Every year, growers across the Cotton Belt lose a significant percentage of their crop to nematodes. In 2003, nematodes cost growers 4.54 percent of their crop, which translated into an economic loss of 1,059,848 bales, valued at more than $413 million. Beltwide losses have increased more than 50 percent in the past 10 years.
Nematodes are a much more insidious problem than most growers recognize. Even the experts say that economic losses have probably been underestimated in the past.
From a historical perspective, many experts think we really didn't know how widespread this problem was and how much damage it was causing. We've made major strides in addressing the situation, but there's still a substantial economic loss every year from nematode infestations across the Belt.
Fortunately, there are steps a grower can take to minimize losses from nematodes. Sampling for nematodes is the best way for a grower to begin addressing this problem. Since nematodes are microscopic pests that often produce no clearly identifiable field symptoms, sampling can:
(1) determine whether or not a problem exists, and
(2) provide a basis for developing a management plan to address the problem.
For the most accurate results, nematode samples should be pulled in the fall after harvest in most states. (California and Arizona recommendations differ slightly.) In addition, the entire process of pulling samples, sending them to the laboratory and interpreting the results should be executed deliberately and carefully. Failure to do so will compromise the sample results and may provide a distorted picture of what is really happening in the field.
A cotton grower who samples for nematodes may be patting himself on the back for being such a good manager, but is he really?
The answer: It all depends on how he's handled the sample. If the sample isn't handled correctly at every step from the field to the laboratory, then it's really just a wasted exercise.
Proper Sampling Technique
Nematode infestations are not normally spread evenly throughout a field. Often, there will be areas of high populations interspersed with areas of low or even non-existent levels. For this reason, it is very important to sample the field multiple times in a random pattern.
The field should first be divided into areas based on cropping history and/or soil type. These areas should then be divided into blocks of no more than 10 acres. If poor growth has been observed in certain areas of the field, those areas should be designated as separate blocks, regardless of the size. Finally, each block should be sampled at least 20 times in a random pattern, the samples thoroughly mixed and about a quart of mixed soil representing each block placed in plastic, sealed bags and labeled accordingly.
If the soil type changes throughout the field, or a grower has noticed areas of crop stress during the year, he should tighten his sampling grid and make sure to sample areas that are suspect. In addition, a grower should provide the laboratory with the cropping history of each field.
After working diligently to obtain a representative sample, it is equally important to properly handle the samples and interpret the results.
Handling Samples Correctly
Laboratory analysis can only detect living nematodes. If the sample is exposed to extremes in temperature, or the soil is allowed to dry, the nematodes present in the sample may die. The results of the analysis would then be very misleading.
Nematode samples should be handled like a carton of milk. You don't want to freeze it, but you don't want to let it get hot either.
I recently visited a lab and saw several boxes of nematode samples sitting on the loading dock on a Saturday. The lab doesn't open until Monday. It was 84 degrees. There's no way those samples could have remained viable for that length of time sitting out there in the hot sun.
The information that those growers got back from those samples was totally useless.
A nematode sample that is exposed to extremes in temperature or allowed to dry out has lost its value. The organisms present in the soil sample will die, and laboratory procedures cannot detect nor measure their presence. In addition to handling concerns, the actual sampling process should be methodically planned and executed for the best possible results.
After the results of the laboratory analysis are received, it is important to consider them carefully, and in combination with other factors so that a grower can minimize the chance of misinterpreting the results.
Interpreting the Results
Laboratory reports differ from one lab to another, and even the measurements are not standardized. While some labs report numbers in juveniles per cc of soil, others may report them in numbers per pint of soil.
Make sure that “apples are being compared to apples,” i.e.: If the lab is reporting results in terms of numbers per pint of soil, make sure that the recommended treatment threshold numbers are stated in the same terms. If not, convert one of the measurements.
Provide a copy of the field's cropping history to the laboratory so that technicians can use it to help formulate recommendations.
Remember that even under the best of circumstances, sampling may simply “miss” nematodes that are present in the field, and/or laboratory analysis may do the same. Sampling is still considered the most reliable method of detecting nematodes, but it is not necessarily 100 percent accurate. If crop productivity continues to decline for unexplained reasons, a grower should continue to suspect the presence of nematodes.
Don Blasingame is a retired Extension plant pathologist with Mississippi State University. He now serves as group leader for the National Cotton Council's Beltwide Nematode Committee.