Several fungi infect soybean seeds, reducing seed quality as well as seed germination. Diseases affecting soybean seed are a major issue for producers of seed and food-grade soybeans.
This disease is caused by several fungi in the Diaporthe/Phomopsis complex, but the primary fungus is P. longicolla. These fungi are present in all production fields in Ohio. Seed decay symptoms can be seen as the pods begin to mature. Severely infected seeds are shriveled, cracked, and appear whitish and chalky. Severely infected seeds will not germinate because the fungus has killed the embryo. When the fungus has only infected the seed coat, some soybean seeds will germinate but become diseased later.
Seed lots with germination of less than 70% should not be used as seed. Treatment of seed with fungicides will kill the fungus when it is present only in the seed coat, allowing for good germination. Seeds that contain infected embryos will not germinate.
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| Figure 43. Symptoms of green-stem syndrome combined with Phomopsis pod blight on soybean. Late season rains promoted development of Phomopsis, small black dots on pods along with extensive feeding by bean leaf beetle. Green stem can develop as a result of poor pod set or soybean virus infection such as bean pod mottle virus. | Figure 44. Phomopsis seed rot. This seed disease is the result of late season rains after seeds have matured. |
This fungus can be a major pathogen of badly weathered or physiologically damaged seed or in seed pods damaged by insect feeding. Alternaria can contribute to reduced germination. We often find this as an inhabitant of the seed coat. Following a drought, many seed samples were identified with Alternaria seed rot.
Purple or pink streaks on the seed coat are the primary symptom of the seed pathogen, Cercospora kikuchii. Discoloration can range from streaks to large blotches. The fungus overwinters on soybean straw in the field. Spores are splashed onto cotyledons and stems. The fungus grows into the pod and spreads through the hilum. This disease does not reduce yield but does reduce seed quality. It does not reduce germination, but seeds infected with C. kikuchii may produce weak seedlings.
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| Figure 45. Purple seed stain caused by Cercospora kikuchii. |
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