4 Scouting for Nematodes
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, 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 will require
assistance from the nematode advisory service. All the other nematodes
must be identified through an assay.
Soybean Cyst Nematode
Females are visible on soybean roots three to six weeks after planting. 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) (Figure 5). 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.