Testing, Treating For Nematodes:
A Wise Production Practice

This article prepared by Thomas A. Lee, Jr.,
Professor and Estension Plant Pathologist,
Texas A&M Research and Extension Center at Stephenville,
appeared in "Plant Disease News," Volume 3, No. 5, July 25, 1994.

In some Texas Southern High Plains cotton fields, yields can increase by as much as 50 percent when root-knot nematodes are controlled. Dr. James Starr, plant pathologist at Texas A&M University, says although not all fields are infested to such a degree, yield losses in the region average around 10 to 11 percent of total yield potential.

"In our area, root-knot nematodes probably cause much more damage than we give them credit for," Starr says. "Fortunately, losses are less severe in other parts of Texas."

The pattern is typical throughout the Cotton Belt -- individual regions suffer significant yield losses, and other areas where the nematodes are less prevalent, have much lower losses. The Texas threshold level for root-knot nematodes is very low; however, Starr says, even barely detectable levels are sufficient to cause serious damage.

The time of year the field is sampled, the type of soil in the field, and the environmental conditions under which the crop is grown are all important factors in determining the amount of crop loss that can be expected.

"If you measure the nematode population in the fall of the year preceding planting, then the threshold level will be much higher than in the spring -- generally several hundred nematodes per pint of soil," he says. Root-knot nematodes are found primarily in coarse-textured sandy soils. Such soils make the cotton more susceptible to nematode damage. Silty, clay-type soils are less likely to have this problem.

Detecting a damaging population can be difficult. The infestation generally causes the cotton growth to be stunted. A lightly infested field will look just a little off, while a severely infested field will have large areas where the cotton is stunted and yellow.

When sampling, Starr suggests than an individual sample represent an area no larger than approximately five acres. Each sample should be composed of 15 to 20 subsamples collected from within the overall sample area. These subsamples are mixed together to form one sample.

"If a grower walks into a field and digs soil out of one spot, he would not get a true representation of the nematodes present," Starr says. Larger fields should be handled differently. If the field is significantly larger -- 50 acres or so -- a grower needs to evaluate the field based on its soil type and cropping history to determine how many samples he should collect. If a field has some obvious problem areas, those spots should be sampled separately from that portion of the field that may be producing very well.

Fields that are split between separate crops the previous season should also be sampled separately. Samples should be taken in the fall, at the harvest of the preceding crop, when root-knot nematode populations will be at their highest.

"In collecting the samples, remember that this is a biological specimen, and it must be handled that way," Starr says. "A good rule of thumb is to treat it like a carton of milk. This is not a fertility sample that you can allow to become very dry, or allow to freeze. It must be protected from excessive heat, cold, or dryness."

Once samples have been collected, they should be sent to local experts for processing and identification of nematodes present. "This might be your state Department of Agriculture, the Agricultural Extension Service for your region, or a private consultant," Starr explains. "If you don’t know what is available in your area, I would suggest you ask your local chemical supply dealership or county Extension office for assistance."

In Texas, send samples to the Texas Plant Disease Diagnostic Laboratory, Texas A&M University, 1500 Research Parkway, College Station, Texas 77843-2589.

When the lab reports for the samples are returned, and damaging levels of nematodes have been detected, corrective steps should be taken.

"In some cases, crop rotation is a very effective control measure, if it fits into the production system. It is important to remember that nematodes can cause problems, but they can be controlled, which can in turn increase profits."