No. 12 Article 8/June 16, 2006

More Soybean Seedling Blight Concerns

Several areas of the state have already been affected by seed rot and seedling blight, resulting in the need for replanting. Now is the time to begin looking for soybean seedling blights. Seedling blights are caused by a number of soilborne fungi that infect the roots of soybean seedlings. Wet conditions favor the development of seedling blights in soybeans. Seeds may simply rot in the soil, or the seedling itself may wilt and die shortly after emergence (damping off). Three genera of fungi (Rhizoctonia, Pythium, and Phytophthora) are responsible for most of these rots.

Seedling rot of soybean (photo courtesy of Department of Crop Sciences, University of Illinois).

Although you can typically expect to see root rot diseases such as Pythium and Phytophthora in low, poorly drained areas, they are not always specifically limited to those areas, especially taking into consideration the wet weather conditions we have experienced thus far in the growing season. The diseases have similar symptoms: brown rotted roots with an absence of secondary roots. Also, both diseases typically kill the seedling. They are undistinguishable in the field and must be identified in a lab to be separated. This is important for future management options. While fungicide seed treatments are available that effectively control both Pythium and Phytophthora, only Phytophthora can be managed by the use of resistant varieties.

Other seedling root rots to look for now are Fusarium and Rhizoctonia root rot. These two fungi cause what is termed a "dry rot." Both can cause a reddish brown canker, usually at the soil line, and a dry-type rot of the roots. Like the wet rots, the dry rots are virtually undistinguishable in the field. And to further complicate the situation, the two fungi are often found together. Both Fusarium and Rhizoctonia seedling blight are less environmentally limited than Pythium and Phytophthora. In other words, you won’t just find them in the wet spots in the field. Given optimal growing conditions, seedlings can recover fairly well from the dry rots. Fungicide seed treatments are an effective option for management.

Seedling blights of soybean. Left: Pythium damping off. Center: Healthy seedlings. Right: Red lesions of Rhizoctonia. (Photo courtesy of Department of Crop Sciences, University of Illinois).

Control of seedling blights is based on using good agronomic practices (well-prepared seedbed, high-quality seed), planting resistant varieties when appropriate, and using selected fungicide seed treatments. For a list of these seed treatments, see "Pythium Rots" in issue no. 9 of the Bulletin this year. Any practice that encourages rapid emergence of seedlings will help minimize the impact of seedling blights. Plant resistant or tolerant varieties of Phytophthora (none are available for Rhizoctonia or Pythium), and apply preventive seed or soil treatments based on the knowledge of the disease or diseases present.--Suzanne Bissonnette

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