University of Illinois Extension

University of Illinois Extension

Hort Answers

Fungal Disease

Pythium Blight [Turfgrass]
Pythium spp. [water molds]

Pythium blight symptoms.
Pythium blight symptoms.
5 (1 = very little damage 5 = plants killed)

All cultivated turfgrasses are susceptible to the various species of Pythium.

Plants Affected

Due to its appearance, the disease is also called grease spot, spot blight, and cottony blight. Symptoms include round to irregular, dark, water-soaked. "greasy" or slimy, sunken patches of matted grass, up to 6 to 12 inches wide, which develop in hot or cool, very wet, calm weather. Elongated streaks may develop following water drainage or mowing when the grass is wet. Diseased areas quickly fade from reddish brown to light brown as the grass dies. A fluffy, white to purplish gray, cobwebby mold may cover blighted grass when the air is saturated with moisture. Enzyme-linked immunosorbent assay (ELISA) kits are available to quickly and accurately identify this pathogen in the field.

Life Cycle

The disease develops very rapidly and can be severe during hot weather and on heavy, poorly drained soils.


Improve surface and subsurface drainage wherever possible. Avoid over-watering, thick thatch, excessive nitrogen fertilization, and compacted soil. Do not mow when the grass is wet. Make preventive applications of fungicides during prolonged, hot, wet weather starting when night temperatures reach 68 F. Repeat applications are needed at 5- to 21-day intervals as long as the weather stays hot and humid.

Related Resources
Home, Yard & Garden Pest Guide
Illinois Commercial Landscape and Turfgrass Pest Management Handbook
U of IL - Distance Diagnosis through Digital Imaging
U of IL - Plant Clinic