How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

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 and in the symptoms they cause.

DAMAGE

Root knot nematode larvae invade roots or 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. These eggs hatch and larvae invade other roots and tubers. Root knot nematode feeding reduces the vigor of plants and causes blemishes on tubers. 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, resulting in formation of numerous stubby roots. The major problem caused by this nematode species is transmission of tobacco rattle virus, which causes corky ringspot disease on developing tubers.

SYMPTOMS

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 or warts 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.

FIELD EVALUATION

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, population levels may be high enough to cause damage to subsequent potato crops. If nematode species have not previously been identified, soil samples should be taken and sent to a diagnostic laboratory for 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.

MANAGEMENT

If any Meloidogyne chitwoodi are found in samples taken in Modoc and Siskiyou counties, damage can be expected. In fields where Columbia root knot nematode has been a problem in the past, a fumigation is warranted unless soil samples have levels of more than 500 larvae/1000cc. In these cases, fumigation with Vapam or Telone in high organic matter soil is not recommended. For other species there are no precise guidelines for deciding what population levels will cause significant damage. Fields infested with stubby root nematodes and tobacco rattle 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.

Prevention. 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. Crop rotation can be useful in reducing nematode populations. 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 populations of root knot nematodes when incorporated as green manure. At present there are no nematode resistant potato varieties available.

Cultural. Fields that are left fallow but kept weed-free usually have an 80 to 90% per year reduction in root knot 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.

PUBLICATION

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Nematodes
B. B. Westerdahl, Nematology, UC Davis
Acknowledgment for contributions to the nematode section:
U. C. Kodira, Plant Pathology, UC Davis

Top of page