Plant Pathology: Extension: Plant Disease Library

Pythium

Important diseases:  Damping-off, Cottony blight on turf, Cottony leak of cucurbits.

Pythium is the most important damping-off fungus. It also causes seed rots, root and crown rots, and soft rots. It has an extremely wide host range. Pythium is a water-mold and is favored by saturated soil conditions and high soluble salts.

The classic "damping off" symptom is the collapse of stems at the soil line causing the plant to fall over. Black lesions may be present on affected stems. Rots are watery and slimy in nature. Affected roots are soft and honey-brown in color. A diagnostic feature of Pythium infection is the sloughing of the outer root cortex leaving only the thread-like root stele intact.

Pythium produces fine, white, profusely branching, aseptate (without cross walls) mycelium. Sporangia are produced on the ends or sides of the hyphae (fungal thread-like material). Thick, double-walled oospores (overwintering or resting spore) also are produced. Oospores and sporangia can be seen within infected plant tissue. If tissue is stained with acid fuchsin, spores will stain pink. Pythium can be easily confused with Phytophthora.