Brown Patch on Lawns

Dr. John F. Baniecki
WVU Extension Service
Plant Pathology and
Entomology Specialist
August 2000

Outbreaks of this disease caused by the fungus, Rhizoctonia solani, occur from June through September and are dependent upon the presence of a susceptible host and favorable climatic conditions. Greenskeepers often speak of these climatic conditions as "Brown Patch Weather." Certain cultural practices tend to increase the severity of brown patch. These conditions are:

1. late evening watering, especially on poorly-drained soil,
2. too liberal fertilization with nitrogenous fertilizers,
3. close mowing, and
4. leaving clippings on the lawn.

Symptoms are circular areas six inches or greater in diameter, usually first appearing as wilted areas in early morning. The spread may stop during the day as the dew dries on the leaves, but resumes growth during the night. As the disease spreads, its growth is marked by a "smokey ring" of blackened grass leaves surrounding the large central areas of light-brown color. Generally, only the lower grass blades are affected. Lawns that are wet at nightfall are much more likely to be attacked than dry ones, and the spread of the disease is more rapid in warm weather when the fungus is actively growing both day and night. When the average air temperature reaches 73^°F, the fungus penetrates leaves through mowing wounds. After penetration, it remains relatively inactive until the day temperatures rise above 80^°F and night temperatures remain above 70^°F. If coupled with high humidity, the fungus can blight a lawn in 6 to 8 hours. It ceases activity when the temperature reaches 90^°F. Moisture on grass leaves greatly increases the rate at which the fungus spreads.