It's no secret that cotton acreage took a big hit in 2007 in the upper Southeast, with much of the reduced acreage going into corn. On the sandier, lighter soils characteristic of the region, the next hit may come from root-knot nematodes.

Nematodes are soil-borne microorganisms that require a live plant host to survive. Root-knot nematodes produce specialized feeding structures in the roots of cotton and other host plants, causing changes in the plant cell. These changes produce swelling around the nematode, causing galls or knots.

These galls are both a blessing and a curse for cotton farmers. A blessing in that they make identification of the problem simple. A curse in that by the time the damage is recognized, cotton is likely to have reduced production and quality.

Root-knot nematodes, despite their distinctive galls, or knots, can still be difficult to diagnose, because they tend to not be uniform in a field, instead usually appearing in clusters spread sporadically throughout the field.

Veteran North Carolina Crop Consultant Danny Pierce says root-knot nematodes, like insects, are a numbers game for farmers. He started consulting in 1981 and has now spent over 25 years working the predominantly sandy soils in and around Goldsboro, N.C.

“Root knot is the major nematode problem on cotton in the sandy coastal plain of eastern North Carolina where I work. Ninety percent of this root knot problem I see is in cotton following cotton. This problem is easily detected by taking nematode samples prior to planting. The North Carolina Department of Agriculture will analyze these samples for three dollars each. This analysis gives you how many root knot per 500cc's of soil and recommendations,” Pierce explains.

“In our area of North Carolina, this problem is more severe on sandier soils with the same populations of nematodes. For example, 300 root knot nematodes per 500cc on deep sandy soils is more of a problem than 600 root knot on loamy sand with clay close to the top of the ground. Even with twice the population of nematodes in the loamy sand, this population will have less of an impact on the cotton plants than the lower population in deep sand. Keep this in the back of your mind when interpreting reports.”

The proliferation of corn acreage in North Carolina may ratchet up the root-knot pressure even more. Prior to the onset of modern corn hybrid varieties, corn wasn't considered a good host for root-knot nematodes. Now, the newer, more prolific corn varieties on the market are an ideal host for root-knot nematodes, leaving many upper Southeast growers with some challenges for 2008.

Across the board in the Southeast, corn acreage was up. Much of that corn proved to be poorly suited to the lighter, sandier soils that dominate the upper Southeast. As a result, many growers are likely to go back to other, more productive crops — primarily cotton.

In many ways breaking the cotton after cotton rotation with a year of corn will benefit the next cotton crop. Unfortunately, reducing root-knot nematodes will not.

Corn is typically not damaged by root-knot nematodes, giving growers a false sense of security when planting subsequent crops. Though not damaged by the microscopic worms, cotton is an ideal host plant for three of the four major species of nematodes that attack row crops in the Southeast (M. incognita, M. arenaria, and M. javanica).

Concern over the potential buildup of root-knot nematodes following corn is not confined to the upper Southeast. The furor over ethanol has spurred corn production nationwide. In Arkansas, veteran Plant Pathologist Terry Kirkpatrick says, “Everyone these days seems to love corn — including nematodes. As we move corn acreage into former cotton acreage, we'll see an increase in root-knot nematodes,” he says.

Though management practices may vary a bit from one part of the country to the next, in North Carolina, Pierce says growers who have done the proper surveys and determined they do have root-knot nematodes in the soil have five options for management:

  • Telone II — This is a very effective nematicide, but since most of the cotton I work on is no-till or strip-till this is not an option. It does not fit into this agronomic cropping system.

  • Avicta — This is a relatively new seed treatment, but does not work well on moderate to high populations of root knot. It is very user friendly and growers love it.

  • Temik — This systemic insecticide/nematicide has been the standard for decades. Higher rates are used as a nematicide, but do not work well on moderate to high populations of root knot.

  • Resistant varieties — There are very few cotton varieties that are resistant to root knot. This resistance is fair at best and many growers don't like other characteristics of these varieties.

  • Rotation — This is the best and only cost effective control of moderate-high populations of root knot.

Of these options only the last one is really cost-effective and viable, according to Pierce. A root-knot resistant soybean variety works great for this rotation, he says. With the cost of production of cotton so high, and even higher where root knot populations are moderate to high, and yields lower than normal in these nematode infested fields, this is the only cost effective option,” Pierce adds.

“High yields of cotton are a necessity for profits, but are next to impossible in these fields. Planting a root knot resistant variety soybean will almost always bring the root knot populations down to a very low level. Cotton can then be planted the following year. The recent high prices of soybeans and low prices of cotton have made this an even more attractive nematode management practice, the North Carolina crop consultant says.

North Carolina State Plant Pathologist Steve Koenning adds, “Seedling diseases and plant-parasitic nematodes account for the greatest yield losses in cotton, although occasionally other diseases may impact yield.

“The root-knot, Columbia lance, reniform, and sting nematodes are increasingly responsible for cotton yield suppression. The primary tactics for management of these nematodes are the use of nematicides, and cultural practices such as rotation. Resistance to nematodes in cotton is generally lacking, although cultivars tolerant to some of these nematodes has been identified.”

The increase in winter wheat acreage in the Southeast in the fall of 2007 is a good indicator that at least some percentage of the land that went from cotton to corn may now go to wheat and double-crop soybeans. That combination would be a huge benefit to subsequent cotton acreage.

While cotton acreage is down across the Southeast, the long-term future for exports looks good and few doubt that cotton will make a return to the Southern scene. When it does, a good way for growers to take maximum advantage is to reduce root-knot nematode populations in their fields, starting with the 2008 crop.