Nematodes

Nematodes are microscopic roundworms that live in diverse habitats. Plant-parasitic nematodes occur in soil and plant tissues. They feed on plants by puncturing cell walls and sucking the cell contents with a needlelike mouthpart called a stylet.

Among nematodes that affect carrots, root-knot nematodes (RKN) are the most important and are widely distributed throughout California. Second-stage juveniles (J2) hatch from eggs, move through soil, and infect plants. They are the only infective stage and seek out and penetrate host plants near their root tips. During their development in carrot roots, third- and fourth-stage juveniles and adult females remain sedentary while inducing root galls and increasing in size. The enlarged posterior end of adult females typically breaks through the root surface and produces an egg mass just outside or beneath the root surface. Several hundred eggs are embedded in a protective gel.

Needle nematode is important in the Imperial Valley, whereas stubby root nematode occurs statewide. Both are migrating ectoparasites (i.e., they feed on but do not penetrate the roots) and lay eggs singly in the soil. They go through four molts, and all juvenile stages probably feed on roots. Both needle and stubby root nematodes must be extracted from the soil to be seen under high magnification.

Root-knot nematodes can cause substantial damage and are of significant concern in California. Their parasitic activity can damage the growing root, resulting in a forked, galled, or stunted taproot. In addition to the reduced marketability, deformed roots tend to pick up excess soil during harvest, increasing the tare transported to the processing factory. Removal and disposal of excess soil increases cost. Furthermore, root-knot nematodes may cause stand and yield reduction. Heavy stubby root and needle nematode infestations can also stunt root development, resulting in uneven growth and yield reduction.

The symptoms described below suggest a nematode problem, but only root-knot nematode galls are diagnostic. Other symptoms can also result from diseases or abiotic causes.

Injury of the growing tip of the taproot by any species of nematode often causes forking of the taproot, stubbing, fasciculation (bunching) of the roots, and a predisposition to wilting. Typically, this taporoot injury occurs within the first few weeks after seed germination. Stubby root nematode may cause thick, short roots. Needle nematode can damage a root's growing point with its long stylet, which causes root elongation to stop, forking, root branching, swelling of root tips, and sometimes root tip necrosis.

The galls (firm, beadlike bumps on the surface) caused by root-knot nematodes, occur 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.

If a previous crop was a host for nematodes that can affect carrots, the nematode numbers might be high enough to cause damage to a carrot crop that follows. Identifying the nematode species present is critical to make appropriate management decisions. Send soil samples to a diagnostic laboratory to identify plant-parasitic nematodes.

The following sampling procedure may be used.

1. Divide the field into sampling blocks 5 acres or less, each representing cropping history, crop injury, or soil texture.
2. Take the soil samples within the root zone (3 to 12 inches deep). Take several subsamples randomly from a block, mix them thoroughly, and make a composite sample of about 1 quart for each block.
3. Place the samples in separate plastic bags and include symptomatic plant roots if available.
4. Seal the bags and place a label on the outside with your name, address, location, current and previous crop, and the crop you intend to grow.
5. Keep samples out of the sun, store them cool (but do not freeze), and transport them 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.

To avoid infesting new fields with plant-parasitic nematodes, good sanitation practices are essential. Clean machinery and equipment thoroughly with water to prevent the movement of infested soil.

Clean fallow (i.e., weed-free) during years when the land is left unplanted is marginally effective in reducing numbers of root-knot nematodes. Crop rotation is often not sufficient for the control of root-knot nematodes because of the co-occurrence of several species and their wide host range. Rotation crops that can reduce numbers of root-knot nematodes include:

• cotton, particularly cultivar Acala NemX HY which is resistant to the race 3 of M. incognita, the only root-knot nematode that can infect cotton, and
• Mi-resistant tomato cultivars (resistant to M. incognita).

Note that such rotations can increase the occurrence of the cotton (melon) aphid or other virus-transmitting pests of concern.

Most crops grown in the Imperial Valley, except cabbage and cauliflower, are susceptible to needle nematode.

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. However, 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).

Resistance to damage caused by root-knot nematodes (forking and galling), by M. incognita and M. javanica has been developed in new carrot breeding lines, but such cultivars are not yet commercially available.

The root-knot nematodes M. incognita, M. javanica, and M. arenaria do not penetrate roots at soil temperatures below 59 to 64°F. However, the temperature threshold for root invasion by M. hapla is at about 55 to 57°F. Therefore, schedule planting when soil temperatures are below this level to minimize forking and galling of taproots.

Damage thresholds depend primarily on nematode species, soil type, soil temperature, and cultivar susceptibility. If the environmental conditions are conducive to parasitic nematode activity, pesticide application may be warranted whenever root-knot nematodes or needle nematode are present. Several new non-fumigant nematicides are under development. After damage has occurred there are no control measures available to restore health of the taproot.