How to Manage Pests
UC Pest Management Guidelines
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 (except M. hapla males), but the other stages are sedentary. During the development in carrot roots, later-stage juveniles and adult females increase in size. The enlarged posterior end of adult females breaks through the root surface and produces 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 need to be extracted from the soil to be seen under high magnification.
Root knot nematodes can cause substantial damage and are of major concern in California. Their parasitic activity can damage the growing root tip and result in a forked, distorted, or stunted taproot. In addition to the reduced marketability, deformed roots tend to pick up excess soil, increasing the tare transported to the shed. Removal of excess soil increases cost. Furthermore, root knot nematodes may cause stand and yield reduction. Heavy infestations of stubby root and needle nematodes can produce similar field symptoms.
The symptoms described below are suggestive of a nematode problem, but are not diagnostic as they can result from other diseases or abiotic causes. Injury of the growing root tip by nematodes often causes forking of the taproot, stubbing, fasciculation (bunching) of the roots, and a predisposition to wilting. Typically this occurs within the first few weeks after seed germination. 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.
If a previous crop was a host for nematodes that can affect carrots, the nematode population density 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 plant-parasitic 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 and include symptomatic plants if available. Seal the bags and place a label on the outside with your name, address, location, the current or 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.
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. However, such rotation can increase the occurrence of the cotton (melon) aphid (Aphis gossypi), which is known to transmit viruses.
Most crops grown in the Imperial Valley, except cabbage and cauliflower, are susceptible to the needle nematode. In general, to avoid infesting new fields with plant-parasitic nematodes, clean machinery and equipment thoroughly with water, and prevent movement of infested soil.
Soil solarization can temporarily reduce many soilborne diseases (including those caused by plant-parasitic 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 Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds (UC ANR Publication No. 21377).
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 do not penetrate roots at soil temperatures below 59–64°F. Therefore, planting when soil temperatures are below this level will minimize forking of taproots. The temperature threshold for root invasion by M. hapla, however, is at about 55–57°F.
Damage thresholds depend primarily on nematode species, soil type, soil temperature, and cultivar susceptibility. If the environmental conditions are conducive to nematode parasitic activity, treatment is warranted whenever root knot, needle, or stubby root nematodes are present. No material is registered for postplant use against plant-parasitic nematodes on carrots. Once taproots are damaged, control measures are futile.
UC IPM Pest Management Guidelines: Carrot
B. B. Westerdahl, Nematology, UC Davis