Side-by-side comparison of a healthy cotton plant root next to a diseased cotton plant root.
Nematodes inhabit a broad range of environments and encompass many different species; some are helpful while others are harmful. The soilborne nematodes that feed on plants are so tiny that they can only be seen under a microscope, thus soil sampling is key to determining species and populations. Two economically important nematodes in cotton cultivation are the southern root-knot nematode (Meloidogyne incognita) and the common reniform nematode (Rotylenchuslus reniformis).
What are nematodes?
Nematodes are microscopic roundworms that can be found in most soils, including cultivated fields. The two most prevalent types of nematodes in cotton production, root-knot and reniform, are plant-parasitic nematodes, meaning they primarily feed on roots of vascular plants. These nematodes have a stylet used to puncture plant cells and ingest nutrients, impeding root development and nutrient uptake, which leads to the reduction of cotton yield.
What are the signs of root-knot nematodes (RKN)?
RKN can be found in all cotton production areas throughout the Cotton Belt, but they are more prevalently associated with coarse soil textures, such as sandy soils. While RKN is typically distributed unevenly in infested fields, concentrated populations can cause severe crop loss, from 75 to 100 percent.
Plants affected by nematodes will be nutrient deficient, so they will look stunted and typically wilt under environmental stress. Nematodes also leave plants more susceptible to fungi and provide sites of entry for seedling diseases that can result in plant death. The nematode life cycle is typically completed in 21 to 30 days, depending on soil temperatures.
Midseason and fall soil sampling are the best methods of determining if nematodes exist. During the growing season, RKN limits the development of the taproot and causes galls to develop on secondary roots. The galls, or tumors, compete for nutrients, often resulting in the abortion of the taproot and secondary root.
Just after harvest when soils are not too wet or too dry is the perfect time to sample soils for RKN populations. Since RKN are not uniformly distributed, the entire section or field must be sampled to get a true appraisal. Economic thresholds are 0.5-1 juvenile nematode/cubic centimeter of soil.
What are the signs of common reniform nematode?
Common reniform nematode are found in areas east of New Mexico, commonly in medium-to-fine-textured soils, or those with less than 50 percent sand. Reniform nematodes are usually distributed more uniformly in the field than RKN. Yield loss from reniform can range from 10 to 25 percent, but losses greater than 50 percent can occur in drought-stressed environments.
Under optimal growing conditions, foliage of infected plants may not even express symptoms. Foliar symptoms often mimic potassium deficiency of yellow tissue with brown margins. However, plants uniformly stunted can experience one to two nodes delay in fruiting initiation.
While reniform nematodes lack root galls or fusing of root, egg masses can be observed on gravid female nematodes protruding from the root. Increased occurrences of Verticillium wilt often result, blocking vascular tissue and limiting water and nutrient uptake. Populations of reniform nematode are several times higher than southern root-knot nematode, generally due to more feeding sites from less root damage. Economic thresholds are two (preplant sampling) and 10 (winter sampling) per cubic centimeter of soil.
How can nematodes be managed?
Once the presence, species and economic thresholds of nematodes have been identified, growers have several options for reducing their spread. Any of the following methods can be supplemented by chemical controls, seed treatments or even biochemical for additional levels of protection from nematodes.
- Cultural practices are the first line of defense against the spread of nematodes. For example, deep tillage can facilitate root development and improved access to nutrients. Sanitation, or properly cleaning equipment, also prevents contamination when moving from infested fields to fields without nematode issues.
- Good weed management is also key to managing nematodes. Many weed species support nematodes when a crop is not present, thus controlling them during the fallow season can reduce nematode populations.
- Crop rotation traditionally has been the standard for managing nematodes. Planting nonhost crops in nematode-infested fields has proven to help reduce populations. Peanuts are a nonhost of southern root-knot nematode, but they are affected by the Meloidogyne arenariaspecies of root-knot nematode. Corn is a nonhost of reniform nematode and makes a good rotation partner for cotton. In some areas, grain sorghum also is used as a rotation partner with cotton to reduce nematode populations.
- Native genetic resistance or tolerance is quickly evolving as a go-to solution for growers who plant cotton on nematode-infested acres. However, not all cottonseed varieties offer the same level of protection, so it’s important to select the cotton varieties that best meet your needs and production goals. PhytoGen® brand varieties containing PhytoGen breeding traits to manage RKN-infested fields are a great solution.
PhytoGen brand varieties with RKN resistance reduce nematode galling, feeding and reproduction by disrupting the host/parasite relationship. RKN-resistant varieties reduce the number of nematodes in the soil similar to a nonhost rotational crop, such as grain sorghum. In comparison, nematode-tolerant varieties will allow nematode feeding and reproduction, but merely handle the stress of the host/parasite relationship better than susceptible varieties.
For more specific recommendations in your region, consult your local PhytoGen cotton development specialist or university Extension cotton specialist. Also visit PhytoGen.com to learn more about available PhytoGen brand varieties that are RKN resistant and for the latest varietal progressions.
Additional educational resources
While planting nematode-resistant cotton varieties is the ultimate defense, additional information on identifying and managing southern root-knot nematode and reniform nematode is available at the following websites:
Understanding Cotton Nematodes
Population Distributions and Densities Maps
Recommendations for Sampling Soils
Cotton producers also may order Nematode Fact Sheets and booklets from the National Cotton Council.
