St. Augustine grass is a highly favored warm-season turf, but its lushness makes it susceptible to fungal diseases, especially in hot, humid environments. These pathogens can quickly damage or destroy large sections of a lawn. Chemical intervention with fungicides is often necessary to stop an active outbreak, but the best approach involves identifying the disease and integrating chemical treatment with proper lawn care. This article guides managing the most common fungal threats to St. Augustine grass.
Identifying Common Fungal Threats
Correctly diagnosing the fungal disease is necessary before treatment, as different pathogens require distinct chemical and cultural strategies. Three diseases account for the majority of severe damage in St. Augustine lawns: Brown Patch, Gray Leaf Spot, and Take-All Root Rot. The symptoms and timing of these infections vary significantly, making visual diagnosis a primary step in management.
Brown Patch (Large Patch) is typically active during the cooler transition seasons of spring and fall when nighttime temperatures are consistently between 65°F and 80°F. It creates circular or irregular patches of thinned, light brown grass, often ranging from a few inches to several feet in diameter. Inspection of the leaf sheath reveals a dark, rotting lesion, which allows the leaf to easily pull away from the stolon.
Gray Leaf Spot is a foliar disease that peaks during the hottest months (June through August) under high humidity and temperatures between 82°F and 90°F. Symptoms first appear as small, brownish spots on the leaf blades. These expand into oval or diamond-shaped lesions with tan or gray centers and distinct dark brown or purple borders. Severe infections cause the grass to look scorched, leading to widespread thinning of the turf.
Take-All Root Rot (TARR) is difficult to identify because it attacks the root system and stolons rather than the leaf blades. The disease is most active when soil temperatures are between 60°F and 75°F, typically during spring and fall. Above-ground symptoms are non-specific, appearing as irregular, thinned patches of yellowish grass that can be mistaken for nutrient deficiency or drought stress. A definitive sign is the short, black, and brittle roots, which make it easy to lift the infected stolons out of the soil.
Targeted Fungicide Recommendations
Selecting the most effective fungicide requires matching the active ingredient to the specific disease and ensuring the product is labeled for residential use on St. Augustine grass. The best approach is a rotational strategy using different chemical classes to prevent the fungus from developing resistance. Fungicides are categorized by their active ingredients, and combining or rotating these ingredients provides the most powerful chemical defense.
For Brown Patch control, the most recommended active ingredients are azoxystrobin and propiconazole. Azoxystrobin offers excellent preventative control with a residual period of up to 28 days, making it ideal for scheduled applications before the disease season begins. Propiconazole is used for its curative properties, acting quickly to halt the spread of an active infection, though it may require reapplication every 14 to 28 days. Products containing both ingredients, like Headway G, provide broad-spectrum control and manage overlapping disease pressure.
Gray Leaf Spot management relies on active ingredients highly effective on foliar diseases, such as pyraclostrobin, azoxystrobin, and thiophanate-methyl. Pyraclostrobin and azoxystrobin are recommended for their efficacy, often providing control for up to 28 days. Thiophanate-methyl is a reliable alternative for rotation, providing protection for about 14 days. Rotation between these active ingredients is necessary to maintain effectiveness, especially in hot, humid summer conditions that favor the disease.
Take-All Root Rot is difficult to control with chemicals alone, but systemic fungicides help suppress the soilborne pathogen, Gaeumannomyces graminis. Azoxystrobin and propiconazole, the same ingredients used for Brown Patch, are recommended for TARR, as is flutolanil. Because this fungus attacks the roots, applications must be made preventatively in the spring and fall when soil temperatures favor fungal growth. The application must be watered in immediately to ensure the active ingredient reaches the root zone.
Proper Application and Timing
Effective fungicide application depends on precise timing and technique, which varies based on the target disease. For all applications, calculate the square footage of the treatment area and calibrate the spreader or sprayer to ensure the product is applied at the correct label rate. Liquid applications often achieve better coverage than granular products, but granular options are easier for homeowners to apply with standard spreaders.
Fungicides work best when applied preventatively (before symptoms appear) or curatively (at the first sign of disease). For foliar diseases like Gray Leaf Spot, the fungicide must remain on the leaf surface. Application should occur when no rain is expected for at least 24 hours. For maximum absorption, apply the product early in the morning after the dew has dried but before temperatures become excessive.
Conversely, for soilborne diseases like Take-All Root Rot, the fungicide must be immediately watered into the soil using a half-inch of irrigation. This ensures the active ingredient moves down to the root zone where the fungus is active. Reapplication intervals are generally between 14 and 28 days, depending on the product’s residual activity and the severity of the disease pressure. Following the label’s reapplication window maintains a consistent protective barrier against the pathogen.
Cultural Practices for Long-Term Health
While fungicides offer a rapid solution, long-term health depends on implementing cultural practices that make the turf less hospitable to fungal pathogens. The most important practice is managing moisture, as most lawn fungi thrive in extended periods of leaf wetness. Water deeply and infrequently, scheduling irrigation for the early morning. This allows the grass blades to dry completely before nightfall. Avoid late evening or nighttime watering, which leaves the foliage wet for many hours.
Mowing practices play a significant role in disease prevention and management. St. Augustine grass should be maintained at a taller height, typically between 3 and 4 inches, promoting deeper root growth and overall turf health. Never remove more than one-third of the leaf blade in a single mowing session, as excessive cutting stresses the plant. When a disease is active, bag the clippings to avoid spreading fungal spores across the lawn.
Fertilization must be carefully managed because excessive nitrogen promotes the lush, tender growth preferred by pathogens like Brown Patch and Gray Leaf Spot. Use slow-release forms of nitrogen, and avoid high nitrogen applications when the grass is under stress or when fungi are most active. Periodic core aeration and dethatching improve air and water movement through the soil, reducing the thatch layer that can harbor fungal organisms.