Pythium irregulare favours wet conditions and is most prevalent in areas with annual rainfalls above 350 mm. The pathogen can survive dry conditions or winter conditions as thick-walled oospores. There is also a greater incidence of P. irregulare in reduce tillage systems, soils with higher organic mater and acidic-neutral soils.

Worldwide

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)

P. irregulare is a water mold organism (fungal-like protist).