Nematodes

Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues, and more than one species may occur in a field. They have a wide host range, and vary in their environmental requirements, the symptoms they cause, and the risk to potatoes.

Overwintering root-knot nematode juveniles invade roots and later tubers, establish feeding sites, and develop into the adult stage. Adult females are swollen, sedentary, and lay eggs in a gelatinous matrix on or just below the root surface. Second-stage juveniles (J2) hatch from these eggs and move through the soil to invade other roots and tubers. When no crop is present, the nematodes survive as eggs or J2, which can be found in the soil. Root-knot nematode feeding reduces the vigor of plants and causes blemishes on tubers. The latter can lead to a severe reduction in tuber quality and, as a result, affected potatoes become unmarketable. Lesion nematodes damage roots by feeding and moving through cortical tissues. In addition, Pratylenchus penetrans increases the susceptibility of potato plants to Verticillium wilt and blemishes tubers. Pratylenchus neglectus is common in potato fields but has not been shown to damage potatoes in California. Stubby root nematodes feed on root surfaces, and can result in formation of numerous stubby roots. Yield loss has not been reported. The major problem caused by this nematode species is transmission of tobacco rattle virus, which causes corky ringspot disease on developing tubers.

The symptoms described below are indicative of a nematode problem but are not diagnostic because they could result from other causes as well. In general, aboveground symptoms include stunted, yellowed, chlorotic, and/or dead plants. Infected plants are likely to wilt earlier under temperature or moisture stress. Infestations may occur without causing any aboveground symptoms.

Feeding by root-knot nematode causes characteristic swellings, called galls, on roots. Galls caused by Meloidogyne chitwoodi are small and difficult to see. On heavily infested plants, egg masses appear as tiny round bumps on feeder roots. Meloidogyne hapla causes small distinct galls with proliferation of lateral roots around these galls. Meloidogyne incognita causes more pronounced galls. All three species of Meloidogyne and Pratylenchus penetrans cause bumps, warts, or crosslike fissures on the surface of infected tubers. However, those caused by M. hapla are less distinct. Brown spots develop inside tubers, mostly in the outer 0.25 inch (6 mm), which are visible when a thin layer of tuber is peeled off. Lesion nematodes cause reddish brown lesions on the roots that turn black later. Stubby root nematodes cause numerous short and stunted (stubby) roots, and corky ringspot symptoms on tubers if the nematode is carrying TRV.

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 potato, their numbers may be high enough to cause damage to subsequent potato crops. Soil samples should be sent to a diagnostic laboratory for enumeration and identification.

Take soil samples in fall from within the root zone of the previous crop after harvest or, preferably, just before harvest. Divide the field into sampling blocks of not more than five acres. Each block should be representative of the field's 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. (See UC ANR Publication 3316, Integrated Pest Management for Potatoes, for more details.) 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 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.

Meloidogyne chitwoodi, Columbia root-knot nematode, has caused significant crop damage in high organic matter soils at low densities in Modoc and Siskiyou counties. In fields where this root-knot nematode is a problem, long-term integrated management tactics such as crop rotation, cultural controls, fumigation, and nematicides are necessary to prevent a substantial devaluation of the crop due to nematode-induced tuber blemishes.

Fumigants (metam sodium and 1,3-dichloropropene) and non-fumigant nematicides (ethoprop and oxamyl) are chemical control options for minimizing Columbia root-knot nematode crop damage in soils with low organic matter content. However, in silty clay loam soil with high organic matter when Columbia root-knot numbers exceed 500 second stage juveniles (J2) per 1000cc of soil, neither 1,3-dichloropropene nor metam sodium provide acceptable levels of control.

The following management tactics for Columbia root-knot nematode may increase efficacy in high organic matter soils):

• A combination of fumigation (1,3-dichloropropene) and application of non-fumigant nematicide (oxamyl) can be effective. However, using this combination has not been tested in Californian in high organic matter soils.
• Metam sodium broadcasted and immediately incorporated using a rotary tiller. (Based on one year of research.)
• Multiple depth injection of 1, 3-dichloropropene or metam sodium

For other nematode species there are no precise guidelines for economic threshold levels under California cropping conditions. Fields infested with stubby root nematodes and tobacco rattle virus have been found in Monterey and Kern counties and in the Santa Maria area of Santa Barbara County. Fields with known virus infestations should not be planted to potatoes without an effective control strategy in place for stubby root nematode, which can transmit the virus that causes corky ringspot in potato.

The following measures will help prevent spread of nematodes to uninfested fields: (1) using certified planting material; (2) cleaning soil from equipment before moving between fields; (3) keeping irrigation water in a holding pond so that any nematodes present can settle out and pumping water from near the surface of the pond; (4) preventing/ reducing animal movement from infested to uninfested fields; and (5) composting manure to kill any nematodes that might be present before applying it to fields.

Crop rotation can be useful in reducing nematode numbers except of stubby root nematode and the tobacco rattle virus which have wide host range and so crop rotation is not practical. Alfalfa is not a host of M. chitwoodi, Race 1; cereals are nonhosts of M. hapla; and there are several nematode resistant tomato varieties that can be used if M. incognita is a problem. Research in the Pacific Northwest has shown that cover crops of rapeseed, mustard, oilseed radish, or sudangrass reduce numbers of root-knot nematodes when incorporated as green manure. At present there are no nematode resistant potato varieties available. For managing Columbia root-knot nematode, consult the UC Nematology website.

Fields that are left fallow but kept weed-free usually have an 80 to 90% per year reduction in root-knot nematode populations. Infested tubers left in the field after harvest can be a source of inoculum. Destroy potato plants that subsequently emerge from these tubers to restrict nematode reproduction. Avoid storage of tubers infected by M. chitwoodi as blemishes can increase during storage.