Nematodes

Plant parasitic nematodes are microscopic roundworms that feed on various plant parts. They survive in soil and plant tissues, and several species may occur in a field. They have a wide host range and varied environmental requirements, and they cause a variety of symptoms on affected plants.

Eight species of nematodes are known to feed on small grains (wheat, triticale, barley, oats, and rye) in California, but each is important only in specific areas. Columbia root-knot nematode in northeastern California, and southern root-knot nematode, Javanese root-knot nematode, and peanut root-knot nematode in the central and southern portions of the state, build up on grains, but their damage is mostly on subsequent rotation crops. Barley root-knot nematode in northeastern California is known to reduce yields of barley. In the Imperial Valley and some other warm areas, lesion nematode may damage small grains. Stem and bulb nematode has damaged susceptible oat cultivars in parts of the South Central Coast. If grain were to be planted in spring or summer in fields infested with southern root-knot nematode, Javanese root-knot nematode, or peanut root-knot nematode, the crop could suffer damage. Cereal cyst nematode has been reported on grasses, but it has not been detected in small grains.

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

Feeding by root-knot nematodes causes cell enlargement and proliferation and may result in swellings, called galls, on roots. Galls are not always formed, and when present are very small and may be spindle shaped or in spirals. Sometimes a proliferation of lateral root branches will form. Occasionally, plants infested with Columbia root-knot nematode will have egg masses, which appear as tiny bumps, on the root surface. Plants heavily infested with barley root-knot nematode may be stunted and chlorotic and often fail to develop heads.

Plants infested by lesion nematode are stunted and yellow in patches and have brown leaf tips, fewer tillers, and smaller heads. Stem and bulb nematode can curl newly emerging leaves into spirals, stunt plants, reduce tillering, inhibit head formation, and sometimes kill the plant. The main symptom of cereal cyst nematode is a bushy or knotty appearance on crop roots, sometimes called witches brooming. The root system fails to develop deep roots. After the females mature, young white cysts can be visible on the outside of the root. At heading, plants may develop whiteheads, which cause heads to turn brown prematurely while surrounding heads are still green.

To make management decisions, it is important to know the nematode species present and estimate its level of infestation. If a previous crop had problems caused by nematodes that are also listed as pests of small grains, nematode 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 nematode identification. Crop damage is usually related to the parasitic nematode population density at certain stages of a crop’s development.

Take soil samples from within the root zone of the previous crop soon after harvest or, preferably, just before harvest. Divide the field into sampling blocks of not more than 20 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 the 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 them 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.

Cultural Practices

Sanitation

Use a high-pressure hose to wash soil from equipment before moving from infested to noninfested fields.

Crop Rotation

Not all crop species are susceptible to all kinds of nematodes. Rotate to resistant cultivars of oats or crops other than small grains in fields with barley root-knot nematode. Cayuse, Curt, Indio, Kanota, and Park oat cultivars are nonhosts to barley root-knot nematode. A 1-year rotation with Cayuse oats is reported to reduce barley root-knot nematode’s numbers to levels sufficient to grow a crop of barley or other small grains. Wheat and barley's ability to host Columbia root-knot nematode varies by variety. For example, Steptoe and Briggs barley are poorer hosts than Fieldwin wheat or Crystal barley. Weed hosts should be destroyed during crop rotations. Cereal cyst nematode can reproduce on most of the grassy weeds in a field. Once established, cereal cyst nematode can be difficult to manage because the cysts can survive for many years in the soil, even without a host. Contact your farm advisor for information on the susceptibility of newer varieties.

Planting Date

For southern root-knot nematode, Javanese root-knot nematode, and peanut root-knot nematode, plant in the fall after the soil temperature has dropped below 64°F to decrease nematode numbers and reduce nematode damage. In some cases, planting during cooler weather may help suppress some nematode infestations.

Fallow

Weed-free fallow often reduces the number of nematodes. Irrigation during the dry period stimulates egg hatch, which further reduces nematode numbers if proper weed control is maintained.

Resistant Cultivars

The wheat cultivar Lassik is resistant to southern and Javanese root-knot nematode, including populations of these nematodes that are able to break resistance in cultivars with the resistance gene Mi-1. This resistance gene is also found in Anza-2NS, Express-2NS, and Yecora Rojo-2NS. Consult your farm advisor for more information regarding availability of resistant cultivars for your area.

Treatment Decisions

Nematicide applications on small grains have not been found to be cost effective.