Nematodes

Plant-parasitic nematodes are microscopic roundworms that feed on plant roots. They survive in soil and plant tissues and several species may exist in a field. They have a wide host range, and vary in their environmental requirements and in the symptoms they induce. Apart from the nematodes listed above, several other species that occur in California, such as stubby root, sting, needle, spiral, sheath, stem and bulb, false root knot, and potato rot nematodes, have been reported as pests on sugarbeet in other parts of the world but are not known to be a factor in California sugarbeet production.

Infestations of sugarbeet cyst nematode may be localized or spread over an entire field. In heavily-infested soils, seedling emergence may be delayed or seedlings may be killed before emergence, resulting in a reduced stand. Seedlings infested with sugarbeet cyst nematodes may be predisposed to secondary infection by soilborne fungi. This nematode is widespread in all former and present California sugarbeet growing areas, especially the Imperial Valley, central regions of the Central Valley, the Salinas Valley, and Monterey, Santa Barbara, and Ventura counties.

Meloidogyne incognita and M. javanica are the most damaging of root knot nematode species found in sugarbeet. Meloidogyne hapla is widely distributed, but not reported as a major problem on sugarbeet in California. Meloidogyne chitwoodi is found in Modoc and Siskiyou counties in northern California where its reproductive levels on sugarbeet are similar to those found on barley and less than those found on wheat, but no yield reductions have been reported.

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.

Seedlings infested by sugarbeet cyst nematode may have longer petioles than normal, with green or yellow leaves depending on the severity of infestation. Plants are likely to be stunted and wilted. Typically, storage roots will not be well developed, and will have excessive fibrous roots. Mature female nematodes can be seen on the root surface as tiny, pinhead size, lemon-shaped bodies that are white in the earlier stages and turn into brown, egg-filled cysts on aging.

Heavy infestation by root knot nematodes in sandy soils may cause plants to wilt and collapse. Swellings (galls) can be seen on fibrous roots and the tap root, which may have a warty appearance.

To make management decisions, it is critical to know the nematode species present and their population estimates. If a previous crop had problems caused by nematodes that are also pests of sugarbeet, population levels may be high enough to cause damage to an ensuing sugarbeet crop. If nematode species have not been identified previously, take soil samples and send them to a diagnostic laboratory for identification.

Before planting sugarbeet, take soil samples from within the root zone after harvest of the previous crop or preferably just before harvest. Divide the field into sampling blocks of 10 to 20 acres 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. Include roots in the sample if possible. 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. Farm advisors and PCAs can help you find a laboratory for extracting and identifying nematodes, and help in interpreting sample results.

Thoroughly clean machinery and equipment with water between fields to mitigate the risk of spreading plant-parasitic nematodes to non-infested fields. This is especially important for H. schachtii as eggs in cysts are well protected against adverse environmental conditions.

Sugarbeet Cyst Nematode

The main host crops of H. schachtii are in the families Chenopodiaceae (e.g. garden beets, Swiss chard, spinach, mangold) and Brassicaceae (e.g. broccoli, radish, Brussels sprouts, rapini, cauliflower, kale, bok choy, mustard, canola, kohlrabi, and others). Several common weeds are also hosts such as common lambsquarters, shepherd's purse, pigweed, chickweed, dock, and others.

Rotation with nonhost crops is widely used to control sugarbeet cyst nematode. The interval between sugarbeet and other crops in rotation depends on the severity of infestation and local conditions influencing the nematode. In the Imperial Valley, fields are considered infested with H. schachtii if three or more cysts are found in a pint-size dump sample that is collected at the sugarbeet factory. Non-infested fields cannot be cropped to sugarbeets more than two years in a row and not more than four out of ten years. In infested fields, sugarbeets can be grown only once every four years.

This cropping program has been used effectively for half a century. The reason for the success is the natural decline in the population density of H. schachtii in the absence of host plants. Contact your farm advisor for help in developing a crop rotation program for sugarbeet cyst nematode. Be sure to destroy weed hosts during crop rotations.

Root Knot Nematode

Control of root knot nematodes by crop rotation is very difficult because of their wide host ranges. Nematode-resistant tomatoes can be grown if Meloidogyne incognita, M. javanica, or M. arenaria are present.

Planting when soil temperatures are below 50°F for H. schachtii and 65°F for M. incognita reduces damage and slows nematode population buildup.

Weed-free fallow, which deprives nematodes of food, reduces most nematode populations. Fallow is most effective if soil is plowed and exposed to sun. Irrigation during the dry period further reduces nematode populations if proper weed control is maintained. The importance of the time of year in which to fallow as it relates to rate of decline of the nematode population is not well understood at this time.

The damage threshold in the Imperial Valley for H. schachtii is one to two cyst nematode eggs per gram of soil. Thresholds have not been established in other areas of the state or for root knot nematodes, but consider treatment if nematodes are present and have caused problems in the past.