Nematodes have been referred to in various publications as “the hidden enemy,” the “unseen threat,” and various other names that indicate their microscopic nature and the fact that they live in the soil. Unfortunately, although nematodes are small, they can cause big problems for cotton growers.
The good news for growers in the Mid-South is that only two nematode species, root-knot and reniform nematodes, are economically important in cotton. The bad news is that either species is capable of suppressing cotton yield by as much as 25 to 30 percent. Recent surveys indicate that one or the other of these species is present in nearly half of the cotton fields in any given season in the states of Arkansas, Mississippi, and Louisiana.
These nematodes are root parasites. Crop damage and yield suppression are direct results of injury to root systems due to nematode feeding and reproduction.
Nematode damage to cotton is much less obvious than the damage that occurs from insects. Because nematodes require a living host in order to reproduce, plant death is unlikely to occur as a result of nematode feeding. More commonly, nematodes cause a progressively debilitating situation where infected plants grow and develop slowly and produce less lint than healthy plants.
The absence of dead plants or other obvious symptoms in the field may help to explain why nematodes are usually underrated by growers as economic threats to their crop. But estimates by the Cotton Disease Council indicate that nematode damage to cotton accounts for an annual loss in production of almost 300,000 bales in Arkansas, Louisiana, and Mississippi, alone.
Currently, reniform nematodes are the most serious nematode threat to cotton production in both Louisiana and Mississippi. In Arkansas, root-knot is the major economic nematode, although reniform has gained ground in the state during the last 10 years.
The first and foremost component of any nematode management program is to determine in which fields nematode problems exist. Not all fields have nematode problems, and a major problem may exist in one field and not in the field across the turn row.
The only certain way to determine if nematodes are present in a field is to test a soil sample that has been collected specifically for nematode assay. Most states, including Arkansas, Louisiana, Mississippi, and Tennessee in the Mid-South have university or Cooperative Extension Service laboratories that provide soil assays for nematodes. Several private laboratories in the region also provide this service.
Advice and guidelines for proper collection and handling of soil samples for nematode assay are available from the laboratories and from local Cooperative Extension Service offices.
While the point of this article is not to give detailed instruction on sampling, a few points should be kept in mind.
First, nematodes are alive. They have to stay alive until they are processed in the laboratory. Dead men tell no tales, and neither do dead nematodes. The accuracy of the results from a nematode assay is only as good as the quality of the sample that was received by the laboratory.
Three “don'ts” are vital. Don't let the sample dry out. Don't let the sample sit in the hot sun. Don't let the sample sit around for a long period of time.
Collect the samples carefully, store them in plastic bags in an insulated container without ice, and send them to the assay laboratory within 24 to 48 hours after collection.
A second consideration in sampling for nematodes is the fact that nematode problems are difficult to quantify. Soil moisture, time of the year, thoroughness of the sampling procedure, sample handling, and laboratory extraction procedures all influence the number of nematodes that are found in a soil sample.
Nematodes are never evenly distributed across fields. In contrast with insects, nematodes do not fly, run, or crawl across the ground to find a host, so they have evolved the ability to reproduce at high levels and to survive for long periods of time in the absence of favorable hosts — in a limited area. This results in high levels of nematodes in parts of the field while adjacent areas of the same field may have few or no nematodes.
The more areas with high nematode numbers that occur in a field, the greater the economic problem. Sample fields thoroughly.
Nematode populations increase during the year in response to the increasing availability of food (the cotton crop). Nematode numbers will be highest and easiest to measure during the last third of the crop season and shortly after harvest. Population levels then decline due to cooler temperatures and the absence of living hosts.
By late fall most years, many of the active nematodes have either died or moved out of the upper soil layers. Nematode eggs which remain attached to roots and root pieces make up much of the overwintering nematode population. Since nematode assay procedures do not accurately detect eggs, late winter and early spring samples generally are not very useful in predicting the severity of the nematode problem that exists in a field.
There are no quick fixes to managing nematodes. Nematode problems in fields don't arise spontaneously, and they don't develop over one season. Fields with severe nematode problems have been in the making for several years, and once nematodes are in a field, they will be there forever.
Nematodes can, however, be managed so their impact on the crop is of little or no economic consequence. Unfortunately, in this day of rapidly developing new technology for crop production and pest management, techniques for nematode control have remained constant. There are no transgenic cotton cultivars with improved nematode resistance. In fact, there are no cotton cultivars available in the Mid-South with any genetic resistance to either reniform or root-knot nematodes (with the exception of Paymaster 1560 BG/RR, which is a little less susceptible to root-knot).
There are a few effective nematicides available, and there are a few alternative crops that may be economically feasible to use in rotation with cotton to help lower nematode populations. Nematodes can be managed, but we don't have the ammunition to simply control them.
The most effective, but perhaps the least practical, method for lowering nematode populations is crop rotation. Planting a non-host or nematode-resistant crop in an infested field can lower nematode population levels from 50 to 90 percent in one season.
