Nematodes

Plant-parasitic nematodes are microscopic roundworms that feed on plant roots. They survive 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 geograph‑ical distribution of the different species is highly dependent on temperature, soil type, and cropping history.

Root knot and stubby root nematodes are the most serious nematode parasites on bell peppers in California. Heavy infestations of root knot nematodes can cause significant reduction in crop stand, and growth and yield of plants. Damage caused by root knot nematode infestations is more severe in light-textured soils that have 50% or more sand, such as loamy sands and sandy loams, where plants with damaged roots quickly become stressed for water. Stubby root nematode can reduce growth and yield of plants and prefer lighter soil types.

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

Nematode infestations damage the plant roots, and therefore symptoms reflect poorly functioning root systems. Aboveground symptoms of severe root knot and stubby root nematode infestations include patches of chlorotic, stunted, necrotic, or wilted plants. Nematode-infested plants are more susceptible to moisture or temperature stress and exhibit stress symptoms earlier than other plants. Furthermore, root systems that have been damaged by nematodes are often more susceptible to infection by soil-inhabiting fungi such as Fusarium and Verticillium species.

Feeding by root knot nematodes results in characteristic galls on roots. Severely galled roots may appear malformed and the root system shortened and thickened. Roots of plants infested with stubby root nematode are likely to have numerous, short and stubby lateral roots.

To make management decisions, it is important to know which nematode species are present. If a previous crop had problems caused by nematodes that are also listed as pests of bell pepper or other solanaceous vegetables, population levels may be high enough to cause damage to subsequent crops. If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification.

Take soil samples soon 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 five acres each that represent 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 root systems of suspect plants into plastic bags with soil. Place soil samples in separate plastic bags, seal them, 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 to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.

In fields infested with root knot nematodes, crop rotation may not be feasible because of their extensive host range; care is needed in the selection of rotation crops because some may be good alternate hosts. New resistant varieties of peppers may prove useful. Soil solarization may help to lower the nematodes in the top layers of the soil and avoid an early infestation of the plants. Roots are likely to become infested as the season progresses by nematodes that survived in the deeper soil layers.

Recently, root knot nematode-resistant bell pepper varieties have become available. These varieties are resistant against the root knot nematode species Meloidogyne incognita, M. javanica, and M. arenaria. Two of these resistant varieties are "Charleston Belle" and "Carolina Wonder." "Carolina Cayenne" is a nematode-resistant hot pepper variety. Under high soil temperature conditions (between 82° to 90°F) the level of resistance may decrease. Although results from field studies done in South Carolina are very promising, the usefulness of these varieties under California conditions and with California nematode populations has yet to be evaluated.

Varieties that are resistant to stubby root nematodes are not available.

Clean soil from equipment with water before moving from infested to noninfested fields, and keep fields free of weeds, because some weed species, such as nightshade, are excellent hosts for root knot nematodes.

Weed-free fallow reduces most nematode populations. Fallowing is more effective if soil is plowed and exposed to sun. Irrigation during the dry period stimulates egg hatch and so further reduces nematode populations if proper weed control is maintained.

Solarization can be used in areas with sufficient solar radiation. Typically, the soil is irrigated and then covered with clear plastic for 4 to 6 weeks during the hot summer period. The effectiveness depends mainly on the heat that can be generated under the plastic during a certain period. Although nematodes in upper soil layers can effectively be reduced, re-infestation of roots by nematodes that survived deeper in the soil will occur. Solarization can, however, provide plants a nematode-free environment early on in the growing season when plants are most susceptible to damage. For further information, contact your local farm advisor or see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds.

Damage thresholds for root knot and stubby root nematodes on peppers have not been developed. If present, damage may occur and preplant treatment is warranted.