Description of the Pest
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. They have a wide host range, vary in their environmental requirements, and in the symptoms they cause.
Eight different species of nematodes are known to attack small grains in California, but each is important only in certain areas. Meloidogyne chitwoodi in northeastern California, and M. incognita, M. javanica, and M. arenaria in the central and southern portions of the state build up on grains, but their damage is mostly on subsequent rotation crops. Meloidogyne naasi in northeastern California is known to reduce yields of barley. In the Imperial Valley and some other warm areas, lesion nematode, Pratylenchus thornei, 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 M. incognita, M. javanica or M. arenaria, the crop could suffer damage.
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.
Feeding by root knot nematodes cause 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 there will be a proliferation of lateral root branches. 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, chlorotic, and often fail to develop heads. Plants infested by lesion nematode, Pratylenchus thornei, will be stunted and yellow in patches, have brown leaf tips, fewer tillers and smaller heads. Stem and bulb nematodes can curl newly emerging leaves into spirals, stunt plants, reduce tillering, inhibit head formation, and sometimes kill the plant. The cereal cyst nematode has been found on grasses in some parts of California, but not on small grains. It is potentially very damaging to grains; an infestation would produce aboveground symptoms similar to that of root knot and lesion nematodes. Infested roots are short, and abnormally branched. Adult females (cysts) can be seen on the root surface as tiny, pinhead size, white or brown, lemon-shaped structures.
To make management decisions, it is important to know the nematode species present and their population estimates. If a previous crop had problems caused by nematodes that are also listed as pests of small grains, population levels 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.
Soon after harvest or preferably just before harvest take soil samples from within the root zone of the previous crop. Divide the field into sampling blocks of not more than twenty 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, and 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.
Use a high-pressure hose to wash soil from equipment before moving from infested to noninfested fields.
Rotation with resistant cultivars of oats or to noncereal crops is recommended for fields with Meloidogyne naasi. A 1-year rotation with Cayuse oats is reported to reduce M. naasi populations to levels sufficient to grow a crop of barley. The cultivar Kanota is also known to be resistant. Weed hosts should be destroyed during crop rotations. Contact your farm advisor for information on susceptibility of newer oat varieties.
For Meloidogyne incognita and M. arenaria, fall planting after soil temperature has dropped below 64°F, reduces damage by the nematodes and also nematode population development.
Weed free fallow reduces most nematode populations. Fallow 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.
Genes for resistance to Meloidogyne chitwoodi, M. incognita, and M. javanica have been transferred to experimental lines of bread wheat. Consult your farm advisor for more information regarding availability of resistant cultivars for your area.
Use of chemical control on small grains has not generally been found to be cost effective.