Wet conditions favor the development of seedling blights in soybeans. Seeds may simply rot in the soil or wilt and die after emergence (damping-off). Three genera of fungi (Rhizoctonia, Pythium, and Phytophthora) are responsible for most of these losses.|
Reduced early season plant growth and a pale green color of seedlings characterize Rhizoctonia seedling blight. Fields infected with Rhizoctonia are often mistakenly identified as having herbicide damage due to the unevenness of the stand. This disease is normally associated with moderately wet conditions and warm temperatures. However, it can appear in very wet fields.
The identifying feature of this disease is a small, reddish lesion at or just below the soil line on one side of the stem. This usually is not a girdling lesion and results in a sunken cankered area at the point of infection. Plants generally grow through this problem but may lose some early season growth.
Pythium and Phytophthora seed rot and seedling blights are usually most destructive during wet springs. Both of these fungi produce spores (called zoospores) that swim to plant roots, encyst, and cause infection. Seeds may simply rot in the ground, or seedlings may wilt and die after emergence, their stems becoming dark brown to black. The most obvious symptom of either Pythium or Phytophthora activity in fields is the absence of plants in low or compacted areas. Cool to moderate temperatures favor disease development. Phytophthora can reappear later in the season during the podfill stage to cause further losses on susceptible varieties. Pythium, however, is active only in the early season as a seedling blight.
Control of seedling blights is based on using good agronomic practices (well-prepared seedbed, high-quality seed, and so forth), planting resistant varieties when appropriate, and using selected pesticide applications. 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.--Joe Toman