North Carolina ranks 10^th in the nation for tomato production, with about 3,300 acres of tomato harvested per year and about $52 million in annual sales (Hopper, 2018). Tomatoes are grown in the field, greenhouse settings, and home gardens statewide. Tomato plants can be susceptible to root-knot nematodes, which are microscopic, roundworms that inhabit the soil. Root-knot nematodes are parasitic nematodes, meaning they infect and feed on plants.

Besides root-knot nematodes, there are numerous other types of non-parasitic nematodes that inhabit the soil. These non-parasitic types do not infect plants and are an important part of the soil food web. Only a small proportion of all nematodes (like root-knot nematode) are plant parasites.

Because root-knot nematodes live and infect host plants below the soil line, they can often be difficult to detect and manage. Regardless of whether tomatoes are grown in large scale agricultural systems or in a home garden, root-knot nematodes can debilitate tomato production unless effectively managed.

Root-knot nematodes belong to the genus Meloidogyne. Several species of root-knot nematode infect tomato, including Meloidogyne incognita (Southern root-knot nematode), M. arenaria (peanut root-knot nematode), M. javanica (Javanese root-knot nematode), M. hapla (Northern root-knot nematode), and the recently emerging M. enterolobii (guava root-knot nematode). The most wide-spread root-knot nematode in North Carolina is M. incognita.

The life cycle of root-knot nematodes takes about 25 days to complete, but this length of time may be affected by soil temperature, moisture, or the presence of host plants. Second stage juveniles (Figure 1), which initiate infection in a host, hatch from eggs and travel to plant roots via films of water. Once a host plant root has been located, a J2 enters the plant root and migrates to the vascular system where it begins to feed. The J2s rapidly molt, during which feeding is paused. In most cases, the juveniles will develop into adult females (Figure 2), which produce egg masses that can contain numerous eggs ranging from around 300 to 600 eggs per female (Figure 3), depending on the species. However, when conditions such as poor plant nutrition, nematode over-crowding, or competition occur, J2s may develop into males that exit the plant root. Among the concerns of the root-knot nematode life cycle is their quick generation turn-over rate and ability to overwinter for several years, making management difficult.

Figure 1. Second stage juvenile root-knot nematodes (microscope view)

W. Ye, N.C. Department of Agriculture & Consumer Services

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Figure 1. Second stage juvenile root-knot nematodes (microscope view)

W. Ye, N.C. Department of Agriculture & Consumer Services

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Figure 2. Female root-knot nematode (microscope view).

W. Ye, N.C. Department of Agriculture & Consumer Services

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Figure 2. Female root-knot nematode (microscope view).

W. Ye, N.C. Department of Agriculture & Consumer Services

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Figure 3. Female root-knot nematodes with eggs in galled-roots (microscope view).

W. Ye, N.C. Department of Agriculture & Consumer Services

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Figure 3. Female root-knot nematodes with eggs in galled-roots (microscope view).

W. Ye, N.C. Department of Agriculture & Consumer Services

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Root-knot nematodes have a broad host range, meaning they can infect many other crops in addition to tomato. However, not all root-knot nematode species are able to infect the same crops, although the host ranges of several root-knot species overlap. Among the many host plants are tomato, pepper, sweetpotato, corn, soybean, tobacco, cotton, and numerous species of weeds. Identification of root-knot species may help dictate a suitable crop rotation plan; however, the large number of host plants may limit options for planting in certain situations.

Resistance to specific species of root-knot nematode is available in some hosts, including tomato. Certain tomato cultivars possess the Mi-1 gene, which renders the plant resistant to M. arenaria, M. incognita, and M. javanica. However, the newly emerging root-knot nematode, M. enterolobii, is capable of infecting tomatoes cultivars with the Mi-1 resistance gene.

Signs and symptoms of root-knot nematode infection occur below ground in the roots. Roots will display characteristic galls (Figure 4A, Figure 4B), or swellings, where female nematodes and their egg masses may be found. Using a magnifying glass can help you look for galls on the roots produced by root-knot nematodes.

