Symptoms and Signs

Key points to know

Stunting and yellowing are above-ground symptoms of SCN.
Early senescence of fields is an indirect above-ground symptom of SCN.
There can be serious yield loss due to SCN without any obvious above-ground symptoms.
Root stunting, discoloration, and fewer nodules are below-ground symptoms of SCN.
You can see SCN females with the naked eye.

1. Stunting and yellowing can be above-ground symptoms of SCN

Mid-season yellowing of SCN-infected soybeans

Mid-season yellowing

Photograph of soybeans that have yllowed in July due to SCN infection

Yellowing in July due to SCN infection

Photograph of severe stunting in soybeans due to SCN infection

Severe stunting and yellowing

Closer photograph of severe stunting and marginal leaf yellowing due to SCN infection

Severe stunting and marginal leaf yellowing characteristic of potassium deficiency

Aerial photograph of SCN damage

Aerial view of severe damage

Aerial photograph of SCn damage

Another aerial view of damage

2. Early senescence or maturation of the crop can be an indirect symptoms of SCN

The aerial image in the bottom of the figure below was taken on September 10. The image shows an M-shaped area of the end of the field turning yellow and brown due to the soybeans maturing (senescing).

The map at the top of the figure contains the results of spring grid-sampling of soil in the area of the field shown in the image. There is an M-shaped pattern of higher SCN population densities (darker-colored squares) that corresponds to the M-shaped area of plants maturing early.

Photograph of distribution of SCN infection rates compared to aerial photographs of fields

3. In the Midwest, SCN often does not cause obvious above-ground symptoms, at least not until population densities (numbers) build to high levels. Substantial yield loss can occur in the absence of symptoms.

Photograph of healthy-looking soybeans in SCN-infested fields

Healthy-looking soybeans in SCN-infested fields

The field map below shows SCN population densities (left), areas of visible yellowing (center) and soybean yield (right) in 100 half-acre cells in an SCN-infested field in central Iowa. Areas of obvious symptoms do not match up with highest SCN population densities or lowest soybean yields.

Field map comparing population densities, areas of visible yellowing, and soybean yield

In a field experiment conducted in central Iowa in the 1990s, a resistant and a susceptible soybean variety were grown in an SCN-infested field, and plants were removed and measured every 2 weeks throughout the growing season.

Plant height of the resistant and the susceptible soybean varieties, essentially the same

Canopy weight of resistant and susceptible soybean varieties, essentially the same

The plant height of the resistant and the susceptible soybean varieties were essentially identical throughout the growing season.

The canopy weight (green leaf tissue) was nearly identical for the resistant and the susceptible soybean varieties growing in this SCN-infested field until the last month of the season, when there was a little more leaf tissue on the resistant plants than the susceptible plants.

Reference:
Soybean Cyst Nematode Reduces Soybean Yield Without Causing Obvious Aboveground Symptoms
Wang, J., T.L. Niblack, J. A. Tremain, W. J. Wiebold, G. L. Tylka, C.C. Marett, G. R. Noel, O. Myers, and M. E. Schmidt.
2003. Plant Disease 87:623-628.

Read publication in pdf format»

SCN resistant soybeans produce 10% greater yield than susceptible varieties

Despite no difference in height throughout the season and no difference in canopy weight until the last month of the season, the resistant soybean variety produced 10% greater yield than the susceptible soybean variety.

4. Root stunting, discoloration, and fewer nodules are below-ground symptoms of SCN

The plants on the left below were inoculated with the nitrogen-fixing bacterium, Bradyrhizobium japonicum ("Rhizobium"), but not with SCN. The plants on the right were inoculated with the nitrogen-fixing bacterium and also SCN. The plants were then grown in a greenhouse for 62 days.

Comparison photograph between soybean roots that are and are not infected with SCN

The SCN-infected roots on the right are smaller (stunted), discolored, and have very few nitrogen-fixing nodules compared to the roots not infected with SCN on the left.

5. The adult females of SCN can be seen on roots without magnification.

SCN females

Bottom pointer = N-fixing nodule
Top pointer = SCN females