Pythium

Pythium is not a true fungus but an oomycete, sometimes called a water mould, that behaves much like one in the garden. It survives in soil and plant debris as tough resting spores called oospores, which can persist for years waiting for the right conditions. When soil becomes saturated and warm, the spores germinate and produce swimming zoospores that move through water films in the soil to find and infect roots, stems or seedlings.

On seedlings, infected tissue at the soil line turns dark, water-soaked and thin, causing the classic damping off collapse. On established plants and turf, affected roots appear brown, soft and stripped of fine root hairs, while the outer root layer can slip off easily when gently pulled between fingers, leaving a stringy inner core. In turf, Pythium blight shows up as small, irregular, greasy-looking patches that are dark green to bronze before turning brown, sometimes with a fine white or grey cottony growth visible in the early morning while dew is still present.

This pathogen is common across Australia wherever drainage is poor or irrigation is excessive, and it is most active during warm, humid weather, particularly over summer in southern states and year-round in the tropics and subtropics. Poorly drained garden beds, overwatered pots, compacted turf and low-lying areas that hold moisture after rain or irrigation are all high-risk sites.

Management relies heavily on improving drainage and avoiding waterlogging, since Pythium needs free water to spread. Avoid overwatering, especially in the evening, improve soil structure and drainage, raise mowing height on affected turf, and remove thatch or compacted layers that trap moisture. Sanitise pots and tools between uses and avoid overcrowding seedlings, since dense, poorly ventilated growth holds humidity at ground level and encourages infection.