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
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Mid-season yellowing
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Yellowing in July due to SCN infection
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Severe stunting and yellowing
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Severe stunting and marginal leaf yellowing characteristic of potassium deficiency
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Aerial view of severe damage
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Another aerial view of damage
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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.
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Source: Niblack et al. 2006. Annual Review of Phytopathology 44: 283-303.
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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.
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Healthy-looking soybeans in SCN-infested fields
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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.
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| 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. |
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The plant height of the resistant and the susceptible soybean varieties were essentially identical throughout the growing season.
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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.
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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 abstract»
Read publication in pdf format» |
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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.
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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.
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| 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. |
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5. The adult females of SCN can be seen on roots without magnification.
