Pythium belongs to the Oomycetes, a separate group of microorganisms and is therefore not a real fungus. It survives as oospores, resting spores which are resistant to unfavourable conditions, especially dehydration. Germination of the oospores is stimulated by exudates from roots of a susceptible host nearby. The oospores germinate and form so-called sporangia from which subsequently zoospores are produced. Zoospores are spores with flagellae, which enable the spores to move through water. The zoospores are chemically drawn to the growing roots of young plants and move towards them through the soil water. They infect the roots through small wounds, where the side roots are formed or through other similarly damaged spots.
On the infected plant, new sporangia with zoospores are formed, that allow the disease to spread to neighbouring plants. The zoospores disperse via water. Therefore, this disease spreads most rapidly in wet conditions with a high (>70%) water content of the soil or substrate. Oospores are dispersed when soil particles are moved around by humans or machinery. Pythium can survive very well in the soil or substrate in the absence of plants. Many species are entirely saprophytic, so these do not harm plants.