Source: University of Illinois
Phytophthora root and stem rot of soybean can occur when soil conditions are wet and the soil temperature is above 65ºF. The disease is caused by a soilborne pathogen known as Phytophthora sojae. Symptoms can be observed as wilting of plants, root systems with dark lesions that may be rotted, dark stem lesions extending up from the soil line, and seedlings that have damped-off.
Wet soil conditions are required for Phytophthora rot to be severe, because the pathogen produces spores (zoospores) that must "swim" through the soil to infect soybean plants. The most severely affected areas in a field generally are those that tend to hold moisture for extended periods (low areas, areas with higher clay content, etc.). Infection can take place throughout the season, and adult plants can be killed in severe conditions.