Pythium; Damping-off is the first seedling disease to occur in a growing season because this fungus prefers cold soil temperatures. Dead seedlings may be visible on the ground with infected plants killed before the first true leaf stage. Plants often have a rotted appearance. Leaves of infected seedlings are initially gray-green and then turn brown. A few days later, the plants die. Diseased plants are easily pulled from the soil because of rotted roots.
Phytopthora; The symptoms of Phytophthora damping-off are very similar to those of Pythium damping-off. However, the Phytophthora fungus prefers warm soil (approximately 80°F). If symptoms similar to those caused by Pythium occur when soil temperatures are warm or in late-planted soybean, the disease is more likely caused by Phytophthora. Another way to determine the causal fungus is to check for stem rot. In June or early July, if weather is favorable, Phytophthora infection may continue to develop on the soybean stem, resulting in chocolate brown discoloration from the soil line up, a unique symptom of this disease.
Rhizoctonia root rot; Similar to Phytophthora damping-off, seedling blight by Rhizoctonia normally appears when the weather becomes warm. However, disease caused by Rhizoctonia exhibits different symptoms from those caused by Pythium and Phytophthora. Stem discoloration by Rhizoctonia is usually limited to the cortical layer of the main root and hypocotyl. Infected stems remain firm and dry without rot appearance typical to Pythium or Phytophthora rot. Typical symptoms are localized brown-to-reddish brown lesions on the hypocotyl at the soil line.