Brown Patch is a widespread fungal disease caused by the soil-borne fungus Rhizoctonia solani. It attacks turfgrass when the environment is warm and saturated with moisture. While the fungus does not typically kill the grass crown or roots, allowing the lawn to recover, it causes significant cosmetic damage and weakens the turf. This condition is both treatable and preventable.
Identifying Brown Patch Symptoms
A lawn affected by Brown Patch displays roughly circular areas of damaged, thinning grass, ranging from a few inches up to three feet in diameter. Individual grass blades within these patches often show distinct, irregular lesions with tan centers and dark brown or purplish borders. This unique pattern helps distinguish Brown Patch from other lawn diseases.
The fungus thrives when nighttime temperatures remain above 65°F to 70°F and daytime temperatures reach 80°F to 90°F, coupled with high humidity. For infection to occur, turfgrass leaves must be continuously wet for 10 to 12 hours. In the early morning, a dark, grayish-brown border known as a “smoke ring” may be visible around the patch’s perimeter, representing the actively spreading edge of the fungus.
Immediate Steps for Disease Control
When Brown Patch is confirmed, immediately eliminate the moisture source fueling the fungus. Stop irrigation in affected areas for several days to allow the soil and grass canopy to dry out thoroughly. Limiting foot traffic on diseased patches is also important because movement can physically spread the fungal mycelium to healthy sections of the lawn.
The application of a curative fungicide is the most effective way to halt the active spread of the disease. Products containing active ingredients like azoxystrobin or propiconazole are commonly used. A higher application rate than a preventative one is often necessary to address the existing infection. Liquid spray applications often provide better coverage than granular products, but both require careful adherence to label instructions for timing and reapplication.
Adjusting Cultural Practices to Prevent Recurrence
Preventing recurrence requires modifying the lawn environment to make it inhospitable to the fungus. Proper irrigation timing is a significant change, requiring watering deeply and infrequently between 4:00 am and 10:00 am. Watering during this window allows the grass blades to dry completely as the sun rises, minimizing the extended periods of leaf wetness the fungus needs to survive.
Mowing practices also play a major role in prevention. Raise the mowing height to the upper recommended range for the grass type, such as 2.5 to 3.5 inches for tall fescue. Taller grass encourages deeper root growth and helps the plant better withstand disease stress. Using a sharp mower blade ensures a clean cut, as dull blades create open wounds the fungus can more easily penetrate.
Addressing soil composition and air circulation is important. Excessive thatch, a layer of organic debris thicker than a half-inch, retains moisture and harbors the fungal pathogen. Correcting compacted soil through core aeration improves both drainage and air movement, which reduces the humidity at the turf’s surface.
Fertilization strategy must also be adjusted. Avoid high rates of quick-release nitrogen during the peak disease period of late spring and summer. High nitrogen encourages soft, succulent new growth that is highly susceptible to infection. Lawns should be maintained with moderate nitrogen levels, preferably from slow-release sources, to promote steady, resilient growth without creating an overly vulnerable canopy.