As the 2004 growing season approaches, cotton producers need to start thinking about nematode management options to include in their production system plans. The reniform nematode is rapidly becoming the most economically important pest associated with cotton production in the Southeast United States. In many locations, the damage incurred by this nematode has surpassed the root-knot nematode in both the reduction of plant development and effects on cotton yields. The reniform nematode has been found in the 11 states that make up the southeast Cotton Belt.
Excluding Texas, over 6.2 million acres of cotton are produced in the Southeast. It is estimated that 1.2 million or 19 percent of these acres are infested with the reniform nematode. The percent of acres infested in each state range from 1.4 to 55 percent. In Texas, estimates of infestation exceed 2.7 percent of over 5.5 million acres. The highest infestations occur in Alabama, Louisiana and Mississippi. In Mississippi, 32.4 percent of the cotton producing acres in 51 counties are infested with this nematode.
The reniform nematode will reduce cotton plant growth and lint yield. Symptoms will vary due to the length of time a particular field has been infested with the nematode. Fields in which the nematode has recently been introduced may display areas of stunted and uneven plant growth, giving the field a spotty appearance. Nematode numbers will generally be higher in the areas of poor plant growth. After a field has been infested for several years, the nematode will become more uniform in its distribution. In these fields, no typical foliar symptoms of nematode damage are visible. Overall plant growth is generally reduced across the entire field and yields will continue to decline each year cotton is planted.
The reniform nematode is capable of reducing cotton yields by 60 percent or higher when cotton is planted in fields infested with high nematode numbers. Therefore, steps are necessary to manage this nematode and maintain low nematode numbers. Cotton lint reductions attributed to this nematode are greater than 152,616 bales in Mississippi.
The first step in a management strategy is to determine that the reniform nematode is actually present. The reniform nematode does not produce a characteristic root symptom as with the root-knot nematode. With the lack of obvious or notable root symptoms, the only means to determine if the nematode is present in the soil is with a soil analysis. A representative sample from the field in question should be submitted to a reputable laboratory for analysis. If the reniform is present then an integrated approach with a combination of management tactics are recommended. Management options include resistant or tolerant varieties, rotations with non-host crops and the use of nematicides.
Currently there are no commercially available cotton cultivars with an appreciable level of resistance to the reniform nematode. Although no resistance has been identified, several cultivars have exhibited a degree of tolerance by producing high yields when planted in reniform-infested fields. However, these cultivars are not consistent in their results to the reniform nematode across the Southeast.
The rotation of cotton with a crop that is a non-host to the reniform nematode or with a crop that has resistance to the reniform is an effective management tactic and should be practiced when possible. The inclusion of a non-host such as corn, peanut, sorghum, wheat, or a crop which has resistant varieties such as soybean, in a cropping system, are effective in reducing reniform nematode numbers. Corn is practically immune to the reniform nematode and is considered a good rotation crop in areas where corn can be produced. Fields should also remain weed free during the rotation to prevent nematode increases on weed species. Numerous weed hosts of the reniform nematode have been reported.
The use of a nematicide is the most common management tactic. Nematicides reduce nematode numbers early in the growing season; however, their effects are short term and they must be applied annually.
The fumigant nematicides, such as Telone II and Vapam, are non-selective biocides that are applied prior to planting the crop. Once applied to the soil the material will volatilize forming a vapor that will move through the air spaces in the soil and mix with the moisture film around the soil particles killing nematodes and other soil microorganisms. To be effective, the soil temperature must be high enough to ensure volatilization of the chemical and moisture content low enough to allow movement of the gas through the air spaces of the soil. The fumigant nematicides generally require a seven- to 14-day waiting period once applied before the crop is planted.
The non-fumigant nematicide, Temik 15G, is also effective in reducing reniform numbers. Temik 15G can be applied in the seed furrow at planting. The active ingredient will kill nematodes on contact and also be absorbed into the developing seedling and provides systemic protection from nematodes and early season insects. However, results vary due to soil types, soil moisture, and environmental conditions found across the southeast.
Post-plant applications of Temik 15G, as a side-dress treatment five to six weeks after planting, in combination with at plant applications have also been effective to increased cotton yields. Side-dress applications, however, have not been consistent in university tests across the southeast. Vydate C-LV when applied at as two foliar sprays starting at the sixth true leaf stage plus seven to 14 days later in combination with an at plant application of Temik 15G has consistently reduced reniform numbers later in the growing season and increased cotton yields. The reduction in nematode numbers associated with post-plant nematicide applications coincides with the cotton physiological growth stages of flower set and boll production.
The availability and yield response attributed to the use of nematicides vary from location to location. Specific precautions and requirements should be obtained from the appropriate state agency in each location that a nematicide is to be used.
Editor's Note: Dr. Gary W. Lawrence is an associate professor of nematology in the Department of Entomology and Plant Pathology at Mississippi State University.