Brown patch is a common fungal disease that targets St. Augustine grass, caused by the soil-borne fungus Rhizoctonia solani. This pathogen thrives in high humidity and moderate temperatures, becoming active when warm-season grasses are entering or exiting dormancy, typically in the fall and spring. Correctly identifying and treating this infection is the first step toward restoring turf health.
Confirming the Presence of Brown Patch
Accurate identification is essential because Brown Patch symptoms can be mistaken for other turf issues. The disease typically manifests as circular or irregularly shaped patches of discolored grass, ranging from one to ten feet in diameter. These patches are often initially yellow, progressing to reddish-brown, and finally becoming straw-colored as the leaf blades die.
A key diagnostic feature in St. Augustine grass is observing the leaf blades at the edge of an active patch. The fungus primarily attacks the leaf sheath, the part of the blade closest to the soil line. If you gently pull on the affected blade, it will separate easily from the runner (stolon) due to a black or dark rot at the juncture.
The stolons and roots of the grass remain largely unaffected, allowing for quicker recovery once conditions improve. Another symptom visible in the early morning is a distinct, dark, water-soaked border, often called a “smoke ring,” at the perimeter of the patch. The disease develops most rapidly when air temperatures are between 75 and 85 degrees Fahrenheit and the grass foliage remains wet for extended periods.
Active Treatment Strategies for Current Infections
Once Brown Patch is confirmed, immediate action is necessary to halt the disease progression. The most direct method involves applying turf-specific fungicides, which interrupt the life cycle of the Rhizoctonia solani fungus and prevent spreading.
Both systemic fungicides (absorbed by the plant) and contact fungicides (remaining on the leaf surface) are used for control. Effective active ingredients include azoxystrobin, propiconazole, and myclobutanil. It is important to rotate between different chemical classes, or Fungicide Resistance Action Committee (FRAC) groups, with repeated applications to prevent resistance.
Fungicides should be applied as soon as symptoms are noticed, following label directions precisely for application rates and intervals (often 14 to 28 days). Applying the product in the early morning ensures absorption before temperatures rise. Immediate cultural adjustments must also be made to stress the fungus.
During an active outbreak, avoid all nitrogen fertilizer applications, as nitrogen promotes the soft leaf growth the fungus attacks. Irrigation should be immediately reduced or temporarily stopped to allow the soil and grass blades to dry out thoroughly. The fungus requires a prolonged period of moisture, often 48 hours or more of leaf wetness, to thrive.
Cultural Practices for Long-Term Prevention
Preventing future Brown Patch outbreaks requires modifying the lawn environment to make it less hospitable to the fungus. The most significant factor to control is moisture, managed through correct watering practices. St. Augustine grass should be watered deeply and infrequently, aiming for about one inch of water per week, rather than short, frequent sprinklings.
The best time to water is in the early morning, preferably before 10 AM, to ensure the grass blades dry completely before nightfall. Watering in the late afternoon or evening extends leaf wetness, creating an environment for fungal growth. Improving air circulation by pruning back nearby shrubs or trees also reduces humidity around the turf, helping the foliage dry.
Mowing practices also play a role in prevention by maintaining the grass’s overall health. St. Augustine grass should be maintained at a height between 2.5 and 4 inches to reduce stress on the plant. Mowing with a dull blade or cutting wet grass can injure leaf tips, providing entry points for the pathogen.
Fertilization management is another preventive tool, as the timing of nitrogen application affects disease severity. High nitrogen applications, especially in the late summer or early fall when moisture is often high, can fuel the fungus. Use slow-release nitrogen sources or a balanced fertilizer outside of peak disease periods to maintain steady growth without creating susceptible turf.
Finally, managing the soil environment helps reduce the fungus’s ability to survive and spread. Excessive thatch, the layer of undecomposed organic matter between the soil and the grass blades, retains moisture and harbors the fungus. Periodic core aeration or mechanical dethatching improves soil drainage and reduces the thatch layer, decreasing the likelihood of future infections.