• the soybean acreage is increasing and, consequently, soybeans are planted more frequently in specific fields, and
• many races of Phytophthora exist in Wisconsin soils.

The disease is caused by the fungus Phytophthora sojae (syn.=Phytophthora megasperma f. s.p glycinea). The Phytophthora fungus can kill plants at all stages of growth, or infected stands may survive but are less productive than healthy stands. Infection by Phytophthora sojae generally occurs in fields with poor internal drainage, but it can occur in normally well-drained fields that are saturated for 7-14 days due to excessive precipitation or irrigation.

The nodules formed by beneficial nitrogen-fixing bacteria are often destroyed and the plants become yellow, partially due to N deficiency. The soybean variety, the race(s) of the Phytophthora fungus present, and the growing conditions will determine whether the killing stem rot phase or the root rot phase will be most prevalent.

This photo shows a stem lesion progressing from the soil line -- a key symptom of infection by the fungus Phytophthora sojae. Photo © Craig Grau