Nematodes

Plant-parasitic nematodes are microscopic roundworms that live in soil and plant tissues, and feed in or on roots. Several species may occur in the same field. Certain nematode species infest a wide variety of crops and other plants, while other nematodes are only able to feed on a few closely related plants.

Certain cyst nematodes and root-knot nematodes are pests of cole crops. The abundance and geographical distribution of each nematode species varies depending on cropping history, soil temperature, and soil type.

Cyst nematodes occur in all cole crop-growing regions of California. They can severely damage any cole crop in any type of soil. Sugarbeet cyst nematode is more widespread in California than cabbage cyst nematode. High numbers of either species, particularly at seeding or transplanting, can stunt plants, reduce yields, and delay crop maturity.

When abundant, root-knot nematodes can cause significant yield reductions. They are most prevalent and damaging in moist, coarse-textured soils (sandy, loamy sand, and sandy loam), warm interior valleys, and warm-season crops. However, cyst nematodes cause the most damage to cole crops in California.

Aboveground symptoms caused by nematodes are not diagnostic, because certain nutrient deficiencies, plant pathogens, and root-feeding insects cause similar symptoms. Symptoms of nematode infestation include yellowing of foliage and slowed or stunted growth. Infestations may also occur without causing any aboveground symptoms.

Cyst nematodes cause:

• patches of stunted or dying plants,
• yellowing of foliage, and
• reduction in head and curd size.

Cyst nematodes do not form root galls. With careful observation, the pinhead-size, lemon-shaped females are visible on root surfaces. Young females are white, then become brown and eventually turn into black cysts (egg-filled bodies).

Root-knot nematodes cause:

• Gnarled roots.
• Galls on roots:
  • M. hapla forms numerous spherical, small galls.
  • Other Meloidogyne species form galls that are irregular in shape and often coalesce.
• Slow or stunted growth beginning with the seedling stage.
• Patches of stunted plants apparent by midseason.
• Wilting occurs earlier in the day than noninfected plants during periods of moisture stress (e.g., warm and windy conditions).

To make management decisions, it is critical to know which nematode species are present and how numerous they are in soil samples.

For root-knot nematodes, diagnostic laboratories usually report the number of second-stage juveniles (J2) per 100 cubic centimeters of soil, since only this stage can be identified in soil samples.

• Later stages (J3, J4, and adult females) are always embedded in roots.
• Eggs and first-stage juveniles (J1) may be present in soil, but their extraction is difficult.

For cyst nematodes, extraction techniques are targeted towards extracting the cysts themselves (which can be filled with eggs) from the soil. The extracted cysts may be squashed to release their eggs. Laboratories therefore report cyst nematode infestations as the number of cysts per 100 cubic centimeters of soil, or the number of cyst nematode eggs per gram of soil.

If a previous crop was damaged by nematodes, and soil testing reveals the presence of nematode species that are pests of cole crops, their numbers may be high enough to damage cole crops. Once nematodes have infested a field, continuous management is necessary.

If nematode presence or the species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification.

1. Divide the field into sampling blocks of not more than five acres. Each block should be as uniform as possible regarding cropping history, crop injury, or soil texture.
2. Take samples after harvest, or preferably just before harvest, within the root zone of the previous crop. Take several subsamples randomly from a block, mix them thoroughly, and make a composite sample of about 1 quart (1 liter) for each block.
3. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current or previous crop and the crop you intend to grow. Any other relevant information (such as the type or date of last nematicide application, the type of crop injury observed, crop cultivar, soil type, etc.) should be included in your records, but is not needed by diagnostic laboratories.
4. If plants with symptoms are available, place the roots in the same bag with soil.
5. Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory.
6. Contact your cooperative extension advisor to help you use the best sampling procedures for your situation, find a laboratory for extracting and identifying nematodes, and for help in interpreting the test results.

Sanitation

Prevent nematodes from moving into noninfested fields by doing the following:

• Thoroughly clean all equipment with water to prevent the spread of the nematodes in plant residue and soil.
• Do not allow irrigation or rainwater to flow from an infested field to other fields; impound or properly divert runoff.
• Prevent animal grazing and movement from infested fields into noninfested fields.

Cultural Practices

To increase crop tolerance to nematode feeding, reduce plant stress with proper fertilization and irrigation (see the production guides in the More Information section for more details). Plow under infested plants after harvest to prevent further reproduction of nematodes. Control weed hosts of these nematodes, including pigweeds (hosts of sugarbeet nematode) and weedy mustards.

Crop Rotation

Cyst nematodes have a relatively narrow host range and can be managed by rotation with nonhost crops. Crucifers are the only hosts for cabbage cyst nematodes. Sugarbeet cyst nematodes are hosted by crucifers, beets, spinach, and weeds in those plant families.

The higher the nematode numbers at harvest, the longer the period of rotation required to reduce nematode numbers before planting a susceptible crop. In Southern California, several years between host crops may be necessary. Longer rotations may be necessary in Northern California. See Sugarbeet Pest Management: Nematodes for more details.

Crop rotation is not very effective against root-knot nematodes because of their wide host range. Strawberry may be a suitable rotation crop in fields with root-knot nematodes because they are nonhosts to M. incognita and most populations of M. javanica. The northern root-knot nematode, M. hapla, does reproduce on strawberry.

Nematicide Application

Economic thresholds have not been established for nematodes in cole crops. Decide whether to apply a nematicide by doing the following:

1. Examine roots for presence of cyst nematodes or root galls before harvest.
2. Consider applying a nematicide whenever the nematodes listed as pests of cole crops are present in the field.

Contact your local UC Cooperative Extension advisor for advice on a specific situation. Nematicide efficacy varies depending on the method of application and soil conditions at the time of application.