Brown Patch is a turfgrass disease caused by the fungus Rhizoctonia solani, affecting both cool-season grasses like tall fescue and perennial ryegrass, and warm-season varieties such as St. Augustine and Zoysia. This soil-borne pathogen causes distinct damage that is often confused with drought stress or other lawn issues. Correctly identifying the visual characteristics of Brown Patch is the first step in managing its spread and preserving the health of a lawn. The disease becomes most active during periods of high heat and humidity, which makes recognizing its unique appearance especially important during the summer months.
General Characteristics of the Patches
The most noticeable symptom of Brown Patch is the formation of large, roughly circular areas of damaged grass across the lawn. These patches typically range in size from a few inches to several feet in diameter, sometimes reaching up to 10 feet across. The affected turf within these spots appears thinned out, with the grass blades turning a light brown or straw color.
A specific identifying feature, particularly on closely mowed grass, is the presence of a “smoke ring.” This is a thin, dark gray or purplish border at the outer margin of the patch, separating diseased grass from healthy turf. This ring is composed of active fungal growth and is most easily observed early in the morning when the grass is covered with dew. The smoke ring often disappears as the turf surface dries out.
Identifying Symptoms on the Grass Blade
While the patches are visible from a distance, a definitive diagnosis requires a close inspection of the individual infected grass blades, especially those along the margin. The fungus primarily targets the leaf sheath, the lower part of the blade closest to the soil surface. On wider-bladed grasses, distinct lesions develop, appearing as tan or light brown spots with dark brown or purplish borders.
These lesions are often described as having an irregular shape. The tips of the grass blades may remain green even as the lower parts of the plant are severely damaged. In the early morning, fine, cottony strands of fungal mycelium may also be visible woven between the affected grass blades. This web-like growth confirms the active presence of Rhizoctonia solani.
How Brown Patch Differs from Other Lawn Issues
Distinguishing Brown Patch from other common lawn problems is based on the unique visual signature of the fungus. Drought stress, for example, results in a more uniform browning or a general bluish-gray cast across large sections of the lawn, not distinct circular patches. A drought-stressed lawn feels dry and crunchy underfoot, and the grass blades curl inward.
Brown Patch also differs significantly from Dollar Spot, another fungal disease that creates small, sunken spots. Dollar Spot patches are much smaller, usually only the size of a silver dollar or up to six inches across. The lesions on individual blades are bleached-white or tan with a distinct hourglass shape. Insect damage, such as from grubs, creates irregular patterns of browning, and the damaged turf can be easily lifted from the soil because the roots have been eaten. The presence of the dark “smoke ring” and the large scale of the patches are the most reliable visual cues.