How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Nematodes are microscopic roundworms that live in diverse habitats. Plant parasitic nematodes live in soil and plant tissues and feed on plants by puncturing cell walls and sucking the cell contents with a needlelike mouthpart called a stylet. Nearly 90 species of nematodes have been reported in association with carrots.

Among nematodes that affect carrot, root knot nematodes are the most important and are widely distributed throughout California. Root knot nematodes enter the roots as second stage juveniles where they remain to feed. Second stage juveniles are motile but the other stages are sedentary. After feeding in carrot roots, adult females become swollen and produce an egg mass just outside the root or just beneath the root surface. Adult root knot nematodes and their egg masses are visible at 10X magnification.

Needle nematode is important in the Imperial Valley, whereas stubby root nematode is found statewide. Both of these nematodes are migrating ectoparasites (i.e., they feed on but do not penetrate into the roots) and lay eggs singly in soil. They go through four molts and all juvenile stages probably feed on roots. Both needle nematode and stubby root nematode are smaller than adult root knot nematode, they are found in the soil and require greater magnification to be seen.

DAMAGE

Root knot nematodes can cause substantial damage and are of major concern in California. They can stunt the plants and cause stand and yield reduction. Heavy infestations of stubby root and needle nematodes can produce similar field symptoms.

SYMPTOMS

The symptoms described below are suggestive of a nematode problem but are not diagnostic as they can result from other causes as well. Carrots affected by nematodes often exhibit forking of the tap root, stubbing and fasciculation (bunching) of the roots, and a predisposition to wilting. In addition, root knot nematodes induce characteristic galls on feeder roots; small, hard-to-see galls are generally associated with Meloidogyne hapla, whereas, relatively large, conspicuous galls are usually associated with other Meloidogyne species. The stubby root nematode causes short roots with a stubby appearance. Needle nematode feeding causes cessation of root elongation, forking, root branching, swelling of root tips, and sometimes root tip necrosis.

FIELD EVALUATION

If a previous crop was a host for nematodes that can affect carrots, the nematode population may be high enough to cause damage to a carrot crop that follows. It is critical to know the nematode species present to make management decisions. If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification of nematodes.

The following sampling procedure may be used. Divide the field into sampling blocks of not more than 5 acres each that are representative of cropping history, crop injury, or soil texture. Take the soil samples from within the root zone (6 to 18 inches deep). 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 along with symptomatic plants, 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 as soon as possible to a diagnostic laboratory. Contact your farm advisor for more details about sampling, locating laboratories that process nematode samples, and interpreting sample results.

MANAGEMENT

Cultural Practices. Clean fallowing (i.e. weed free) during years when the land is left unplanted is effective in reducing root knot nematode populations. Crop rotation is usually not very effective for control of root knot nematodes because of the occurrence of several species and their wide host range. However, cotton is a host for only one root knot nematode species, M. incognita race 3, and the Acala cultivar NemX HY is resistant to this species. Thus, it might be useful as a rotation crop for reducing root knot nematode populations.

Most crops grown in the Imperial Valley, except cabbage and cauliflower, are susceptible to needle nematode. To avoid infesting new fields, clean machinery and equipment with water and prevent movement of infested soil.

Soil solarization can reduce many soilborne diseases, nematodes, and weeds. It requires a 4-to 6-week treatment during the hottest time of the year. The efficacy against plant parasitic nematodes is at best limited to about 12 inches in soil depth. For further information, see UC ANR Publication No. 21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds.

Resistant Cultivars. Good resistance to root knot nematode damage (forking and galling) is available in new carrot cultivars, but these are not yet commercially available.

Date of Planting. The root knot nematodes M. incognita, M. javanica, and M. arenaria become inactive at soil temperatures below 64°F. Planting when soil temperatures are below this level will minimize forking of tap roots caused by feeding by these three nematode species. M. hapla, however, is still active below 64°F.

Treatment Decisions. Damage thresholds have not been established. Treatment is warranted whenever root knot, needle, or stubby root nematodes are present. No materials are registered for postplant use on carrots and once tap roots are damaged, control would not be feasible.

PUBLICATION

UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438
Nematodes
B. B. Westerdahl, Nematology, UC Davis
A. T. Ploeg, Nematology, UC Riverside
J. O. Becker, Nematology, UC Riverside

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