Pythium irregulare causes damping-off and root rot of plants. P. irregulare has hundreds of plant hosts and infects under moist conditions that favour the movement of zoospores. Infections can occur under field and greenhouse conditions.

The pathogen grows and colonizes a plant by producing hyphae. When hyphae from opposite mating types meet, they fuse and produce oospores, which serve as overwintering structures. Upon germination, an oospore either forms hyphae or develop a zoosporangium, which produces motile zoospores that swim and infect plants. When reaching the root cortex, zoospores form cysts that then germinate, infect and invade the plant root. The infection progresses with the growth of hyphae. (Purdue Extension)