feeds to protect against the spread of "mad cow" disease. Germany is expected to adopt a similar policy and the rest of the European Union could follow suit.

The restriction will place Europeans in the market to find a protein source for their feeds. This is good news for soybean growers, says Allan Lines, Ohio State University ag economist. If more countries pass similar regulations, prices could approach the $5/bu USDA predicted for the 2000 crop.

A possible problem is the reluctance of Europeans to accept genetically modified (GMO) crops, he says. Since production of GMO soybeans has not been approved in some South American countries, they could become the preferred supplier over the U.S.

Another concern stems from how Europe will use its supply of meat and bone meal. If it's not used there, it likely will be sold cheaply to other countries that haven't banned it, creating competition for soybean meal as a feed additive. That could temporarily dent soybean exports, says Lines.

Scientists Identify New Corn Viruses Two new corn viruses have been identified by researchers at Ohio State University.

Maize necrotic streak virus was found in Arizona. Initially thought to be infected with the maize dwarf mosaic virus, the corn underwent 18 months of study and researchers have yet to find how the disease is transmitted.

In lab tests, Ohio State researchers have been unable to infect plants through common crop insects - the most likely method of transmission. Tests to infect the plants through a fungus and even by placing the virus directly on the leaves failed. Soil transmission is the only known method of infecting other plants but as yet scientists have failed to identify what in the soil causes the virus to spread.

"It may not be a big problem if it's not easily transmitted in nature," says Peg Redinbaugh, research plant molecular biologist. "If the virus is primarily transmitted through the soil, like we think, then it could be just a local problem."

The second virus, found in Georgia, has similar symptoms to other corn diseases, namely maize mosaic virus, maize chlorotic dwarf mosaic virus and maize rayado fino. Scientists haven't determined how this unnamed virus is transmitted, either. If it's spread by planthoppers or leafhoppers like the three diseases it resembles, it will likely be confined to the South due to the geographical limitations of those insects, says Redinbaugh.

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