Soybean Disease Atlas

2^nd Edition

Southern Soybean Disease Workers

NEMATODE DISEASES

Fig. 28

There are approximately 100 species of nematodes that are known to feed on soybeans, but only a few species are considered to be of major economic concern on the crop. The nematode genera and species that are the most damaging to soybeans include: the root-knot nematodes (Meloidogyne incognita and M. arenaria), the soybean cyst nematode (Heterodera glycines), the lance nematode (Hoplolaimus columbus), the reniform nematode (Rotylenchulus reniformis), and the sting nematode (Belonolaimus longicaudatus).

Of these, the soybean cyst nematode is the most economically important in soybean production.

Fig. 29

Nematodes can be spread from one place to another - from diseased to healthy plants or from infested to noninfested fields - by both active and passive means. Under their own power, they can only move a few feet per year. However, any activity that results in the movement of nematode-contaminated soil, water, or plant materials may introduce nematodes into new areas. The life cycle of most nematodes is relatively short and is usually completed within 3-4 weeks. Consequently, a few hundred nematodes in the early season may increase to hundreds-of-thousands per pound of soil by the end of the growing season.

Nematodes damage plants through their feeding by puncturing and killing root cells with a spike-shaped structure called a stylet (Fig. 28c). If a nematode population or community (a mixture of single species populations) is large enough, this root feeding may impair the ability of the plant to absorb water and nutrients and to stand erect in the soil. This may lead to yield suppression or, in severe cases, the death of the plant. The aboveground result of nematode damage is usually visible within a field as scattered areas of stunted plants with yellow or yellowing foliage (Fig. 28). These areas generally recur each year and may enlarge over time. Unfortunately, many other factors including soilborne fungal pathogens, root-feeding insects, or soil factors such as pH or nutrient imbalances, poor drainage, or improper use of pesticides may also cause similar above ground symptoms.

The presence root-knot and cyst nematodes on soybeans can usually be diagnosed in the field by a visual inspection of the root systems. Root-knot nematodes induce the formation of characteristic swellings or galls that are easily distinguishable from the nitrogen-fixing nodules that are a normal part of the root system (Fig. 28a & 29). The presence of the soybean cyst nematode may also be detected "on the spot" if the soybean roots have been carefully removed from the soil so that the yellow or white female stages or the less visible brown cysts (Fig. 28d) remain attached to the roots. For all other known soybean-parasitic nematodes, diagnosis is based upon their recovery and identification from soil and root assays performed in the laboratory. (See section on procedures for collecting soil samples.)

Management of several of the more damaging nematode species such as root-knot and soybean cyst nematode has most often been accomplished through the use of nematode-resistant cultivars. Unfortunately, the effectiveness of resistant cultivars diminishes over time if they are continually grown in infested fields. Most states now suggest management strategies that utilize cropping sequences that include both resistant and susceptible soybean cultivars, as well as non-host crops, in the cropping scheme.

Chemical nematicides have also been used successfully to manage nematode populations. However, because of the narrow profit margin for soybeans, their expense is often prohibitive. This means control centers on non-chemical management practices such as crop rotation, deep tillage and fallow plowing which will reduce severity and incidence of nematode diseases.