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How to Manage Pests

UC Pest Management Guidelines

Root-knot nematodes cause various types of swellings, bumps and warts on tuber surfaces (right) and blemishes inside (center).



Scientific names:
Northern root knot nematode: Meloidogyne hapla
Columbia root knot nematode: M. chitwoodi (Race 1 & 2)
Southern root knot nematode: M. incognita
Lesion nematode: Pratylenchus penetrans and P. neglectus
Stubby root nematode: Paratrichodorus sp.

(Reviewed 8/07, updated 4/12)

In this Guideline:


Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues, and more than one species may occur in a field. They have a wide host range, and vary in their environmental requirements and in the symptoms they cause.


Root knot nematode larvae invade roots or tubers, establish feeding sites, and develop into the adult stage. Adult females are swollen, sedentary, and lay eggs in a gelatinous matrix on or just below the root surface. These eggs hatch and larvae invade other roots and tubers. Root knot nematode feeding reduces the vigor of plants and causes blemishes on tubers. Lesion nematodes damage roots by feeding and moving through cortical tissues. In addition, Pratylenchus penetrans increases the susceptibility of potato plants to Verticillium wilt and blemishes tubers. Pratylenchus neglectus is common in potato fields but has not been shown to damage potatoes in California. Stubby root nematodes feed on root surfaces, resulting in formation of numerous stubby roots. The major problem caused by this nematode species is transmission of tobacco rattle virus, which causes corky ringspot disease on developing tubers.


The symptoms described below are indicative of a nematode problem, but are not diagnostic because they could result from other causes as well. In general, aboveground symptoms include stunted, yellowed, chlorotic, and/or dead plants. Infected plants are likely to wilt earlier under temperature or moisture stress. Infestations may occur without causing any aboveground symptoms.

Feeding by root knot nematode causes characteristic swellings, called galls, on roots. Galls caused by Meloidogyne chitwoodi are small and difficult to see. On heavily infested plants, egg masses appear as tiny round bumps on feeder roots. Meloidogyne hapla causes small distinct galls with proliferation of lateral roots around these galls. Meloidogyne incognita causes more pronounced galls. All three species of Meloidogyne and Pratylenchus penetrans cause bumps or warts on the surface of infected tubers. However, those caused by M. hapla are less distinct. Brown spots develop inside tubers, mostly in the outer 0.25 inch (6 mm), which are visible when a thin layer of tuber is peeled off. Lesion nematodes cause reddish brown lesions on the roots that turn black later. Stubby root nematodes cause numerous short and stunted (stubby) roots, and corky ringspot symptoms on tubers.


To make management decisions, it is important to know the nematode species present and their population estimates. If a previous crop had problems caused by nematodes that are also listed as pests of potato, population levels may be high enough to cause damage to subsequent potato crops. If nematode species have not previously been identified, soil samples should be taken and sent to a diagnostic laboratory for identification.

Take soil samples in fall from within the root zone of the previous crop after harvest or, preferably, just before harvest. Divide the field into sampling blocks of not more than five acres. Each block should be representative of the field's cropping history, crop injury, or soil texture. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about 1 quart (1 liter) for each block. (See UC ANR Publication 3316, Integrated Pest Management for Potatoes, for more details.) Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current/previous crop and the crop you intend to grow. Keep samples cool (do not freeze), and transport them as soon as possible to a diagnostic laboratory. Contact your farm advisor to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.


If any Meloidogyne chitwoodi, Columbia root knot nematode, are found in samples taken in Modoc and Siskiyou counties, damage can be expected. In fields where Columbia root knot nematode is a problem, long-term integrated management tactics such as crop rotation, cultural controls, fumigation, and nematicides are necessary to prevent a substantial devaluation of the crop.

Acceptable levels of control are not reached using 1,3-dichloropropene or metam sodium when Columbia root-knot exceeds 500 larvae per 1000cc of soil in silty clay loam soil with high organic matter.

Alternative management options for Columbia root knot nematode (high organic matter soils):

  • A combination of fumigation using 1, 3-dichloropropene and application of oxamyl can be effective. However, using this combination in high organic matter soils has not been tested in California.
  • Metam sodium broadcasted and incorporated using a rotary tiller, or application using multiple depth injection shanks may provide control based on one year of research.
  • Multiple depth injection of Telone II may also provide increased efficacy.

For other species there are no precise guidelines for deciding what numbers will cause significant damage. Fields infested with stubby root nematodes and tobacco rattle have been found in Monterey and Kern counties and in the Santa Maria area of Santa Barbara County. Fields with known virus infestations should not be planted to potatoes.

Prevention. The following measures will help prevent spread of nematodes to uninfested fields: (1) using certified planting material; (2) cleaning soil from equipment before moving between fields; (3) keeping irrigation water in a holding pond so that any nematodes present can settle out and pumping water from near the surface of the pond; (4) preventing/ reducing animal movement from infested to uninfested fields; and (5) composting manure to kill any nematodes that might be present before applying it to fields.

Crop Rotation. Crop rotation can be useful in reducing nematode populations. Alfalfa is not a host of M. chitwoodi, Race 1; cereals are nonhosts of M. hapla; and there are several nematode resistant tomato varieties that can be used if M. incognita is a problem. Research in the Pacific Northwest has shown that cover crops of rapeseed, mustard, oilseed radish, or sudangrass reduce populations of root knot nematodes when incorporated as green manure. At present there are no nematode resistant potato varieties available.

Cultural. Fields that are left fallow but kept weed-free usually have an 80 to 90% per year reduction in root knot populations. Infested tubers left in the field after harvest can be a source of inoculum. Destroy potato plants that subsequently emerge from these tubers to restrict nematode reproduction. Avoid storage of tubers infected by M. chitwoodi as blemishes can increase during storage.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider information relating to the impact on environmental quality Not all registered pesticides are listed. Always read label of product being used.
  (Vapam) 37.5–75 gal 48 0
  (Metam Sodium) 50–100 gal 48 0
  (Sectagon) 30–75 gal 48 0
  COMMENTS: Contact your farm advisor for advice on the most effective application method for a particular situation. Fumigants such as metam sodium and 1, 3-dichloropropene are a prime source of volatile organic compounds (VOCs), which are a major air quality issue. Fumigate only as a last resort when other management strategies have not been successful or are not available.
  (Mocap) Maximum label rate 72 0
  COMMENTS: Contact your farm advisor for information on maximizing the effectiveness of this material.
  (Telone II) Label rates 7 days 0
  COMMENTS: Helps to control lesion nematode, which is associated with Verticillium wilt and stubby root nematode, which is associated with corky ringspot. Fumigants such as metam sodium and 1, 3-dichloropropene are a prime source of volatile organic compounds (VOCs), which are a major air quality issue. Fumigate only as a last resort when other management strategies have not been successful or are not available.
Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
B. B. Westerdahl, Nematology, UC Davis
Acknowledgment for contributions to the nematode section:
U. C. Kodira, Plant Pathology, UC Davis

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