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Corn+Soybean Digest

Are Nematodes Gaining Ground?

Watch for a buildup in the Southeast

Recent droughts and lingering low prices have soybean farmers in the Southeast looking at survival as their top priority in 2001. They're also targeting any pest that keeps them from squeezing more bushels from every acre.

As a pest group, nematodes are pretty much off the radar screen. More obvious pests — like weeds and insects — get most of the management attention.

The fact is, nematode problems in soybeans may be getting worse. Farmers might not know it because the effects of drought may have masked impacts on crop growth.

Soybean cyst nematode (SCN) is still the number-one nematode pest, but other species like reniform, rootknot and Columbia lance are showing up more than ever in field surveys.

Universities have a long-standing recommendation for effective SCN management, reports John Mueller, nematologist at Clemson University's Edisto Experiment Station at Blackville. He believes that Race 14 and perhaps other races may be building up because farmers are planting Race 3-resistant varieties continuously and almost ignoring crop rotation.

Changes in cropping patterns and soybean management systems may also be causing some nematode buildups. For example, there's little doubt that the increase in cotton acreage has brought about more problems in soybeans from reniform and Columbia lance nematodes.

In many counties, cotton is planted year after year on the same acres. Farm equipment then spreads these nematodes to soybean fields. Also, several agronomists point out that heavy use of Roundup Ready soybean varieties has resulted in cleaner fields, but in turn has caused more problems, particularly with rootknot and lance.

Since most of these varieties were developed by seed companies based in the Midsouth, few in Maturity Groups IV-VIII have resistance to these nematodes.

While varietal resistance and crop rotation are the first line of defense for SCN, control of other nematodes is not so straightforward. For example, Mueller points out that rootknot nematodes can reproduce on corn roots. That eliminates corn as a rotation crop in a rootknot-infested field. Cotton is also not an option for fields with rootknot, lance or reniform infestations.

What can farmers do if they suspect problems from these nematodes? Conduct soil nematode assays in the fall, just after harvest.

South Carolina farmers have been urged for years to take their nematode samples in conjunction with soil sampling for fertility status. It's not happening, though, on much of the acreage.

Jason Norsworthy, Clemson soybean specialist, conducted a recent survey of soybean farmers and found that only 16% take nematode samples, while 63% take soil samples. For just a few cents per acre, a farmer would know which nematodes are present. That would assist with crop and variety selections for each field.

Varietal resistance to rootknot, reniform, SCN and lance are all available in adapted varieties. But farmers must obtain this information from state extension services or seed company representatives.

Using soil assay samples to find out what species are present, and then using rotation/varietal resistance, will help prevent buildups of species or races for which there are few, if any, controls available.

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