Field history can provide valuable clues to the identity of nematode and other pest problems. A nematode infestation in a new field usually begins in a small area. It gradually intensifies in the original spot and is spread through the field by cultivation, harvest, erosion, and other factors, which spread, infested soil or plant parts. Therefore, the total effect of a recently introduced nematode is a gradual production decline for the field, as the percentage of the field that is involved and the severity of damage at any given area in the field increase over the years.

A nematode assay is often necessary to complete a diagnosis. A variety of methods may be used to extract nematodes from soil and plant tissues. When they are identified and their relative population densities are known, the amount of risk to the crop from plant parasitic nematodes can be determined. Such risk levels are determined through long term experience of the nematologists with nematodes and with the crop in both growers' operations and in controlled experiments. There are many crop/nematode combinations for which we do not have adequate research data to establish accurate risk levels. In these cases, the most important function of a nematode assay is to determine the kinds of nematodes present. For severe pests on high-value crops, the presence of barely detectable populations is often sufficient to justify use of an appropriate control measure such as a resistant cultivar, a nematicide, or a change of nematicide to one more effective for the particular nematodes present.

Nematodes can injure crops; one or more of them occur in most soils. Although some can cause significant losses when present in low numbers, most do not cause economically significant damage unless their numbers are unusually high or the plant is also subject to unusual levels of stress caused by other factors. Carefully integrating as many as possible of the following components into the management program will help keep most nematode pests below damaging levels, and may simplify the decisions that must be made about selecting and applying nematicides when they are needed.

Prevention

Preventing a nematode problem is far better than trying to treat one after it is established. Many serious nematode pests are widespread, but a few are quite limited in distribution. If peanut root knot nematode is in one field but not in a neighboring field, one can avoid carrying that serious pest problem into the uninfested field by knowing that nematodes are spread in contaminated soil and plant parts. Quarantine refers to a governmental regulatory action taken to prevent importing a pest into a previously uninfested area. It is an attempt to use official regulations to enforce the common sense rules that a grower should apply on his own to prevent spread of pests which have limited distribution. Since most nematodes enter new areas or spread within an area by movement of soil or infested plants, most quarantine regulations and voluntary sanitation measures are intended to control the movement of contaminated soil and plant material. For instance, potato cyst nematode can be spread in bits of soil carried with soybean seed from infested land. Good sense dictates working areas that are not infested with nematodes before moving to those that are infested to avoid carrying contaminated soil or plants on the machinery to the uninfested field. Ornamental cuttings to be rooted should be taken only from uninfested plants or portions of plants from above ground, which have never been rooted, in a potentially contaminated soil. This prevents propagating populations of root knot and burrowing nematodes that might seriously reduce growth and would cause the plants to be unfit for shipment to many potential markets because of quarantines.