Different species of root-knot nematodes produce similar symptoms; therefore, it is not possible to differentiate species based on symptoms alone. However, galls may be larger in more aggressive species such as M. enterolobii and smaller for species such as M. hapla. Further, it is possible for multiple species to inhabit the same field.

Nematodes are rarely evenly distributed in the field and instead are often found in clusters. Therefore, symptoms within a field are usually found in ‘hot spots’ or patches, where nematodes are more concentrated. Above-ground symptoms, when present, are often associated with high nematode populations, and may include stunting, chlorosis (yellowing), and wilting of plants (Figure 5). In cases of moderate to severe infestation, plants are more susceptible to drought, and their ability to uptake water and nutrients decreases. A decline in yield is often observed with root-knot nematode infection in more susceptible tomato varieties, especially under high nematode population levels. Above-ground or foliar symptoms may not always be present when nematode populations are low.

In addition, root-knot nematode root infection can increase susceptibility of the host plant to other pathogens, such as Fusarium crown and root rot, when these pathogens are also present in the field.

Figure 4A. Root-knot nematode (M. enterolobii) symptoms on ‘Rutgers’ tomato roots. Other species of root-knot nematode produce similar galling symptoms

T. Schwarz, NC State University

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Figure 4A. Root-knot nematode (M. enterolobii) symptoms on ‘Rutgers’ tomato roots. Other species of root-knot nematode produce similar galling symptoms

T. Schwarz, NC State University

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Figure 4B. Root-knot nematode (M. enterolobii) symptoms on ‘Rutgers’ tomato roots. Other species of root-knot nematode produce similar galling symptoms

T. Schwarz, NC State University

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Figure 4B. Root-knot nematode (M. enterolobii) symptoms on ‘Rutgers’ tomato roots. Other species of root-knot nematode produce similar galling symptoms

T. Schwarz, NC State University

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Figure 5. Above-ground symptoms of root-knot nematodes. The row on the left are tomatoes infected with root-knot nematode. The row on the right are nematode-free plants

J. Desaeger, University of Florida - IFAS

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Figure 5. Above-ground symptoms of root-knot nematodes. The row on the left are tomatoes infected with root-knot nematode. The row on the right are nematode-free plants

J. Desaeger, University of Florida - IFAS

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Following visual assessment of galling symptoms, confirmation of root-knot nematode infection can be accomplished through assessment of a soil or plant tissue sample by a diagnostic or nematode assay laboratory. Diagnosis of nematode issues in symptomatic plants may be performed by diagnostic labs such as the North Carolina State University Plant Disease and Insect Clinic, while quantification of nematode populations can be performed by labs such as the N.C. Department of Agriculture & Consumer Services (NCDA&CS) Nematode Assay Laboratory or private nematode assay labs.

In some cases, identification of the nematode to the species level may assist in effectively managing the pathogen. Although many species of root-knot nematode infect tomato, these species do not always infect rotational crops in the same manner. For example, the Northern root-knot nematode, M. hapla, will infect tomato cultivars not possessing the Mi-1 gene, but this nematode species cannot infect corn and only poorly infects cotton. Identification of the nematode to species requires examination of the morphology by a trained technician or by molecular assays, such as DNA tests, which are not always available depending on the species of concern. Inquiring with the diagnostic laboratory will help determine if species identification can be performed.

Soil samples for identification and quantification of nematode populations should be taken following the NCDA&CS Nematode Assay Laboratory sampling guidelines, which are briefly summarized here.

To quantify nematode populations in the field, collect 20 to 30 soil samples from a field of 5 acres or less using a soil probe. For areas larger than 5 acres, collect additional soil cores, or divide the field into smaller sampling areas. Collecting samples in a grid like or zig-zag pattern (Figure 6) across the field can be beneficial, since nematodes tend to be located in clusters in the field. Collect soil cores at depths of 4 to 8 inches. Combine the individual soil cores in a clean plastic bucket, mix gently, then remove a smaller portion of soil from this larger sample to submit to the Nematode Assay Laboratory.

Figure 6. Example of zig-zag pattern to follow when sampling for root-knot nematodes.

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Figure 6. Example of zig-zag pattern to follow when sampling for root-knot nematodes.

