4. SCOUTING FOR NEMATODES

North Carolina Cooperative Extension Service

To Table of Contents: Scouting Soybeans (North Carolina Agricultural Extension Service publication AG-385)

The most accurate way to detect nematodes is through a soil assay. However, nematode damage--especially that caused by root-knot and cyst nematodes--can be detected during routine scouting. Look for areas in the field where the plants are stunted or off color. Carefully dig up the plants with the roots intact. Shake off the dirt and examine the roots. Root-knot nematodes cause swellings (galls) on the root. The northern root-knot nematode produces tiny galls that are often difficult to detect except by close examination of the root system, which is usually highly proliferated. The soybean cyst nematode produces cysts that are usually visible on a carefully dug root system after it has been allowed to dry for several seconds.

When stunted plants are found and it is not possible to diagnose the cause in the field, a soil sample should be collected and submitted for a nematode assay. The problem may be caused by one or more other species of nematodes that can be as damaging as the root-knot and cyst nematodes but that do not produce easily recognizable symptoms.

Eight species of plant-parasitic nematodes are of economic importance on soybeans in North Carolina: they are the soybean cyst, southern root-knot, peanut root-knot, northern root- knot, sting, Columbia lance, lesion (Pratylenchus brachyurus), and reniform nematodes. It is important to identify the species and, in the case of the soybean cyst nematode, the race in order to make good control decisions. The soybean cyst and root-knot nematodes are not difficult to identify in the field, but determining the species of root-knot nematodes and the races of both kinds of nematodes may require assistance from the nematode advisory service. All the other nematodes must be identified through an assay.

Recommendations for pesticides that are effective in managing nematode problems in soybeans can be obtained from the local Extension agent or the North Carolina Agricultural Chemical Manual, also available from the county Extension office.

Soybean Cyst Nematode

Females are visible on soybean roots three to six weeks after planting (Figure 5). To examine the roots for cyst nematodes, carefully dig plants from the edge of the affected area, gently tap the soil from the roots, and allow the roots to dry for a few minutes. A hand lens or magnifying glass (10X to 15X) is helpful in observing the females or cysts because they are small (less than 1 mm long). Races can be determined only with a bioassay.

Root-Knot Nematodes

Galls--abnormal enlargements of the roots--are the characteristic symptom of root-knot nematodes (Figure 6). The size and shape of the galls vary with the soybean variety and with the number and species of nematodes. Generally, microscopic observation, bioassay, or both are needed to determine the species. Some clues, however, are useful in determining the dominant root-knot species present in the field. A high population following a corn crop is usually evidence that the southern or peanut root-knot nematode is present. If large numbers of root-knot nematodes are present following a peanut crop, they are probably northern or peanut root-knot nematodes.

Back to IPM Home Page

Back to IPM Contents