Pythium Blight

Pythium aphanidermatum and other Pythium spp.

Cool-Season Grasses Highly Destructive Oomycete (Water Mold)

Symptoms

Greasy, water-soaked patches that collapse and mat within hours. Cottony white mycelium is visible in early morning and can spread rapidly in streaks following drainage patterns.

Initial patches are small (1–4 inches), water-soaked, and dark green to purplish
Affected areas collapse and turn tan to brown within 24–48 hours
Cottony, white to gray mycelium is visible in early morning — a diagnostic feature
Disease spreads in streaks following surface drainage, foot traffic, and mowing patterns
Can destroy large areas of turf in 24–48 hours under ideal conditions

Signs

Dense cottony white to gray mycelium covering blighted areas — most abundant and diagnostic in early morning before heat dissipates it
Mycelium often visible spreading in streaks following drainage or mowing patterns
Sporangia (lemon-shaped, transparent) and zoospores visible under a compound microscope in water mounts from infected tissue

Biology

Pythium spp. are oomycetes (water molds) that produce oospores surviving in soil and thatch for years. Sporangia and zoospores spread in free water. Infection requires free moisture on leaf surfaces and high temperatures. Disease can spread explosively under ideal conditions.

Favorable Conditions

Temperature range: 85–95°F days, >68°F nights

Daytime temperatures above 85°F with night temperatures above 68°F
Relative humidity above 90% or prolonged leaf wetness
Poor drainage, low-lying areas, compacted soils
High nitrogen fertility promoting lush growth
Overseeded thin stands in late summer

Cultural Practices

Improve surface and subsurface drainage — avoid low-lying wet areas
Avoid evening irrigation; water early morning only
Reduce nitrogen inputs in summer — limit lush growth
Ensure adequate air circulation; remove obstacles blocking airflow
Avoid mowing infected areas — clean equipment before moving to healthy turf
Topdress to improve surface drainage on putting greens

Related Reports

No published reports yet for this disease.

Reports will appear here as they are peer-reviewed and published.