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How to Manage Pests

The UC Guide to Healthy Lawns

Pythium blight (Grease spot) — Pythium spp.

Turf killed by Pythium blight
Photo by Ali Harivandi

Turf with Pythium blight
Photo by A. H. McCain

Click on images to enlarge.

Hosts

All turfgrass species

Symptoms

Pythium blight infestations start out as small, circular spots of dead grass that run together as the infection spreads. When it is hot and conditions are favorable, this disease can spread rapidly in a 24-hour period. Infected leaf blades blacken and rapidly wither and later turn reddish-brown. Leaves lie flat, stick together, and appear greasy. Masses of white, cottony mycelia may appear. Pythium blight occurs in parts of the lawn where it is wet and often runs with the pattern of water drainage.

Conditions favoring disease

Pythium blight tends to develop in low spots that remain wet. Temperatures between 80° and 90°F favor disease development. High nitrogen levels also contribute to the disease.

Prevention and management

The key to preventing and managing Pythium blight is proper irrigation. Avoid overwatering and improve soil aeration and drainage. Do not mow when the leaf blades are wet and avoid applying excess nitrogen during hot, humid weather. Reduce shading. Fungicides are available but are usually not warranted in home lawns.

For more information on lawn diseases, refer to:
Pest Notes: Lawn Diseases: Prevention and Management


Statewide IPM Program, Agriculture and Natural Resources, University of California
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Agriculture and Natural Resources, University of California

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