Nematodes

Nematodes are typically microscopic elongated roundworms that occur in nearly every aquatic and terrestrial environment. The vast majority of soil-dwelling nematodes cannot harm plants but play an important role in the soil food web and in nutrient cycling. Plant-parasitic nematodes obtain their food only from living plant tissues. They live in soil and in plant tissues and feed on plants by puncturing cell walls and withdrawing the cell contents with a needlelike mouthpart called a stylet. Most plant-parasitic nematode species are similar in appearance. However, correct identification is important when developing an IPM strategy because they differ in damage potential, environmental requirements, and host range.

Among nematodes that affect cilantro and parsley, root-knot nematodes are the most important species and are distributed throughout California. The development rate of root-knot nematodes is temperature related. Crop damage is greatest in warm regions with coarser-textured soils such as sand or sandy loam. At soil temperatures of 77 to 82°F, the optimal range for the development of root-knot nematode species, they complete their life cycle within 3 to 4 weeks.

Each female produces an egg mass that contains several hundred eggs embedded in a protective gel. The eggs are immediately ready to hatch. In the presence of sufficient soil moisture and adequate soil temperature, second-stage juveniles (J2) hatch from eggs and invade host roots typically near their tips. Once inside the root, the juveniles soon become immobile while establishing a feeding site that sustains them through three additional molts to the adult stage. This life style is classified as sedentary endoparasitic.

Stubby root nematodes occur statewide. They are migratory ectoparasites that prefer to feed on root tips but do not penetrate into the roots. They go through four molts and all juvenile stages feed on roots. The adult females lay eggs singly in soil.

The following foliar symptoms may be indications of a nematode problem but could also result from other biotic or abiotic causes such as fungal diseases, insect injury, irrigation problems, or nutrient deficiencies. Aboveground symptoms for plant-parasitic nematodes may include patches of variable crop growth with stunted, chlorotic, and excessively wilted plants. Often the number and size of leaves is reduced. Nematode-infested plants may grow less vigorously, be smaller in size, and produce lower yield and diminished quality compared to plants in nematode-free soil. Young seedlings are particularly sensitive to nematode attack.

Plant cells that contribute to the root-knot nematode's nutrition enlarge and the root deforms, showing characteristic galls or knots. These may grow to about one inch in diameter and harbor one or more of the developing nematodes. Root-knot nematode infection severely reduces water and nutrient uptake and transport in the plant.

Injury of the growing root tip by stubby root nematodes often causes stubbing and fasciculation (bunching) of the roots. Typically this occurs within the first few weeks after seed germination.

The severity of crop damage is related to the nematode species, their population density at planting, and soil temperature. It is critical to know these factors to make rational management decisions. If field history indicates that a previous crop had problems caused by nematodes that are listed as pests of cilantro or parsley, nematode numbers may be high enough to cause damage to seedlings. If nematode species have not been identified previously, take soil samples and send them to a diagnostic laboratory for identification. For many nematode-crop associations, including cilantro and parsley, damage thresholds have not been established for California conditions.

Use the following sampling procedure.

1. Divide the field into sampling blocks that are representative of cropping history, crop injury, or soil texture.
2. Take the soil samples from within the root zone.
3. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about 1 pint (approximately 500 cm³) for each block.
4. Place the samples in separate plastic bags and include symptomatic plants (with roots) if available.
5. 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.
6. Keep samples cool, but do not freeze (ideally kept at 54° to 59°F). Transport the samples as soon as possible to a diagnostic laboratory.
7. Contact your farm advisor for more details about sampling, locating laboratories that process nematode samples, and interpreting sample results.

Clean machinery and equipment thoroughly with water to prevent movement of infested soil.

Rotation with nonhost crops can reduce nematode numbers. Before selecting a rotational crop, make sure it is truly a nonhost crop for the species or race of root-knot nematode present in your field.

Clean fallowing (i.e., weed-free) during years when the land is left unplanted is effective in reducing most nematode populations. However, special attention must be paid to weed control, both in nonhost rotations and fallowing. Many common weeds are hosts to root-knot nematodes. These include black and hairy nightshade, yellow and purple nutsedge, and pigweeds.

Deep plowing, fallowing, and solarization may further reduce nematode numbers.

Soil solarization may be useful, especially in the desert production areas of Southern California. It 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, at best the efficacy is 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 21377).

The root-knot nematodes M. incognita, M. javanica, and M. arenaria do not penetrate roots at soil temperatures below 60 to 64°F. Therefore, plant when soil temperatures are below this level to minimize the early attack on seedlings. The temperature threshold for root invasion by M. hapla, however, is at about 55 to 57°F.

Striving for optimum growing conditions by addressing other abiotic and biotic plant stress factors such as soil moisture, nutrition, insect pests, and other diseases may minimize nematode damage.

There are no current treatment guidelines for root-knot nematode or stubby nematode in cilantro and parsley. Do not plant cilantro or parsley in infested fields.