St. Augustine grass (Stenotaphrum secundatum) is a popular warm-season turf known for its thick, dense growth and appealing blue-green color across the southern United States. Its vigorous nature makes it an excellent choice for lawns in hot, humid climates. However, this environment makes the grass highly susceptible to destructive diseases and pests. A sudden decline in turf health, manifesting as yellowing or dead patches, signals that a biological or environmental stressor is at work. Understanding the distinct symptoms is the first step in saving your lawn.
Identifying Damage Caused by Fungal Diseases
Two primary fungal pathogens frequently damage St. Augustine grass, with symptoms varying based on the season. Brown Patch, or Large Patch (Rhizoctonia solani), is a cool-season disease active during the spring and fall when moisture is high. This fungus creates roughly circular patches of yellowed or brown turf, often ranging from one to five feet in diameter.
To confirm Brown Patch, examine the leaf blades near the soil line. The fungus attacks the leaf sheath, causing it to rot, allowing the affected blade to be easily pulled away from the runner. Patches are often circular, sometimes leaving a “doughnut” pattern where the center recovers. Management involves reducing irrigation, applying a systemic fungicide, and avoiding excessive nitrogen fertilizer.
The second major threat is Gray Leaf Spot (Pyricularia grisea), which thrives in the heat and humidity of mid-summer and early fall. This fungus directly attacks the leaf blades, creating distinct lesions key to diagnosis. Spots begin as small, brown specks that expand into oblong or diamond shapes. Mature spots have a gray or tan center surrounded by a dark purple or brown border.
Severe infection causes the lawn to take on a scorched or drought-stressed look, with widespread yellowing and thinning. Gray Leaf Spot is accelerated by high nitrogen fertilizer and extended periods of leaf wetness. Fungicides are the primary treatment, but cultural adjustments, such as watering early in the morning so blades dry quickly, are also important.
Pests That Target St. Augustine Grass
The Southern Chinch Bug (Blissus insularis) is the most destructive insect pest, capable of killing large patches of turf quickly. These tiny insects use piercing-sucking mouthparts to extract sap from the grass runners and crowns. As they feed, they inject a toxic substance that blocks water and nutrient flow, causing the grass to yellow and die.
Chinch bug damage first appears as irregular patches of yellowing turf that quickly turn brown, often starting in the hottest areas, such as along driveways and sidewalks. This pattern is frequently confused with drought stress, but chinch bug damage will not recover after watering. The dead patches expand outward as the insects move toward the healthy turf border.
To confirm an infestation, inspect the grass at the margin between the dying and healthy turf, looking at the soil surface and the base of the plant. The flotation test is a definitive method, involving inserting a bottomless can into the soil and filling it with water. Chinch bugs, if present, will float to the surface within five minutes. Rapid treatment with a labeled contact insecticide is necessary, as the grass will not recover once the crowns are killed.
While chinch bugs are the major killer, other pests like sod webworms and grubworms also cause damage. Sod webworms feed on the leaf blades, causing superficial chewing damage. Grubworms, the larvae of beetles, feed on the roots, which is diagnosed if the turf lifts easily from the soil like a loose carpet.
Non-Living Stressors and Cultural Mistakes
St. Augustine decline is often caused by environmental stress or homeowner maintenance errors rather than pests or pathogens. Improper mowing height is a common mistake; the grass should be maintained at 3.5 to 4.5 inches. Scalping, or cutting the grass too short, severely stresses the plant, damages the crown, and leaves the turf vulnerable to sun exposure and disease.
Using a dull mower blade is detrimental because it tears the grass blades instead of cleanly cutting them, creating wounds that serve as entry points for fungal diseases. Removing more than one-third of the blade height in a single session also stresses the plant, hindering photosynthesis.
Irrigation errors significantly contribute to turf decline. Overwatering, especially during cooler periods, encourages shallow root growth and creates moist conditions required for fungal pathogens like Brown Patch to flourish. Conversely, underwatering leads to drought stress, which resembles chinch bug damage and leaves the turf weak and susceptible to insect attack. The preferred practice is deep, infrequent watering, aiming for one to one and a half inches of water per week, applied in the early morning.
St. Augustine grass is intolerant of dense shade, requiring four to six hours of direct sunlight daily for optimal health. In shaded areas, the grass thins and weakens, making it vulnerable to issues like Take-All Root Rot. Soil compaction, often caused by heavy foot traffic, starves the roots of oxygen and restricts access to water and nutrients.
A Step-by-Step Diagnostic and Action Plan
When patches of St. Augustine grass begin to yellow or die, a systematic diagnostic process helps identify the true culprit. The first step involves assessing recent maintenance practices, including mowing height and the watering schedule. If the grass is cut too short or watered at night, correcting these cultural errors is the immediate priority.
Next, focus on confirming the presence of the Southern Chinch Bug, as this pest requires the fastest action. Perform the float test or visually inspect the grass at the yellow-to-green border to determine if the insects are actively feeding. If chinch bugs are confirmed, an immediate application of a labeled insecticide is necessary, targeting the affected area and surrounding healthy turf.
If insects are not present, the problem is likely fungal or environmental, requiring a closer look at the leaf blades and patch patterns. Circular patches with blighted sheaths that easily pull away from the runner point toward Brown Patch, which requires fungicide treatment and reduced irrigation. Diamond-shaped spots on the leaf blades indicate Gray Leaf Spot, requiring a fungicide application and temporary cessation of high-nitrogen fertilizer. Following this sequence allows homeowners to move quickly from observation to targeted treatment.