St. Augustine grass is a warm-season turf prized for its deep blue-green color and thick, carpet-like growth habit. Despite its popularity, this turf is highly susceptible to various fungal diseases. These pathogens thrive in specific environmental conditions, such as high humidity and excessive moisture. Recognizing the signs of infection and taking prompt action is the first step toward preserving the health and appearance of the lawn. Managing these diseases requires a multi-faceted approach combining cultural practices with targeted chemical treatments.
Identifying the Specific Fungal Threat
Accurate identification of the specific fungus is important because treatment strategies vary significantly between different diseases. The three most prevalent and destructive fungal threats are Brown Patch, Gray Leaf Spot, and Take-All Root Rot (TARR).
Brown Patch, caused by the fungus Rhizoctonia solani, typically presents as circular patches of yellow or brown grass, ranging from a few inches to several feet in diameter, often in the spring and fall when temperatures are below 80°F. A distinguishing symptom is the dark, rotting appearance at the base of the leaf sheath, which causes the blades to pull easily away from the stolon.
Gray Leaf Spot, caused by Pyricularia grisea, becomes most active during the hot, humid summer months. This infection is characterized by small, distinct lesions that are oval or diamond-shaped, featuring a gray or tan center with a prominent purple or brown border. If the infection is severe, the entire lawn may take on a scorched, dull appearance, mimicking drought stress.
Take-All Root Rot (TARR), caused by Gaeumannomyces graminis var. graminis, is a root disease that first appears as a general yellowing and thinning of the grass in irregular patches. Since the fungus attacks the root system, the stolons of the infected grass are easily lifted from the soil, and the roots may appear dark brown or black and rotted. Unlike Brown Patch, the leaf sheaths on TARR-infected grass are generally not damaged.
Cultural Practices to Halt Fungal Spread
Modifying the lawn’s environment is the primary way to manage fungal diseases, as fungi require extended periods of moisture on the leaf surface or in the soil. Proper watering techniques are a first-line defense. Lawns should be watered deeply and infrequently, soaking the soil to a depth of four to six inches to encourage deep root growth. The best time to irrigate is in the early morning, ideally just before sunrise, allowing the grass blades to dry completely before the cool evening hours and preventing the prolonged leaf wetness fungi favor.
Mowing practices also play a significant role. St. Augustine grass should be maintained at a height between three and four inches, which helps to reduce stress and retain soil moisture. It is important to mow only when the grass is dry, as mowing wet grass can spread fungal spores across the lawn via the mower blades.
Addressing soil health is essential by alleviating compaction and reducing thatch. Aeration helps to improve drainage and air movement into the soil, while dethatching removes the thick layer of dead organic matter that can harbor fungal pathogens.
Selecting and Applying Fungicides
When cultural controls are insufficient, chemical treatment with fungicides becomes necessary. Fungicides are generally categorized as either contact, which forms a protective barrier on the leaf surface, or systemic, which are absorbed by the plant and move within its tissues.
The successful long-term management of fungal diseases depends on preventing the development of fungicide resistance. This is achieved by rotating between products that contain different active ingredients. Fungicides work best when applied preventatively, before symptoms become severe, or at the very first sign of disease activity.
For Brown Patch, applications are often timed for the cooler, humid conditions of fall and spring, while Gray Leaf Spot treatments are generally needed in the summer heat. Always read the product label carefully, as some products, particularly those targeting root diseases like TARR, need to be watered into the soil immediately after application for maximum effectiveness.
Long-Term Management and Prevention
Sustained prevention shifts focus from treating symptoms to maintaining a resilient environment that discourages fungal growth. Soil health is a major factor; performing a soil test to determine pH and nutrient levels is recommended. Take-All Root Rot is often favored by high soil pH, which can sometimes be counteracted by applying peat moss to the affected areas.
Fertilization practices must be carefully adjusted, as high-nitrogen, quick-release fertilizers can encourage the flush of soft, succulent growth that is highly susceptible to fungi. Instead, use balanced fertilizers that include potassium, and avoid excessive nitrogen applications in late summer and fall when Brown Patch is active. Consistent monitoring allows for early detection and targeted treatment, reducing the need to spray the entire area.