Phytophthora sojae is an Oomycete which causes a serious root and stem rot of soybeans. The pathogen can survive for long periods in infected plant tissue or soil as sexual spores called oospores. Asexual reproduction results in the formation of ovoid sporangia that are produced in abundance on rotten root tissues when the soil is flooded. Motile spores called zoospores are formed within the sporangia. Sporangia formation and zoospore release can occur within a few hours in flooded soils. The zoospores lack cell walls and swim by means of two flagella of unequal length. The longer whiplash flagellum is directed forward and the shorter tinsel flagellum extends behind.

Following their release, the zoospores swim in a helical path. The zoospores of P. sojae are attracted by chemicals (the isoflavones daidzein and genistein) released by soybean roots and germinating seeds. Once the zoospores reach the soybean root they adhere to the surface, shed their flagella, and form a cell wall (encyst). The encysted zoospores then germinate and the hyphae penetrate into the root and begin rotting the tissue.