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| Figure 15. Closeup of brown stem canker caused by Phytophthora sojae. This is the only soybean disease that will have a canker from the base of the stem up the plant. |
Several fungi can cause stem cankers on soybeans. They girdle the stems, reducing nutrient and water flow to developing pods. The amount of yield loss due to these diseases depends on how early cankers form during pod fill and the number of plants in a field that are affected.
This is the same Phytophthora that causes seedling damping off; it can attack soybean plants throughout the production season when the soil is saturated. The distinct characteristic of Phytophthora stem rot is the brown to tan discoloration from the soil line up the stem. Once the pathogen colonizes the main tap root, it begins to colonize the stem. Affected plants will have few if any roots. Although saturated soil is necessary for infection, development of the disease occurs under almost any environmental condition.
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| Figure 13. Late-season symptoms of Phytophthora root and stem rot. Note brown canker at the base of the soybean stem. | Figure 14. Late-season symptoms of Phytophthora root and stem rot. Note overall yellowing and wilting of plant. This is an advanced stage of decline. |
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