Seedling blight occurs after seedlings emerge. Seedling blight by Pythium is very similar to that by Phytophthora in terms of symptoms. One normally cannot separate the two without further laboratory tests. When seedling blight occurs, dead seedlings are visible on the ground. Infected plants dead before true leaf stage will have a rotted appearance. If leaves are present, infected seedling leaves will have a gray-green color before turning brown. A few days later, the plants die and have a rotted appearance. Diseased plants are easily pulled from the soil because of rotted roots. Seedling blight by Phytophthora can be differentiated from Pythium after V2 growth stage or later. Plants infected by Phytophthora have a brown discoloration extending from root up the stem. The two fungi attack soybeans in different temperature regimes. Soybeans planted in cold, wet soil are most likely to be attacked by Pythium. If disease occurs in warm conditions (around 80F), it is more likely caused by Phytophthora.
Seedling blight by Rhizoctonia also can cause seedling diseases. Seedling disease by Rhizoctonia is different from those caused by Pythium and Phytophthora. Unlike Pythium and Phytophthora damping-off, stem discoloration by Rhizoctonia is usually limited to the cortical layer of the main root and hypocotyl. Infected stems remain firm and dry. Typical symptoms are localized brown to reddish brown lesions on the hypocotyl and lower stem that do not extend above the soil line. The reddish brown color is a key symptom in diagnosing the disease. Seedling blight by Rhizcotonia normally appears as the weather becomes warm (80F) and it is more often seen in late planted soybean fields.
Cool early May temperatures have contributed to good stands in emerged soybean fields and no replanting has been reported, although seedling blight has been observed.
If stand reduction happens in a soybean field that needs replanting, producers should determine if a fungal disease is involved – before replanting. A fungicide seed treatment may be needed for damping-off caused by fungal pathogens. Identification of seedling disease is essential in fixing the problems as different fungicides are effective in controlling different seedling diseases. The information is also useful for seed treatment of next year’s soybean.