Brown patches in St. Augustine grass usually trace back to one of a handful of culprits: drought stress, insect feeding, fungal infection, cold damage, or chemical injury. The trick is that each cause leaves a slightly different pattern, and because St. Augustine is the dominant lawn grass across the Gulf Coast and much of the Southeast, nearly every common turfgrass problem shows up in it at some point. Figuring out which problem you have matters, because the fixes for one cause can actually worsen another.
Drought Stress and Underwatering
St. Augustine grass is a warm-season species that thrives in heat, but it still needs consistent moisture. When water runs short, the grass responds by losing green color, starting with a dull blue-gray tint before turning straw-brown. You will often see footprints stay visible for minutes after you walk across a drought-stressed lawn because the wilting blades lack the turgor to spring back. The browning tends to be widespread and relatively even rather than concentrated in discrete circles or patches.
Not all St. Augustine varieties handle dry spells the same way. Research comparing diploid and interploid genotypes found that some newer interploid hybrids maintained above 50% green cover without supplemental irrigation in trials where common cultivars like Floratam, Raleigh, and Palmetto could not keep up.
Leaf shape plays a role too. Studies mapping the physical traits linked to drought tolerance found that varieties with narrower, shorter leaves lose less water through transpiration, while denser, more prostrate growth habits help shade the soil and slow evaporation from the surface.
If you suspect drought, the fix is straightforward: water deeply but infrequently, aiming for about three-quarters of an inch per session so moisture reaches the root zone rather than just wetting the surface. Early morning is best because evening watering keeps blades damp overnight, which invites fungal problems. Brown areas caused purely by drought will green up within a week or two once consistent irrigation resumes, assuming the roots are still alive. If the stolons snap rather than bend when you tug on them, those sections are dead and will need to be re-sodded.
Chinch Bugs
If the browning starts as irregular yellow patches along a sunny edge of the lawn, especially near a driveway or sidewalk, chinch bugs are the first suspect. The southern chinch bug is the single most damaging insect pest of St. Augustine grass in the United States, costing Florida homeowners and lawn-care professionals millions of dollars every year.1EDIS / University of Florida. Managing Southern Chinch Bug in Warm Season Turfgrasses These tiny black-and-white insects, only about a fifth of an inch long as adults, pierce grass stems and suck out the plant juices while injecting a toxin that blocks water movement through the blade. The result looks a lot like drought damage at first, which is why so many people respond by watering more. That does not help because the insects, not the lack of water, are the problem.
To check for chinch bugs, push a bottomless coffee can into the soil at the edge of a yellowing area and fill it with water. Within five minutes, chinch bugs will float to the surface if they are present. You can also part the grass near the boundary between brown and green turf and look for the small, fast-moving insects at the base of the blades. They are most active from late spring through early fall and prefer hot, sunny spots. Heavily thatched lawns are more vulnerable because thatch gives the bugs cover.
Treatment usually involves a targeted insecticide, though resistance has become a genuine problem in some populations. Reducing thatch buildup and avoiding excessive nitrogen fertilization, which produces the lush growth chinch bugs favor, can help keep numbers manageable.
Sod Webworms
Tropical sod webworm larvae are another destructive pest of warm-season turfgrasses in the southeastern United States, particularly on newly established sod, residential lawns, and athletic fields.2EDIS. Tropical Sod Webworm Herpetogramma phaeopteralis Guenée (Insecta: Lepidoptera: Crambidae) The damage pattern differs from chinch bugs. Early-stage larvae only scrape the upper surface of the blade, so the grass looks slightly off-color or translucent at first. As the caterpillars grow larger, they chew through entire sections of the leaf blade, leaving a ragged, uneven appearance that eventually exposes brown thatch in expanding patches.3Journal of Integrated Pest Management. Tropical Sod Webworm (Lepidoptera: Crambidae): a Pest of Warm Season Turfgrasses
Because sod webworms feed at night and hide in the thatch during the day, you may not see them unless you look after dark with a flashlight. The larvae prefer dry, hot areas of the lawn. One useful field sign is the presence of small, tan-colored moths that zig-zag low across the grass at dusk; those are the adults, and where the moths land, eggs follow. The good news is that webworm feeding generally does not kill the roots or stolons, so the grass can recover if the infestation is caught before it becomes severe.3Journal of Integrated Pest Management. Tropical Sod Webworm (Lepidoptera: Crambidae): a Pest of Warm Season Turfgrasses Left unchecked, however, the bare patches often fill in with weeds before the grass can regrow.
