After Labor Day, some soybeans started to turn yellow, especially those in maturity groups II or earlier. However, you also may have observed premature defoliation in soybean fields planted with later maturity groups, such as group III or later. Typically, only some areas within fields have defoliated plants; in the rest of the field, the plants are still green. Such defoliation can be caused by abiotic and biotic factors such as diseases. If defoliation is caused by disease, infected areas offer a good opportunity to spot the disease, which can be helpful in future prevention.
Iowa State University
Yang, X. B., "Premature defoliation in soybean" (2000). Integrated Crop Management News. 2020.
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