Seedling Soybean Diseases Continue in Replant
It seems to be late in the year to be talking about seedling diseases, but this year is a little different. In the last week we have had several contacts on seedling disease problems in replant soybean. In addition, some fields also may have stand issues related to disease that are being affected by stormy weather patterns that injure the plants. This results in stress that lowers the plant's defenses, triggering infection by our common seedling disease pathogens (Fusarium, Pythium, Phytophthora, and Rhizoctonia).
Over the last couple weeks many parts of the state have experienced stormy conditions and/or heavy rainfall which favors seedling diseases in soybean. Earlier in the year when it was cooler, Pythium was the main cause identified in problem fields. As soil temperatures have warmed, I suspect that conditions are more favorable for Phytophthora to be the cause in really wet fields.
Phytophthora can kill plants at any stage of development, but Pythium typically does not kill plants much past the V5 growth stage. Fields that have had hail or wind damage could be affected by Rhizoctonia and /or Fusarium if they are not in heavy moisture areas. These two are usually a more common problem in well-drained soil types.
As replanting usually occurs with warmer soil temps, it is often not recommended to treat the seed as emergence occurs rapidly. The problem with some fields is that continuous rain has occurred since replanting and the extremely wet soil conditions favor the fungus more than emergence. If you are considering any replant at this point (even though it is really late), I would suggest a seed treatment if you have had a history of stand issues during wet conditions. If you're planting in a field that has never had a stand problem, treatment is usually not needed.
In any situation where you have had a seedling disease/stand problem it is important to get it identified correctly. Field identification can be difficult and many symptoms are similar among seedling diseases. I encourage you to get a diagnosis of what the problem is in your fields so that proper management actions can be taken in the future.
Seed treatment and the use of resistant varieties (for Phytophthora) are the management actions which are modified based on the field history. In fields where a seed treatment fungicide was used and seedling disease is still developing, this can be due to the wrong treatment being used or excessive moisture leading to reduced product performance under extreme conditions. This can be the case with very wet conditions.
More information on Phytophthora management and product rates is in NebGuide G1785, Management of Phytophthora Root and Stem Rot of Soybeans.
For more information on diagnosing and managing crop plant diseases found in Nebraska see the Plant Disease Management section of CropWatch.
Extension Plant Pathologist