Fungal Diseases That Brown St. Augustine Grass
Several fungal infections cause browning, and they tend to look different from each other in ways that help with identification.
Brown Patch and Large Patch
Brown patch is a foliar disease, meaning it mainly damages the leaf blades themselves without killing the underlying shoots and stolons. Large patch, by contrast, causes leaf sheath rot and can kill affected shoots entirely.4Crop Science. Screening St. Augustinegrass Genotypes for Brown Patch and Large Patch Disease Resistance Both are caused by species of the fungus Rhizoctonia. Large patch typically develops as small round areas of orange or tan turf that can expand to several meters in diameter if unchecked.5Crop Science. Review of the biology and management of large patch of warm‐season turfgrasses
Large patch tends to flare up in spring and fall when the grass is entering or exiting dormancy and soil temperatures are moderate. Brown patch is more of a summer problem, favored by warm, humid nights. Both are aggravated by overwatering, poor drainage, and excess nitrogen. If you pull up a blade at the edge of a large-patch circle, the leaf sheath will often peel away easily and feel slimy at the base. With brown patch, the leaves themselves show tan lesions with darker borders, but the sheath and runners remain firm.
Fungicide applications can help manage active outbreaks, but cultural practices do more in the long run: water in the morning so the canopy dries before nightfall, avoid fertilizing when disease pressure is high, and improve soil drainage if you have chronically wet spots.
Take-All Root Rot
Take-all root rot (TARR) is a particularly frustrating problem because it attacks below ground, where you cannot see it until the damage is severe. The disease is caused by fungi in the Gaeumannomyces group, and research from Texas has identified multiple species involved, with some being more aggressive than others at producing symptoms.6PubMed. Complexity of Gaeumannomyces species causing take-all root rot of St. Augustinegrass in Texas Infected roots turn dark brown or black and become short and stubby. Above ground, you see yellowing that fades to brown in irregular patches, and the grass pulls up with very little resistance because the root system is so compromised.
TARR is often misdiagnosed as chinch bug damage or simple drought stress, and the misdiagnosis matters because treating for insects or just adding water will not solve a root-rot problem. The disease is favored by high soil pH, so one of the primary management strategies is applying sulfur or acidifying fertilizers to bring the soil pH down. Excessive irrigation and herbicide stress can also make the grass more susceptible.
Cold Weather and Freeze Damage
St. Augustine grass has limited cold tolerance, and winter hardiness is considered a major limiting factor for growing it in the transition zone, which includes areas from central Texas through the Carolinas where winters are cold enough to be borderline.7Euphytica. Assessing freeze-tolerance in St. Augustinegrass: temperature response and evaluation methods In mild winters, St. Augustine simply goes dormant and turns tan or straw-colored, greening up again in spring. In hard freezes, however, the stolons and growing points can be killed outright.
What makes freeze damage tricky is that the grass can lose its cold tolerance rapidly during mid-winter warm spells. Research on freeze tolerance found that a significant loss of hardiness occurs when plants are subjected to deacclimation events, meaning a few warm days in January can make the grass far more vulnerable to the next cold snap.8Euphytica. Assessing freeze tolerance in St. Augustinegrass: II. acclimation treatment effects This is why a lawn may survive one deep freeze without trouble and then die from a milder one that comes after a warm stretch.
If your St. Augustine grass is brown in late winter or early spring and you live in a region that experienced hard freezes, the best approach is patience. Wait until mid-spring, six to eight weeks after the last frost, and see what greens up. Stolons that are still pliable and have green nodes when you peel them open will likely recover. Stolons that are mushy or snap apart are dead and those areas will need new sod or plugs.
Herbicide and Chemical Injury
St. Augustine grass is sensitive to a surprisingly long list of common lawn herbicides. A study evaluating herbicide injury across multiple sites found that fenoxaprop-ethyl, metsulfuron-methyl, MSMA, and quinclorac all caused significant yellowing at every test location within ten days of application.9Texas Journal of Agriculture and Natural Resources. Evaluating Herbicidal Injury to St. Augustine Grass in Sod Production Several other products, including 2,4-D combinations and triclopyr blends, caused damage at two out of three locations. And some of these herbicides did not just yellow the grass temporarily; imazapic and metsulfuron-methyl reduced regrowth at all tested sites, meaning the injury lingered well beyond the initial application.
This is a common scenario: someone spots a broadleaf weed in their St. Augustine lawn, reaches for a general-purpose “weed and feed” product designed for cool-season grasses, and ends up burning out large sections. The label matters enormously. Products containing atrazine are generally safe for St. Augustine but are banned or restricted in many states. Products containing MSMA, once widely used in the South, are now severely restricted by the EPA. If your browning appeared within a week or two of applying any lawn chemical, herbicide injury should be high on the list. The browning from chemical damage is typically uniform across the treated area rather than patchy, and it often shows a distinct boundary where the application stopped.
