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 feed by puncturing cells and withdrawing the contents with a needle-like 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.

The following 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/or excessively wilted plants. Often the size of leaves and number of flowers are reduced. Nematode-infested plants may grow less vigorously, be smaller in size, and produce lower yield and diminished fruit quality compared to cucurbits in nematode-free soil. Young seedlings are particularly sensitive to nematode attack.

Root knot nematodes (Meloidogyne spp.) are by far the most important plant parasitic nematodes in cucurbits. All cultivated cucurbits are very susceptible to the southern (M. incognita) and Javanese (M. javanica) root knot nematodes while they are poor hosts to the northern root knot nematode (M. hapla). In the presence of sufficient soil moisture, second stage juveniles (J2) hatch from eggs and invade host roots at soil temperatures above 64°F. 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. Plant cells that contribute to the nematode's nutrition enlarge and the root deforms, showing characteristic galls or knots. These may grow to one inch or more in diameter and harbor one or more of the developing nematodes. Galls severely reduce water and nutrient transport in the plant. The development rate of nematodes is temperature related. 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. Crop damage is greatest in warm regions with coarser-textured soils such as sand or sandy loam.

Lesion nematodes are migratory endoparasites that invade roots and move and feed within the root cortex. In contrast to root knot nematodes, they are able to leave the host if conditions become unfavorable. Infestation may cause reddish brown to dark brown lesions on roots. Lesion nematode species are not known to cause economic damage on cucurbits in California. However, the root lesions may predispose plant tissues to invasion of fungi and bacteria that can accelerate root decay.

All other potentially crop-damaging nematodes are ectoparasites; they feed on the outside of the root. Stubby root nematodes prefer to feed on root tips. Symptoms include short feeder roots, stunting, and yellowing of plants. Theneedle nematode has been found only in the Imperial Valley where it can cause damage to cucurbits. Its relatively long mouth stylet can damage the growing point of roots, which may cause terminal root swelling, cessation of root elongation, and root necrosis. Pin nematodes, Paratylenchus species, occur in production fields but are not known to cause significant damage to cucurbits in California.

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 a previous field or crop had problems caused by nematodes that are listed as pests of cucurbits, population levels may be high enough to cause damage to seedlings. If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification.

Take soil samples from within the main root zone (2 to 16 inches deep). 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 several subsamples randomly from a block, mix them thoroughly, and make a composite sample of about 1 pint (or about 500 cm³) for each block. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, 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 the soil. Ideally the samples should be kept at 54 to 59°F in a cooler and transported as soon as possible to a diagnostic laboratory. Contact your UC farm advisor for more details about sampling, to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.

• Rotation with nonhost crops can reduce nematode population levels. Before selecting a rotational crop, make sure it is truly a nonhost crop for the species/race of root knot nematode present in your field.
• Deep plowing, fallowing, and solarization can further reduce nematode population levels, especially root knot nematodes. Soil solarization may be useful, especially in the desert production areas of Southern California, but it is neither as effective nor as reliable as chemical fumigation.
• 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
  • pigweed
• 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.

None available

Start looking for symptoms caused by nematodes during the vegetative growth stage. If symptoms are found, investigate if they are caused by root knot nematodes. Use this information to plan for future plantings.

Studies in California have shown that the damage threshold for root knot nematodes (M. incognita) in cantaloupe at planting is near the detection level (1.6 J2/100 cm³). A preplant density of 40 J2/100 cm³ is likely to cause at least 30% yield loss. Although this kind of research has not been done on squash, pumpkin, or cucumber, threshold levels for M. incognita, M. javanica, and M. arenaria in these crops is probably similar to those in cantaloupe. There are no current treatment guidelines for lesion, stubby root, or needle nematodes.

To mitigate nematode damage, apply nematicides preplant or at planting. Consider postplant applications only for supplemental suppression of plant parasitic nematodes.