A brown patch in an otherwise green lawn is frustrating, partly because the cause isn't obvious from the curb. The same dead-looking spot can come from a fungus, from drought, from grubs eating the roots, or from the family dog. Around San Antonio, where we deal with heavy clay soil and long stretches of heat, a few of these show up again and again. Read the patch correctly and the fix gets a lot simpler.
Quick answer
Brown patches usually come from one of four things: a fungus called large patch (common in St. Augustine), drought stress on our clay soils, grubs chewing the roots, or dog urine. Figure out which one you have first, because the fix for fungus is the opposite of the fix for drought. When in doubt, tug on the grass: if it lifts up like a loose carpet, you likely have grubs.
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Large Patch Fungus: The Usual Suspect in St. Augustine
Most San Antonio lawns are St. Augustine, and St. Augustine is prone to a fungal disease called large patch (older references call it brown patch). It shows up as roughly circular areas that turn yellow then tan, often with a darker ring around the edge. Tug on the blades at the margin and they pull free easily, with rotted-looking bases.
Large patch loves cool, wet conditions, so it tends to flare in spring and fall rather than the dead of summer. Overwatering and watering in the evening make it worse, because the grass stays damp overnight. The first move is to stop feeding it: water early in the morning only, and cut back on nitrogen until the disease settles down. A targeted fungicide application can stop an active outbreak, and that timing matters more than the product.
Drought Stress and Heat
Our clay soils hold water, but they also seal up and shed it when they dry out and crack. During a San Antonio August, a patch that gets a little less water than the rest of the yard (near a sidewalk, on a slope, over a buried utility line) browns out first. Drought-stressed grass usually wilts and folds before it browns, and footprints stay pressed in the lawn instead of springing back.
Texas A&M AgriLife recommends watering deeply and less often rather than a light sprinkle every day. Aim to wet the soil to about six inches, then let it dry before the next round. Shallow daily watering trains roots to stay near the surface, which is exactly what you don't want when the heat sets in.
- Footprints linger instead of bouncing back
- Blades fold or curl along the midrib
- Spots near pavement, slopes, or high ground brown first
- Soil is hard and cracked under the dead area
Grubs Eating the Roots
White grubs are the larvae of beetles, and they feed on grass roots just under the surface. The giveaway is that the brown grass lifts up like loose carpet, because the roots that anchored it are gone. You may also see more birds, armadillos, or skunks digging in that area, since they are after the grubs.
A few grubs per square foot is normal and not worth treating. Heavy populations are. If you peel back a square foot of sod and count a handful or more of the C-shaped larvae, a properly timed grub control product is warranted. Timing is everything here too, since the young grubs near the surface are far easier to control than the big ones.
Pet Urine and Other Spot Causes
Dog urine burns grass because of its concentrated nitrogen and salts, leaving small round dead spots that often have a ring of extra-green, lush grass around the edge. That green halo is the tell that separates pet damage from fungus. Watering the spot well right after the dog uses it dilutes the damage.
A few other things masquerade as disease: fertilizer spilled in one spot, a gas can or hot equipment left on the lawn, or even shade from a new structure. Walk the patch and think about what is different about that exact location before you reach for a chemical.
How to Tell Them Apart Fast
Start with the tug test. Grass that lifts like carpet means grubs. Grass that pulls free with rotted bases at the margin of a circle points to fungus. Grass that is still firmly rooted but wilted and crispy points to drought. A small round dead spot with a green ring is almost always a pet.
Getting the diagnosis right saves you money and time, because treating drought stress with a fungicide does nothing, and dumping more water on a fungus makes it spread. If you have looked at the patch and still aren't sure, that is a good moment to have someone walk the lawn and call it.