Nematodes

Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues and several species may occur in a field. The host range varies according to the species, with some being able to infest a wide variety of crops and others being limited to a narrow crop range. Symptoms of nematode infestation also vary according to the nematode species and crop type, and are often non-specific (yellowing, stunting). Root knot nematode species, however, cause typical galling on roots of infested plants. The geographical distribution of the different species is highly dependent on temperature, soil type, and cropping history.

Root knot nematodes are known to cause economic damage in most areas of California. The needle nematode has been reported only from the Imperial Valley where it can cause serious damage. Damage to plantings can delay maturity and reduce head size. Although high numbers of spiral and stunt nematodes have also been suggested to cause yield loss in lettuce, this remains to be proven.

Infestations may occur without resulting in obvious aboveground symptoms. The symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well.

Root knot nematodes feed within the roots and cause characteristic swelling of roots (galls). Meloidogyne hapla, the northern root knot nematode, generally occurs in cooler regions than the other three Meloidogyne species that prefer hot summer climates. Galls formed by Meloidogyne hapla are spherical, distinct, and generally smaller than those caused by the three "warm-climate" species. Plants infested as seedlings may be stunted, with patches of stunted plants becoming evident by midseason.

The needle nematode lives in the soil and feeds on root tips, leading to swelling at the root tip and often root branching. Proliferation of lateral roots and sometimes death of root tips are observed. Aboveground symptoms of needle nematode infestation include downward cupping of cotyledons at the seedling stage, as though they were wilting. Leaves may turn grayish green with chlorotic outer margins. Stunting and yellowing have been associated with large numbers of Rotylenchus sp. and Merlinius sp.

To make management decisions, it is important to know which nematode species are present and at what levels. If a previous crop had problems caused by nematodes that are also listed as pests of lettuce, their numbers may be high enough to cause damage to subsequent crops. If nematode species have not previously been identified, soil samples should be taken and sent to a diagnostic laboratory for identification.

Soil samples should be taken after harvest or preferably just before harvest, within the root zone of the previous crop. 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 quart (1 liter) for each block. Place the samples in separate plastic bags, 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 to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.

Sanitation, cultural controls, crop rotation are acceptable methods.

Plow under infested plants after harvest to prevent further reproduction of nematodes. Reduce stress on plants by proper fertilization and irrigation. In areas with hot summers, soil solarization can provide partial control of root knot nematode; when combined with composted chicken manure it may provide more complete control. Solarization can also reduce populations of soilborne fungi pathogens and weed seed.

Crop rotation is not very effective against root knot nematodes due to their wide host range. Strawberries may be a suitable rotation crop in fields in which the species of root knot nematode has been identified. They are nonhosts to M. incognita and most populations of M. javanica, but allow reproduction of M. hapla. Rotation will not be effective against needle nematodes in the Imperial Valley, because their host range includes most crops grown in the Valley.

Thoroughly clean soil from all equipment with water to prevent the spread of infestation. Do not allow irrigation water to flow from an infested field to other fields without impounding. Prevent animal grazing and movement from infested to uninfested fields.

Damage thresholds have not been developed for nematodes on lettuce. Treatment for root knot nematode is advised if any are detected in sandy, loamy sand, or sandy loam soils, or where high numbers are found on any soil type, especially in spring and summer plantings. Consider treating for needle nematode before lettuce is planted in the Imperial Valley whenever this pest is present in soil samples, especially for fall planting when high soil temperatures favor nematode activity. Contact your farm advisor for advice on specific situations.