Pythium seedling blight and root rot of soybean
Pythium species cause pre- or postemergence damping off. Infected seeds appear rotted and soil sticks to them. Infected seedlings have water-soaked lesions on the hypocotyl or cotyledons that develop into a brown soft rot. Diseased plants are easily pulled from the soil because of rotted roots.
The oomycete pathogens survive as oospores either in plant residue or in soil. Saturated soil is critical for infection for all Pythium species. In general, Pythium species that are prevalent in the northern United States infect plants at lower temperatures (50 to 60°F), and Pythium species in the South infect plants at warmer temperatures (85 to 95°F), although there are exceptions. Diseased plants are more common in no-till fields or low-lying areas of the field that are prone to flooding.
Planting in cold, wet soils should be avoided to reduce infection by Pythium species that infect at low temperatures. Where Pythium is a problem, seed treatments targeting oomycetes can provide some protection. Resistance to metalaxyl/mefenoxam has been documented.
Gallery Images: M. Chilvers, K. Wise, A. Robertson, G. Munkvold, and T. Mueller