How to Manage Pests
UC Pest Management Guidelines
Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues and several species may occur in a field. The host range varies according to the species, with some being able to infest a wide variety of crops, and others being limited to a narrow crop range. Symptoms of nematode infestation also vary according to the nematode species and crop type and are often non-specific (yellowing, stunting). Root knot nematode species, however, cause typical galling on roots of infested plants. The geographical distribution of the different species is highly dependent on temperature, soil type, and cropping history.
Two genera of plant parasitic nematodes, Heterodera (cyst nematodes) and Meloidogyne (root knot nematodes), are considered important pests of cole crops. Cyst nematodes occur frequently in cole crop-growing regions of California and can severely damage the crop on any soil type; all cole crops are susceptible to cyst nematodes. The sugarbeet cyst nematode is more widespread in California than the cabbage cyst nematode. Heavy infestations can reduce yields and/or delay crop maturity. Root knot nematodes occur in a wide range of soil types, but are most common and cause greatest damage in coarse-textured sandy, loamy sand, and sandy loam soils. Infestations of root knot nematodes are much more damaging in warm interior valleys or in warm-season crops than in winter coast plantings because the warmer temperatures result in greater nematode reproduction and subsequent greater damage. Heavy infestations of root knot nematodes can cause significant yield reductions.
The symptoms described below are indicative of a nematode problem, but are not diagnostic because they could result from other causes such as diseases, insect injury, or nutrient deficiencies. Infestations may occur without causing any aboveground symptoms.
Symptoms of cyst nematode infestation include patches of stunted or dying plants, yellowing of foliage, and reduction in head and curd size. White, pinhead size, lemon-shaped females and brown cysts can be seen on the root surface upon careful observation. Cyst nematodes do not induce gall formation on the roots.
Root knot nematodes spend most of their life cycle in galls on roots. Galls formed by M. hapla are spherical and distinct, whereas the other root knot nematode species may induce galls that are irregular in shape and often coalesce. Plants infested as seedlings may be stunted, with patches of stunted plants seen by midseason. Nematode-infested plants wilt earlier under temperature and moisture stress.
To make management decisions, it is important to know which nematodes are present and to estimate their population. If a previous crop had problems caused by nematodes that are also listed as pests of cole crops, population levels may be high enough to cause damage to subsequent crops.
If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification. Divide the field into sampling blocks of not more than five acres each that are representative of cropping history, crop injury, or soil texture. 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. 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. If plants with symptoms are available, place the roots in the same bag with soil. Keep samples cool (do not freeze), and transport 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.
(2) Do not allow irrigation water to flow from an infested field to other fields without impounding.
(3) Prevent animal grazing and movement from infested to uninfested fields.
Cultural practices. Plow under infested plants after harvest to prevent further reproduction of nematodes. Reduce stress on plants by proper fertilization and irrigation.
Crop rotation. Cyst nematodes have a relatively narrow host range and can be managed by rotation with nonhost crops. Crucifer family plants are the only hosts for cabbage cyst nematodes. Sugarbeet cyst nematodes are hosted by crucifers, beets, spinach and related weeds. In southern California several years between host crops may be necessary. The higher the nematode population at harvest, the longer the period of rotation required. (See UC ANR Special Publication 3272, Sugarbeet Pest Management: Nematodes, for more details.) Longer rotations may be needed in northern California. In the cooler climates of northwestern Europe and some states in the US (Idaho, Wisconsin), growing specific cruciferous cover crops is being used to manage sugarbeet cyst nematodes. These crops act as a trap crop that lure the nematodes into their roots but do not allow their multiplication. The usefulness of this strategy in the warmer climate of California where sugarbeet nematodes can go through multiple cycles during one growing season remains to be determined.
Crop rotation is not very effective against root knot nematodes because of their wide host range. Strawberries 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. They do allow reproduction of M. hapla, however.
When to treat. Damage threshold estimates have not been developed for nematodes on cole crops. Consider treating whenever the nematodes listed as pests are present in the field. Contact your farm advisor for advice on a specific situation because efficacy of these materials varies depending on method of application and soil conditions at the time of application.
UC IPM Pest Management Guidelines: Cole Crops