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Symptoms and Signs

High population densities of the soybean cyst nematode (SCN) can result in large portions of soybean fields with plants that are severely stunted and yellow (Figures 1 and 2). More frequently, however, few aboveground symptoms of SCN can be observed - yet soybean yield losses of 10-20% or more can be attributed to SCN damage in these fields (Figure 3). This hidden cause of yield loss has presented a major problem for growers who must learn to recognize when their fields are infested with SCN. Reduced yields in SCN-infested fields often are attributed to adverse cultural or environmental conditions rather than to SCN - a tragic mistake that leads to increased SCN infestation and damage in subsequent years. Also, the age and vigor of the soybean plants, the nematode population density in the soil, soil fertility and moisture levels, and other environmental conditions influence the intensity of the symptoms. Soybean cyst nematode damage usually is more severe in light, sandy soils, but will occur readily in all types of soil.

Figure 1	Figure 2
Figure 3

The whitish-yellow adult females of SCN on the outside of roots dug from the field are a telltale sign of SCN infestation that can be easily seen (Figure 4). The SCN females are lemon-shaped, slightly less than 1 mm in diameter, and considerably smaller than Bradyrhizobium root nodules (Figure 5). Reduced nodulation was associated with infection of soybean roots by populations of SCN identified as race 1. Analysis of soil and root samples by a diagnostic lab, however, is the only reliable means to confirm SCN diagnosis. The vermiform (worm-shaped) juvenile and adult males can be extracted from soil samples, as well as the brown cysts (dead female body containing eggs) and freed eggs in the soil. Developing SCN juveniles and females can be observed in stained soybean root samples. Diagnostic labs also can estimate the population density of SCN in a field to provide recommendations of SCN management options. Determination of the race of SCN in a field is a laborious task, and it may or may not be helpful in choosing a soybean cultivar for that site.

Figure 4

Figure 5

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