Root-knot nematodes (Meloidogyne spp.) are microscopic roundworms that are widely distributed throughout North Carolina on many row and vegetable crops, and may cause varying degrees of damage to cotton. Specific root-knot nematode species that affect cotton include the southern root-knot (M. incognita), Javanese root-knot (M. javanica), and the more recently introduced guava root knot nematode (M. enterolobii). Of these, the southern root-knot (M. incognita) is the most wide-spread root-knot nematode in North Carolina. The guava root-knot nematode is an invasive species that has been reported in Florida, Louisiana, North Carolina, South Carolina, and Georgia. Across the globe, other official reports of the guava root-knot nematode include tropical to sub-tropical areas in Asia (China and Vietnam), South and West Africa (Malawi, Senegal, Ivory Coast and Burkina Faso), Central and South America (Mexico and Brazil), and the Caribbean islands, including Cuba, Puerto Rico, Tobago, Trinidad, Guadeloupe, and Martinique.

Root-knot nematodes (RKN) have a very extensive range of hosts, including several field crops grown in rotation with cotton in North Carolina such as tobacco, soybean, sweetpotato, and corn. Additionally, many vegetable crops, potatoes, and numerous weed species are hosts to root-knot nematodes. However, each specific species of root-knot nematode has an associated list of hosts, some of which overlap with other root-knot nematode species. These hosts are outlined in Table 1 below.

The life cycle of root-knot nematodes begins as an egg, goes through several successive juvenile stages, then reaches an adult stage. Eggs are produced in egg sacks at the root surface by female root-knot nematodes, which feed and survive fully inside the roots. The eggs survive in the soil until conditions are conducive for hatching. Infectious juvenile nematodes hatch from the eggs, move through the soil via water films, and invade nearby host roots. Once inside the host root, the juvenile root-knot nematodes will establish permanent feeding sites that will provide them sufficient nutrients for the rest of their life. The majority of the juveniles will become spherical-shaped adult females; a minor proportion will become adult males, which will leave the root and not feed. Adult female root-knot nematodes stimulate the root tissues at their feeding site to enlarge, producing the galls that are characteristic of infection by this nematode. Each female may lay hundreds of new eggs on the root surface, which then hatch in warm, moist soil to continue the life cycle.

The length of the life cycle depends on several factors, including soil temperature, soil moisture, and host plant suitability. under optimal conditions the life cycle takes approximately 25 days. Root-knot nematodes also require adequate soil moisture for egg hatching and movement through soil to host roots. Soil moisture conditions that are optimum for plant growth are also ideal for the development of root-knot nematodes.

Root-knot nematode infestations can often be identified in the field by the presence of galls, knots, or bumps on the root system of cotton plants. These root galls are localized swelling of root tissues initiated through feeding of the nematode inside the root (Figure 1).

Aboveground symptoms are frequently observed in “disease foci” or patches across the field, where the population density of nematodes is moderate to high. Plants in these patches may be chlorotic (yellow) and/or stunted. Other aboveground symptoms may include poor emergence, wilting, and early plant death. Foliar symptoms may indicate secondary infections caused by fungi or bacteria following the root damage caused by root-knot nematode feeding.

Figure 1. Meloidogyne enterolobii root galling symptoms on cotton.

Adrienne Gorny, NCSU

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Figure 1. Meloidogyne enterolobii root galling symptoms on cotton.

Adrienne Gorny, NCSU

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If a cotton crop is suspected to be impacted by root-knot nematodes, plants should be dug up, taking care to preserve the integrity of the root system, as possible. Roots should be washed gently in a bucket of water or under running water to remove soil. Visually inspect root, looking for the presence of root galls (Fig. 1), which indicate infection of the cotton plant by root-knot nematodes. Although the presence of root galls is sufficient to diagnose root-knot nematodes in cotton, root galling symptoms alone will not indicate the level of nematode pressure (nematode population counts) in the field, nor indicate what specific species of root-knot nematode is present. Diagnosis can be confirmed through assessment of soil and root samples for the presence of root-knot nematodes by a plant disease diagnostic laboratory or nematode assay lab. Soil samples for nematode assay services may be submitted to the North Carolina Department of Agriculture & Consumer Services (NCDA&CS), Agronomic Services, Nematode Assay Laboratory, 4300 Reedy Creek Road, Raleigh, NC 27607.

