** IMPACT **
University of California,
Div. of Agriculture & Natural Resources
---University Pest Management Guidelines---
WARNING ON THE USE OF PESTICIDES
Pesticides are poisonous. Follow all precautions and recommendations on the pesticide label. The grower is legally responsible for residues on treated crops. Keep all pesticides out of the reach of children, unauthorized persons, pets and livestock. Consult your County Agricultural Commissioner for correct methods of disposing of unwanted pesticides and containers.
COMMODITY: POTATOES
Record number: 607200111
REVIEWED: 7/92
NEMATODES:
Common Name: Northern root knot
Scientific Name: Meloidogyne hapla
Common Name: Columbia root knot
Scientific Name: M. chitwoodi (Race 1 & 2)
Common Name: Southern root knot
Scientific Name: M. incognita
Common Name: Lesion
Scientific Name: Pratylenchus penetrans and P. neglectus
Common Name: Stubby root
Scientific Name: Paratrichodorus sp.
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. M. hapla causes small distinct galls with proliferation of lateral roots around these galls. M. 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 fieldUs cropping history, crop injury, or soil texture. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about one quart (1 liter) for each block. (See IPM for Potato UCANR Publication 3316 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 M. chitwoodi are found in samples taken in Modoc/Siskiyou counties, damage can be expected and control measures are warranted. 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 excess irrigation water in a holding pond so that any nematodes present can settle out and pumping water from near the surface of the pond; or, planning irrigations to minimize the amount of excess water; (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 Meloidogyne chitwoodi, Race 1; cereals are nonhosts of Meloidogyne hapla; and there are several nematode resistant tomato varieties that can be used if Meloidogyne incognita is a problem. At present there are no nematode resistant potato varieties available.
Cultural. Fields that are left fallow but kept weed-free usually have a 80-90 percent 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.
TREATMENT:
PREPLANT
A. Pesticide (commercial name): METHYL BROMIDE*
Comments: Contact your farm advisor or agricultural commissioner to determine availability of a section 18 registration.
B. Pesticide (commercial name): METAM SODIUM (Vapam, Soil Prep, Sectagon II)
Amount/Acre: 50-100 gal
Comments: Contact your farm advisor foradvice on the most effective application method for a particular situation.
C. Pesticide (commercial name): ETHOPROP* (Mocap)
Amount/Acre: Maximum label
Comments: Contact your farm advisor for rate information on maximizing the effectiveness of this material.
* Permit required from county agricultural commissioner for purchase or
use.
REFERENCE:
Potato Pest Management Guidelines. U.C. Kodira, B.B. Wesxterdahl. July 1992. PUBLICATION: UCPMG6