Rotation to rice for a year can lower the number of either root-knot or reniform nematodes by as much as 80 percent. Unfortunately, root-knot is usually most serious in light-textured, sandy soils where rice production is not always practical. Where reniform nematodes are a problem, rice rotations are more feasible. Reniform nematodes are most often found in soils with higher silt or clay content than those favoring the root-knot nematode, and these soils may also allow profitable rice production.
Another highly effective rotation crop, but one of very limited use in the Mid-South, is peanuts. Peanuts are non-host for both root-knot and reniform nematodes. Peanut-cotton rotations have been very effective in the Southeastern states for managing both nematodes.
Grain sorghum is also a non-host for reniform and a poor host for root-knot. Root-knot population levels under grain sorghum were approximately 60 percent lower than populations following either susceptible soybean or corn in recent studies in Arkansas. Nematode samples assayed in the Arkansas Nematode Diagnostic Laboratory over the last few seasons indicate that a year of grain sorghum can lower reniform nematode population levels by as much as 60 to 70 percent.
Corn has become a popular crop for growers throughout the Mid-South, and corn-cotton rotations are increasing in popularity among growers. The effectiveness of corn for nematode management is a mixed bag. Corn, like grain sorghum, is considered to be a non-host for reniform nematodes, and a year of corn can lower population levels substantially. However, corn is a good host for the root-knot nematode and a year of corn can increase root-knot population levels substantially.
Soybeans in rotation with cotton can also produce either positive or negative results. Soybeans are good hosts for the reniform nematode. Most soybean cultivars allow reniform populations to increase significantly, but a few cultivars are resistant and can lower populations. The situation with root-knot is similar. There are a few root knot-resistant soybean cultivars that lower nematode population levels, but most are in maturity Group V or later.
Susceptible, and even most moderately susceptible soybean cultivars allow considerable nematode reproduction to occur.
Although they are rather expensive, nematicides are many times the most practical method for nematode management in cotton. Three nematicides are currently recommended for nematode control in cotton in Arkansas and most of the rest of the Mid-South. Information and advice on rates, timing, and application methods are available through most county Extension offices.
Telone II, a soil fumigant, is injected into the soil as a liquid where the chemical volatilizes to a gas. Telone II can be highly effective in killing nematodes, but it must be injected 12 to 24 inches deep under the row at least a week prior to planting. The soil should be sealed immediately after injection to avoid loss of the gas prematurely.
Soil texture, temperature, and moisture all affect the efficacy of Telone II. Soil should be moist, but not saturated, in the upper foot, and the fumigant is most effective at soil temperatures between 40 and 80 degrees F at the depth of injection.
Both Temik and Nemacur are non-fumigant nematicides that work by killing or inactivating the nematodes on contact. The systemic nature of these materials is a distinct plus in that they also kill early-season insect pests that feed on the leaves and upper plant parts. Unfortunately this systemic activity has virtually no bearing on nematode control because nematodes do not feed on these parts of the plant.
Non-fumigant nematicides are usually applied into the planting furrow during the planting operation. These materials provide two to four weeks of nematode suppression, allowing the establishment of cotton seedlings with minimal damage from nematode feeding.
Soil texture and moisture also affect the efficacy of these materials, although not to the same degree as with Telone II.
Activity against nematodes is lower in extremely dry soil, and excessive rainfall after application can dilute the effectiveness of these chemicals.
Temik 15G can also be applied as a sidedress application to plants at about the time of pinhead square. Sidedress applications should be considered only in addition to an at-planting application. Waiting until pinhead square to make the first application of nematicide is not an effective way to manage nematodes because too much damage may already have occurred to the young seedlings prior to pinhead square in the absence of an at-planting nematicide application.
As with crop rotation, nematicides are not a cure-all. Proper application of nematicides to cotton fields where either root-knot or reniform nematodes are at economic levels generally improves plant growth, and in most cases, provides an increase in yield. However, nematicides will not work miracles. In fields with extremely high nematode population levels, even the use of a nematicide may not result in acceptable crop performance, and environmental conditions in any given year will affect their efficacy.
Where severe nematode problems exist, growers would be well-advised to consider using crop rotation initially to lower nematode population levels. Nematicides then may provide acceptable degrees of nematode suppression for the next year or for the next several years, depending on the situation.
Nematode management in cotton is an ongoing process. Detection of the problem through soil sampling for nematode assay is the fundamental component of a management program. Of equal importance with initial detection of the problem, however, is regularly monitoring the impact of the management strategy that is being used in each field.
Crop rotation can be a very effective means of getting a severe nematode problem back to a manageable level. The proper rotational sequence can also help maintain nematode populations below economically damaging levels — if the sequence is repeated periodically.
Nematicides generally provide a degree of nematode suppression, and under moderately severe population levels, nematicides generally provide a significant improvement in yield.
However, neither crop rotation nor nematicides eliminate all nematodes from a field. Root-knot or reniform nematodes will begin to increase as soon as cotton is planted into the field, and their populations will increase every year that cotton is grown.
Terry Kirkpatrick is an Extension plant pathologist with the University of Arkansas Southwest Research and Extension Center.