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Effective nematode management begins with field selection, sampling for nematode populations, and starting with nematode-free planting material. It is difficult to completely eradicate nematodes from a site once they are established; therefore, it is crucial to follow recommended guidelines and sanitation practices to ensure the site remains free of root-knot nematodes and highly productive. If nematode populations establish within the site, the following disease management tools can be used to bring root-knot nematode populations to below damaging levels. Producers are highly encouraged to implement more than one tool for durable and robust management.

Exclusion and Sanitation: Due to their microscopic size, root-knot nematodes naturally have limited movement in the soil; therefore, it is important to limit the spread of nematodes to new areas by exclusion and sanitation. Nematodes can easily be moved in contaminated soil on equipment, shoes, garden tools, or vehicles. To reduce the risk of moving nematodes through contaminated soil, equipment and tools should be sanitized by washing with a solution of 10% household bleach, rinsing with clean water, and air drying before moving to unaffected fields or areas. The use of nematode free transplants and planting media is important to exclude nematodes from the greenhouse, garden, and field.

Crop Rotation: Crop rotation to a non-host crop can be a useful tool to help manage root-knot nematodes. By alternating to a crop that the nematodes can neither feed on nor complete their life cycle on, this tool can promote a decline in root-knot nematode populations in the soil. However, effective use of non-host rotational crops for management of root-knot nematodes is dependent upon the species (and sometimes race) of root-knot nematode present in the field, or if the crop possesses any resistance genes. For example, M. hapla can reproduce on tomato cultivars without the Mi-1 gene, but cannot reproduce on corn and only poorly on cotton. Yet M. incognita race 3 reproduces very well on tomato, corn, and cotton. When considering crop rotation, it is useful to also consider whether any cover crops that are used may be a host to root-knot nematodes. Your local county extension agent or crop consultant can assist with questions about using crop rotation on your farm for suppressing root-knot nematode populations.

Host Resistance: There are numerous tomato cultivars with resistance to several of the root-knot nematodes present in North Carolina. A brief list of resistant cultivars is presented in Table 1, and a more complete list of resistant cultivars may be found in the Southeastern US Vegetable Crop Handbook.

Chemical Control: Chemical control options including fumigants and non-fumigant nematicides are available for use in tomato production to suppress root-knot nematodes and minimize yield losses (Table 2). Prior to selection and application of a chemical option, be sure to review all product registrations and labeling. Keep in mind that an additional pesticide license is required to apply soil fumigants. Visit the Southeastern US Vegetable Crops Handbook for more information on fumigants, non-fumigants, and bio-fumigants.

Biocontrol: Several biocontrol agents and products are available to help suppress root-knot nematodes in tomato (Table 2). Biocontrol products are non-chemical alternatives that have shown to reduce root galling, egg masses, and overall nematode populations. However, their use can be limited due to their sensitivity to environmental conditions. Further, the suppressive effects of biocontrol agents may take longer to manifest when compared to those seen with traditional chemical control options.

Grafting: Grafting of a desirable scion onto a root-knot nematode resistant rootstock may be a helpful way to avoid crop damage and yield loss due to root-knot nematodes. Several root-knot nematode resistant root-stock varieties are listed in Table 3, and additional sources of resistant rootstock varieties for grafting can be found at the USDA Vegetable Grafting Portal.

Hopper, Brandon. 2018. NC Tomato Statistics. North Carolina State Extension.

Kemble, J.M., I.M. Meadows, K.M. Jennings, and J.F. Walgenbach. 2020. Southeastern U.S. 2020 Vegetable Crop Handbook. Southeastern Vegetable Extension Workers.

• The NCDA&CS Nematode Assay Lab provides soil detection and diagnostics.
• The NCSU Plant Disease and Insect Clinic provides diagnostic and control recommendations.
• The NC State Extension Plant Pathology portal provides information on crop disease management.
• The Southeastern US Vegetable Crop Handbook provides information on vegetable disease management.
• The North Carolina Agricultural Chemicals Manual provides an up-to-date list of chemicals available for control of nematodes and other diseases and pests.
• APS resource for Root-Knot Nematode has detailed information about root-knot nematode.

This factsheet was prepared by the NCSU Plant Nematology Lab.