Nematode populations vary throughout a field, so multiple areas of the field should be sampled to provide an accurate evaluation of the entire field. Using a soil probe or hand trowel, collect 20 to 30 soil cores from a 5-acre block, at a depth of 6-8 inches, along either a grid-like or a zig-zag pattern across the field. Combine the soil cores in a clean plastic bucket and remove a smaller subsample from this larger sample to submit to the Nematode Assay Laboratory. Detailed sampling instructions and sample submission forms can be found at the NCDA&CS Nematode Assay Laboratory sampling guidelines.

The standard nematode soil assay cannot differentiate between specific root-knot nematode species, due to the highly similar appearance of these nematode species under the microscope. Root-knot nematode species identification can be obtained through a molecular assay at a diagnostic lab. This molecular diagnosis must be requested when submitting a sample and usually requires an additional fee. When requesting a molecular assay, it is helpful to include roots with distinct root galls at the time of sample submission as well as an accurate crop history, information about fertility, herbicides, and cultural practices to aid in diagnosis.

Managing root-knot nematodes can be difficult due to their broad host range. Routine soil sampling for nematode analysis is important to better understand if root-knot nematodes are present in the field and if their population densities pose a risk to the cotton crop. Although soil samples for nematode analysis can be collected anytime in which the soil is not frozen and in good condition, sampling is best conducted in the fall, around harvest time, prior to Thanksgiving when nematode numbers are at their highest. It is valuable to know what the nematode pressures are in a given field to select the proper management tools.

Exclusion and Sanitation

Root-knot nematodes can be moved in infested soil via equipment, vehicles, tools, or shoes that have entered the field. Equipment from nematode-infested fields should be sanitized before moving to unaffected fields. Workers should also sanitize shoes after working in nematode-infested fields. Working in fields with known high populations of nematodes last will reduce time and effort needed for sanitation of equipment.

Resistant Varieties

Host resistance is one of the most cost-effective management tools for managing root-knot nematodes. There are cotton varieties available with resistance to southern root-knot nematode (M. incognita), which are listed in the Cotton Production Guide. To keep these RKN-resistant varieties functioning well and minimize the risk of developing populations of root-knot nematode that overcome the resistance, it is advised to rotate use of resistant and non-resistant varieties over several rotational cycles/years of cotton planting. No commercial cotton varieties with resistance to M. enterolobii are available at this time.

Crop Rotation

Crop rotation may decrease the nematode populations when a non-host crop is used as a rotation. Non-host and poor-host crops for root-knot nematodes, such as southern and guava root knot nematodes include sorghum, grasses, grain, alfalfa, peanuts, and resistant varieties of soybean and corn. Cotton is not economically affected by peanut root knot nematode (M. arenaria), and could be a good rotation option with peanuts (Table 1).

¹Genetic host resistance is available in some cotton varieties to this species. Please see “Resistant Varieties” section. ↲

Conventional Chemical Management

Chemical management options help to reduce populations of root-knot nematodes within the field and prevent infection of host plants. The products in Table 2 below have a label for use in cotton, as of 2023. For annually updated chemical management tools, consult the North Carolina Agricultural Chemical Manual.

¹See label for proper PPE, rates, and restrictions associated with each chemical. Chemical treatments listed are not exhaustive, and several seed treatment and in-furrow chemical options are available from different chemical companies. ↲

²See restrictions for soil type and water proximity on label. ↲

³Seed treatments are typically most effective under low to moderate root-knot nematode population counts. ↲

⁴Vydate for root-knot and reniform nematodes only. ↲

• The American Phytopathological Society information page for Root Knot Nematode has more detailed information about root knot nematodes.

  • The North Carolina Agricultural Chemicals Manual provides an up-to-date list of chemicals available for control of nematodes and other diseases and pests.

• The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations.

• The Extension Plant Pathology Portal provides information on crop disease management.

• The Cotton Production Guide has descriptions of diseases and controls, updated annually.