St. Augustine grass (Stenotaphrum secundatum) is a common warm-season turf known for creating a thick, dense lawn throughout the southeastern United States and the Gulf Coast region. This coarse-bladed grass spreads aggressively via above-ground runners, called stolons, which help it establish quickly and crowd out many weeds. While generally a hardy grass in tropical and subtropical climates, it is particularly susceptible to a specific range of threats that can cause rapid decline and eventual death. The primary culprits behind the demise of St. Augustine grass are aggressive insect pests, pervasive fungal diseases, and poor cultural management practices that weaken the turf’s natural defenses.
Insect Pests That Decimate St. Augustine Grass
The most destructive insect pest for St. Augustine grass is the Southern chinch bug (Blissus insularis). These insects use piercing-sucking mouthparts to draw sap from the grass blades near the soil line. Their saliva contains a phytotoxic substance that blocks the plant’s ability to transport water and nutrients, causing rapid desiccation and widespread death. Damage appears as expanding, irregular patches of yellowing grass that quickly turn brown and die, often mistaken for drought stress. These dead patches grow outward as the insects move to the live perimeter. Infestations are confirmed by finding the tiny, black-bodied adults or reddish-orange nymphs at the edge of the affected area.
Lawn grubs, the larvae of beetles like the Japanese beetle or masked chafer, also severely weaken the grass. These creamy-white, C-shaped larvae feed directly on the root system, preventing the absorption of water and nutrients. Severe root damage causes the grass to wilt and die in patches. In heavy infestations, the turf may be easily lifted from the soil because the roots have been severed.
Sod webworms are also a common problem. The larvae of these moths chew the grass blades near the soil surface, resulting in irregularly shaped brown spots that look like the grass has been closely clipped. This leaf tissue loss depletes the grass’s energy reserves, making it more susceptible to diseases and pests.
Fungal Infections and Lawn Diseases
Fungal pathogens are a major cause of St. Augustine grass death, thriving when environmental conditions create a moist habitat. The most destructive disease is Large Patch, caused by the fungus Rhizoctonia solani. This disease is active during cool or mild weather, typically in the spring and fall, when night temperatures are low but the soil remains moist. The fungus attacks the leaf sheaths and stolons, causing circular or semi-circular patches of grass to turn yellow and then brown.
A tell-tale sign is the dark, rotted appearance of the stolons at the soil level, often with an outer ring of yellowing grass blades. When the stolons are infected, the grass blades detach easily from the runner, leading to the death of the plant section.
Gray Leaf Spot, caused by the fungus Pyricularia grisea, is a serious foliar disease that flourishes in hot, humid summer conditions. It is often encouraged by prolonged leaf wetness from rain or late-day irrigation, and excessive nitrogen fertilization. The disease manifests as distinctive oblong or diamond-shaped spots on the grass blades. These spots have tan or gray centers surrounded by a dark purple or brown margin.
Severe and repeated infections cause significant thinning of the turf, leading to a scorched appearance. This chronic weakening makes the grass vulnerable to other factors that can cause its death, especially in stressed lawns.
Environmental and Cultural Causes of Decline
Improper management practices create stress that leaves St. Augustine grass vulnerable to pests and diseases, often becoming the indirect cause of death.
Water Management
Water management is a frequent issue, as both too little and too much water are detrimental. Underwatered turf suffers from drought stress, causing the leaves to curl and eventually leading to death. Conversely, overwatering promotes prolonged soil and leaf wetness, leading directly to root rot and encouraging fungal diseases like Large Patch. Deep, infrequent watering of about one inch per week is recommended, allowing the soil to dry out between applications.
Sunlight Requirements
St. Augustine grass requires high light levels for energy production, and insufficient sunlight causes decline. While it is considered one of the more shade-tolerant warm-season grasses, it still requires a daily minimum of four to six hours of direct or filtered sunlight. Excessive, dense shade causes the grass to thin out, weaken, and eventually die because it cannot photosynthesize enough to sustain itself.
Mowing Practices
Mowing practices also play a substantial role in turf health. Cutting the grass too short, or scalping, removes too much leaf surface at once. This stresses the plant and depletes the energy reserves stored in the stolons. Most St. Augustine cultivars should be maintained at a taller height, typically between 3.5 and 4 inches, to promote deeper roots and shade the surface stolons.
Chemical Misuse and Weather
Chemical misuse and extreme weather can cause sudden death. Applying certain herbicides intended for other grass types can chemically burn and kill the turf. Similarly, over-fertilization, particularly with high-nitrogen quick-release products, can scorch the grass and create a rapid growth flush that invites Gray Leaf Spot. As a warm-season grass, St. Augustine is sensitive to cold, and prolonged exposure to freezing temperatures can cause widespread winterkill by freezing the root system.