There is no chemical antidote. You water the lawn well to help flush the product through the soil, back off on mowing, and wait. Recovery depends on whether the herbicide killed the stolons or just burned the leaf tissue. Foliar-only damage usually rebounds in a few weeks; stolon damage means re-sodding.
Nematode Damage
Nematodes are microscopic roundworms in the soil that feed on grass roots, and they are one of the more underdiagnosed causes of brown St. Augustine grass. The sting nematode (Belonolaimus longicaudatus) is a particularly damaging species. In controlled experiments, inoculated St. Augustine plants developed only about 44% of the root mass that uninoculated control plants had after seven months, and the infected plants showed measurable reductions in transpiration, which means they could not move water efficiently even when water was available.10PubMed Central. Dynamics of Belonolaimus longicaudatus Parasitism on a Susceptible St. Augustinegrass Host
Above ground, nematode damage looks a lot like drought stress: the grass wilts and browns even though you are watering. That confusion leads people to water even more, which does nothing because the roots are too short and damaged to absorb it efficiently. Diagnosis requires a soil sample sent to a nematology lab, which is available through most state agricultural extension offices for a modest fee. Sandy soils are far more prone to nematode problems than clay soils, so if you have a sandy lawn in Florida or coastal Texas that browns despite adequate watering, nematodes are worth testing for. Chemical nematicides exist but are limited in the residential market, so management often focuses on building overall turf health through proper fertilization, mowing height, and reducing stress.
Nutrient Deficiencies and Soil Problems
Iron deficiency is the most visible nutritional cause of yellowing and eventual browning in St. Augustine grass. It shows up as a characteristic interveinal chlorosis, where the leaf blade turns yellow but the veins stay green, giving a striped look. This is different from nitrogen deficiency, which causes a more uniform pale-green-to-yellow fade across the entire blade. Iron chlorosis is often a soil-chemistry problem rather than a true deficiency: the iron is present in the soil but locked up at high pH, making it unavailable to the roots.
Research on micronutrient applications in St. Augustine grass found that foliar iron is far more effective at greening up the turf than granular iron applied to the soil, which makes sense when the issue is soil pH tying up the nutrient. In the same research, granular manganese actually reduced turfgrass quality by about 20% at one test site, a reminder that throwing more micronutrients at the lawn is not always helpful.11Crop Science. St. Augustinegrass quality and nutrient content in response to granular and foliar iron, manganese, and magnesium
Phosphorus is another nutrient that requires a measured approach. Research on sandy soils determined that if soil phosphorus levels are already above a certain threshold, additional phosphorus fertilization provides no extra growth benefit and is simply wasted.12Crop Science. Phosphorus Requirements of St. Augustinegrass Grown in Sandy Soils Excess phosphorus runs off into waterways and contributes to algal blooms, which is why many Florida counties have passed ordinances restricting phosphorus in lawn fertilizers. A basic soil test, also available through your county extension office, tells you your soil’s pH, phosphorus, potassium, and often micronutrient levels, removing most of the guesswork.
How to Narrow Down the Cause
The pattern and timing of the browning usually tell you where to start looking. Browning that appears in expanding circles, especially in spring or fall, points toward large patch or other fungal disease. Irregular patches in sunny, hot spots along hardscape edges suggest chinch bugs. Browning that is widespread and coincides with a stretch of dry weather is likely drought. A ragged, uneven look to the blades, especially in summer, suggests webworm feeding. Uniform browning appearing a week or two after you applied a product almost certainly means chemical injury.
The below-ground causes, take-all root rot and nematodes, are harder to detect visually. Both mimic drought stress and require closer inspection. With TARR, the roots will be visibly dark and stubby. Nematode damage needs a lab sample. Both tend to be worst in sandy soils and in lawns that have been stressed by other factors.
One thing to keep in mind is that these problems often stack. A lawn weakened by drought is more susceptible to chinch bugs. A lawn stressed by herbicide injury is more vulnerable to take-all root rot. Excessive irrigation aimed at preventing drought opens the door to fungal disease. Treating just one cause while ignoring the underlying conditions that made the grass vulnerable tends to produce a repeating cycle of browning every season. A soil test, a close look at the insects present, and an honest assessment of your watering and chemical habits will usually point you toward the real issue, or combination of issues, driving the problem.