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Corn+Soybean Digest

Watch Those Other Nematodes!

Most herbicide-resistant soybean varieties available for the Southeast possess resistance to soybean cyst nematode (SCN), but few have resistance or tolerance to other nematode pests important in the region.

With crop prices predicted to be low again for 2000, it's more important than ever that farmers select varieties based on performance plus other good traits needed for each field. Buying varieties just because they possess herbicide resistance might not cut it.

A concern of many who advise farmers is that the trend toward almost a monoculture of herbicide-resistant varieties overshadows other resistance traits available in many varieties. Most Roundup Ready varieties have resistance to important pests such as SCN and frogeye leafspot. But the potential for yield losses from secondary nematodes like lance, reniform and root-knot causes some to worry that farmers have blinders on when it comes to selecting varieties.

An extension plant pathologist in South Carolina recently conducted a survey of 500 soybean fields in the state and found that 485 (97%) had detectable levels of parasitic nematodes. Further, he found that, in 152 of those fields (31%), one or more species of nematodes was at or above threshold levels. In other words, roughly a third of the fields were infested enough to require the use of chemical and/or cultural controls.

SCN is recognized as the No. 1 nematode problem across the U.S., but lance, root-knot and reniform nematodes also are prevalent throughout the Southeast. Since these three also attack cotton, it has been proposed that more problems are showing up on soybeans because of the resurgence of cotton in the region.

The lance nematode, first found in South Carolina in the late '60s, has since spread throughout the Coastal Plains of Georgia and Alabama. Where numbers are high, this nematode can cause yellowing and stunting of the crop. Often, infested fields show an up and down growth pattern, with the most infected plants short and yellow, with root systems devoid of nodules and bunched near the soil surface.

Infected plants are usually located in spots in the field and they rarely die. However, they're poor producers of pods and healthy seed. Thus, yield is adversely affected.

There are no varieties with resistance to lance nematode, but several conventional varieties have good tolerance. These include Clemson's Hagood, Dillon and Motte, along with Benning from the University of Georgia. Several varieties have also been found to have lance tolerance. Contact company representatives about seed supplies.

Root-knot nematode problems in the Southeast are commonly caused by two types: the common or southern root-knot and the peanut root-knot. The most important symptom is galls on infected roots. In fact, this nematode can quickly kill a plant if enough galls are present to inhibit water and nutrient flow into and through the root system.

As plants become infected, the leaves show a "flagging" with interveinal yellowing and browning of the top leaves. Infected plants easily succumb to the effects of drought and do not compete well with weeds. It's common to see irregular patches of infected and dead plants across an infested field.

Root-knot control is best accomplished with resistant varieties. Many conventional varieties have resistance to the common root-knot, and a few have resistance to the peanut root-knot. But very few Roundup Ready varieties have resistance. That's because most breeding for resistance to root-knot has been done by public breeders in the Southeast, while most Roundup Ready germplasm has been developed by commercial breeders in the Midsouth, where root-knot nematode is not a significant pest.

The reniform nematode has been showing up more and more across the Southeast, especially in cotton-producing areas. Symptoms include yellowing of leaves, plant stunting and lots of unfilled pods. The roots of infected plants also show symptoms and may die.

Reniform control is best done with resistant varieties in conjunction with rotation to grass crops like corn.

For information on which varieties possess resistance, contact your seed dealer or seed company representative.

To find out which nematode(s) are present in your fields, have your soils tested. Collect samples from each field in the fall, shortly before or after harvest and before a hard freeze. All states have labs that test for nematodes, and county extension offices have information about taking and submitting samples for testing.

Before farmers make their variety decisions, it's extremely important to have soil assay information to determine if resistance to one or more nematode species is needed. Paying attention to this facet of management could pay good dividends at harvest next year.

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