Early Season Soybean Diseases Showing Up in Nebraska
|Figure 1. Fusarium (left) and Rhizoctonia (right) seedling infections. Note the difference in root injury where Fusarium is typically from the tip of the tap root and progresses upward and Rhizoctoinia is typically a lesion at or near the soil surface and the tap root will not be as affected. Lateral root injury will also occur with Rhizoctoinia, but the tap root is often still in good health. (Right photo by Jim Stack, former UNL Extension Plant Pathologist)|
June 19, 2009 Over the past several weeks many parts of the state received heavy rainfall which favors the development of seedling diseases in soybean.
In many cases, Pythium and Phytophthora have led to poor stands and damping off of plants. Both of these fungal pathogens, referred to as water molds, are favored by wet conditions. At this point in the season, no treatments are available; however, ensuring the crop doesn't become moisture stressed will help reduce problems.
If heavy rains continue, we will continue seeing more Phytophthora in some fields. This pathogen can kill plants at any stage of development. Pythium. on the other hand, typically does not kill plants much past the V5 growth stage.
Identifying the Problem
I encourage you to diagnosis the problem in your field so you can take proper management actions in the future. These would include planting resistant varieties (for Phytophthora) and using seed treatments. In some fields, the seed was treated with a fungicide and yet seedling disease is still developing. This can occur with wet conditions or when the wrong treatment or product rate is used. The most common example of a product rate issue is when mefenoxam or metalaxyl is put on at a rate too low for good Phytophthora control.
More information on product rates and management for Phytophtora can be found in NebGuide G1785 Management of Phytophthora Root and Stem Rot of Soybeans.
Fields with less available moisture that have been stressed with stormy conditions tend to have Rhizoctonia and/or Fusarium. Of the two, the most common in Nebraska is Rhizoctinia. It is a soilborne fungus found in many fields, just waiting for the right stress conditions to occur. When fields have a history of Rhizoctinia, treat seed for future plantings with a strobilurin-based fungicide. A complete list of soybean seed treatments is in NebGuide G1852, Seed Treatment Fungicides for Soybeans.
Additional information on Phytophthora and Rhizoctonia diseases of soybean can be found on the Plant Disease Central Web site.
Extension Plant Pathologist