Nematodes cost cotton growers in Georgia millions and millions of dollars each year. The costs come from loss of yield and in the purchase of nematicides. Used judiciously, the value of increased yield more than offsets the cost of the nematicide. Growers next year have a single best chance to protect their crop from nematodes; this chance comes before the furrow is closed in 2025. At that time a grower can chose to use a nematicide, or not. The grower can choose to plant a nematode-resistant variety, or not. The grower can decide if cotton should even be planted in a field, given the cropping history. Once the furrow is closed, all of those decisions are made. The grower can now worry about other things because whatever Team Nematode does or doesn’t do to his Home Team Cotton Crop is largely out of his hands.
Growers may only have that “one chance” to fight nematodes next season, but to make the “best decision” (variety, nematicide, crop to plant in a field) requires that a grower have the best information possible. When it comes to a battle with nematodes in a cotton field, the best information comes from sampling for nematodes at the end of the previous season. For the 2025 season, that means growers should be sampling between now and the onset of cold weather and freezing temperatures.
I walk a lot of cotton fields this time of year and damage from nematodes to a cotton crop is never more evident than it is now. Obvious symptoms include stunting, symptoms of nutrient deficiencies, “tiger striping” on the leaves, and, often, pre-mature cutout. All of these can be considered “general” symptoms in that they may exist because of other causes as well. However, they are a good indication that nematodes COULD be the culprit. You can carefully dig up a symptomatic plant and look for galls on the roots. If present, this is a clear indication that the southern root-knot nematode is a problem. However, if no root galls are observed, it could still be reniform, sting, or Columbia lance nematodes. This is why there is no substitute for pulling soil samples for nematode analysis.
There is no better way to characterize a nematode problem in a field than to pull nematode samples. Results from analysis of these samples allows one to determine not only the type(s) of plant-parasitic nematodes in a field, but also the quantity of each type. Knowing the species present allows one to make more informed decisions about whether (or not) planting a nematode-resistant variety is appropriate. Growers can plant varieties that have resistance to the southern root-knot and reniform nematodes. We do not have varieties with resistance to the sting of Columbia lance nematodes.
Knowing the “quantity” of nematodes in samples, (for example in 100cc or in a pint of soil) is also very important for several reasons. First, the nematode count can be compared to our “economic threshold value”. This value is our best guesstimate as when nematode populations are above that number, growers are more likely to see an increase in profitable yield with use of a nematicide; when numbers are lower than the economic threshold, the grower is less likely to increase profitability with use of a nematicides. For UGA Extension, our economic threshold (per 100cc soil) for southern root-knot nematodes is 100 J2 juveniles, for reniform nematodes is 250, for Columbia lance nematodes is 80, and for sting nematodes is only 1.
Second, knowing the quantity of nematodes allows one to better determine which is the most appropriate nematicide to use. At low levels, a seed-treatment nematicide may be appropriate. For moderate levels a grower would be better off using a nematicide such as AgLogic 15GG or Velum, and possibly supplementing either with Vydate C-LV. Where nematode numbers are very high, there is no substitute for use of an appropriate resistant variety or fumigation with Telone II. Third, knowing how nematode populations may vary across a field allows growers to be more judicious in “site specific” applications of nematicides, either in terms of product or in term of rate.
Data from soil samples collected for nematode analysis are only as good as was the care in collecting the samples. In order to get the best results, growers must consider these following factors.
- Samples should be collected prior to onset of cold weather.
- Samples from specific areas of concern are most indicative when collected directly from the root zone and not in the row middles.
- Where samples are collected to better understand a “bad” spot in the field, a comparative, separate, sample from a “good” area should also be collected.
- Collecting more samples from a field increases cost, but also better allows grower to understand his field. At very least, separate samples should be collected if problem areas occur in a field or where different soil types occur.
- Samples should not be collected if soil is too dry or too wet.
- Samples should be cooled immediately after collection and sent at quickly as possible to a diagnostic lab.
- Gowers are advised to seek help from Extension or consultants to best interpret results.
You as growers are busy at the end of harvest. Sampling for nematodes costs money per sample and in time to collect samples. However, the decision you make in 2025 before you close the furrow with regards to nematode management will affect everything else you do to grow that crop. Sampling nematodes now will help you to a better crop next year. At least consider it….
Dr. Bob Kemerait
UGA Plant Pathology