Lesion nematode damage in white or Irish potato is caused by plant-parasitic roundworms belonging to the genus Pratylenchus. More commonly known as root-lesion or lesion nematodes, this group of nematodes is found in all potato-producing regions of North America and Canada. The most common species found in potato fields in North Carolina is P. penetrans (Figure 1). Known for its broad host-range of over 400 crop plant species worldwide, P. penetrans may also infect alfalfa, bermuda grass, boxwood, corn, cotton, peach, red clover cover crops, rye, soybeans, sweetpotato, tobacco, and tomato in North Carolina. Pratylenchus penetrans primarily feeds within the plant roots, causing damage to both potato roots and tubers. This feeding action may predispose the roots to further infection by secondary microbes such as Vericillium dahliae, a soilborne fungus known for causing Potato Early Dying syndrome.

Figure 1. Adult female lesion nematode, Pratylenchus penetrans

A. Gorny, NC State University

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Figure 1. Adult female lesion nematode, Pratylenchus penetrans

A. Gorny, NC State University

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The foliar symptoms of lesion nematode infection include yellowing (chlorosis) of the leaves and stunting of the plant. This makes identifying lesion nematodes as the source of the problem difficult, since the aboveground symptoms do not differ from general symptoms that are caused by a several different soilborne pathogens or abiotic stresses. Plants must be dug up and the roots inspected to observe damage caused by lesion nematodes. On the feeder roots of white potato, characteristic signs of a lesion nematode infestation are dark reddish-brown and black lesions along the roots which often appear in discrete spots. As the season progresses and nematode feeding continues, lesions can coalesce to become necrotic areas of tissue that may eventually girdle the roots. Using a hand lens or magnifying glass can help you look for black lesions or root damage. On the tubers, nematode feeding may result in larger lesions that are reddish-brown to black in color. Lesions may resemble the symptoms of potato common scab caused by Streptomyces scabies, including necrotic, scab-like patches of tissue on the tuber (Figure 2).

Figure 2. Damage caused by the lesion nematode Pratylenchus penetrans to a potato tuber

A. Gorny, NC State University

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Figure 2. Damage caused by the lesion nematode Pratylenchus penetrans to a potato tuber

A. Gorny, NC State University

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If a potato crop is suspected to be suffering from lesion nematode damage caused by P. penetrans, plants may be dug from the soil and inspected for characteristic feeding damage. A visual diagnosis includes checking the plant roots and tubers for black necrotic lesions. Diagnosis can be confirmed through assessment of soil or root samples for the presence of the nematode by a diagnostic laboratory or nematode assay laboratory. Samples for nematode assay services may be submitted to the North Carolina Department of Agriculture & Consumer Services (NCDA&CS), Agronomic Services Division, Nematode Assay Laboratory, 4300 Reedy Creek Road, Raleigh, NC 27607.

In order to determine nematode pressure, soil samples should be taken from several areas in the affected field. Using a soil probe, collect 20 to 30 soil cores across a 5 acre block, at a depth of 6 to 8 inches from each area using either a grid-like or a “zig-zag” pattern across the area. Combine soil cores in a clean plastic bucket and remove a smaller portion of soil from this larger sample to submit to the Nematode Assay Laboratory. Detailed sampling instructions can be found at the NCDA&CS Nematode Assay Laboratory sampling guidelines.

Lesion nematodes, including P. penetrans, are obligate biotrophs, meaning they require living plant roots to feed and reproduce. They are classified as migratory endoparasites, meaning they mainly move and feed inside plant roots, yet can also move through the soil as well. Their life cycle consists of six life stages: egg, four juvenile stages (often referred to as J1-J4) and the adult stage. The full life cycle of P. penetrans typically takes 4-8 weeks to complete, and may vary depending on soil moisture, soil temperature, and host crop suitability. Female lesion nematodes lay eggs one at a time in plant roots or soil, which develop to first-stage juveniles, then to second-stage juveniles before hatching from the egg. The second-stage juvenile nematodes can penetrate plant roots to feed and continue developing through a third- and fourth-stage, and finally into adults. Adult lesion nematodes may exit and re-enter the roots. Under favorable environmental conditions (presence of a suitable host crop, sufficient soil moisture, and favorable temperatures) lesion nematode populations can increase to over 1,000 nematodes per gram of root. Lesion nematodes can overwinter in infected root tissue, weed hosts, or in the soil at any life stage.

Root-lesion nematodes can interact with and exacerbate secondary pathogenic microbes such as the fungus Verticillium dahliae, known for causing Potato Early Dying syndrome. Lesion nematodes feed by lysing (breaking open) cells which forms cavities allowing them to migrate intracellularly (that is, directly through the cells that compose the root) to continue feeding on healthy tissue. Necrotic lesions can be observed on the root as a result of the damage caused by feeding and movement. The lesions and other feeding wounds caused by P. penetrans provide an opening through which V. dahliae can enter the already compromised root tissue. Symptoms of Potato Early Dying syndrome, also known as Verticillium wilt, include wilting of the plant, chlorosis and early senescence due to infection. This disease complex has become of increasing economic significance and can lead to premature vine death and severe yield losses of up to 50% in potato production. Symptom severity of this disease complex is often more severe when both P. penetrans and V. dahliae are present in teh field, compared to the symptoms when either pathogen is present alone.

Managing P. penetrans can be difficult due to its broad host range and lack of potato cultivars with genetic resistance to this pathogen. Cover crops such as ryegrass or canola can be used to inhibit population buildups of P. penetrans. Many species of lesion nematode can also infect, reproduce, and survive on common weeds. Therefore, good weed management will eliminate a potential food source for these nematodes. The two most effective management tactics for lesion nematode are sanitation and the application of nematicides.

Avoiding the introduction of P. penetrans into the field through sanitation procedures is highly beneficial, as they can be extremely difficult to completely eradicate once populations are established in the field. Lesion nematodes spread between fields readily via contaminated soil, seed pieces, equipment, and wind-blown soil. Careful sanitation by cleaning of equipment moved between fields is an effective way of reducing the spread of lesion nematodes between fields and farms. Sanitize by washing with a solution of 10% household bleach, removing any stuck soil, then rinsing with clean water and drying before moving to unaffected fields or areas. Purchase and planting of high-quality, nematode and disease-fee seed will also reduce risk of introduction of lesion nematode into the field. Routine soil sampling for nematode analysis is important to understand if a field has lesion nematode and monitor population counts.

Another effective management tactic to reduce lesion nematode populations within the field is the use of nematicides. Often applied as a pre-plant soil fumigation, nematicides can reduce lesion nematode population levels to below economic thresholds. Fumigants such as 1,3-dichloropropene and metam sodium, as well as non-fumigant nematicides such as fluopyram are available for use.

The NCDA&CS Nematode Assay Lab provides soil detection and diagnostics.

The NC State University 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.

This factsheet was prepared by the NC State University Plant Nematology Lab in 2021.