September and October catch a lot of San Antonio homeowners off guard. The heat breaks, lawns start to look like they're finally getting a rest, and then circular patches of dead-looking grass appear seemingly overnight. Brown patch fungus is extremely common in St. Augustine lawns during these conditions, and it spreads fast when the weather pattern is right. The good news is that it rarely kills the grass permanently. The frustrating part is that it looks almost identical to problems that do.
Quick answer
Brown patch (Rhizoctonia solani) is a fungal disease that causes circular, tan-colored patches in St. Augustine lawns, most commonly in fall and spring when nighttime temperatures drop below 70°F and the grass stays wet overnight. The edges of the patches have a dark, water-soaked border in the early morning. Unlike drought stress or chinch bug damage, brown patch spares the grass crown, so the turf can recover once conditions dry out. Preventive or curative fungicide applications and improved watering habits are the main tools.
Want it handled for you?
Seeing circular tan patches in your St. Augustine lawn this fall? Schedule a treatment with Prescription Lawn Services and we'll confirm the diagnosis and apply the right fungicide before the damage spreads.
See how our lawn care program fits into your prescription plan.
What Causes Brown Patch
Brown patch is caused by Rhizoctonia solani, a soil-borne fungus that becomes active when nighttime temperatures drop to the 68 to 75°F range and the grass remains wet for extended periods. Those are exactly the conditions that arrive in San Antonio in September and October, and again in March and April. The fungus spreads through infected thatch and can move via foot traffic, mowing equipment, and water runoff.
It is not a sign of neglect. Well-maintained lawns get brown patch. Conditions that make it worse include excess nitrogen fertilization going into fall (lush, soft grass is more susceptible), poor drainage, evening watering that keeps leaves wet overnight, and heavy thatch accumulation.
How to Identify Brown Patch
The classic pattern is circular or roughly circular patches of tan-to-brown grass, ranging from a few inches to several feet across. In the early morning, look for a dark, water-soaked ring at the outer edge of the patch. This "smoke ring" effect is one of the most reliable field identifiers for active brown patch and disappears as the dew burns off.
A key diagnostic: pull a blade from the outer edge of the patch. With brown patch, the blade pulls away cleanly from the crown, and the crown itself remains white and firm. If the crown is rotted or the blade pulls up roots and all, you may be dealing with something else. In severe cases where the crown is eventually affected, the turf can die and require resodding, but most brown patch cases spare the crown and allow recovery.
Brown Patch vs. Drought Stress vs. Chinch Bugs
All three can produce brown patches, and getting the diagnosis right matters because the treatments are completely different. Brown patch tends to appear in circular patterns, happens in cool moist conditions, and the blades pull cleanly from the crown. Drought stress is diffuse (the whole lawn looks dull or blue-gray, not in distinct patches) and responds to watering. Chinch bug damage concentrates in hot, sunny areas during summer and the patches expand from the edges outward, with bugs visible on a soap-flush test.
Season matters too. Chinch bugs are a summer problem (June through September). Brown patch hits in fall and spring. If your patches appeared after the first cool stretch in September and nights have been humid, brown patch is the likely culprit.
- Brown patch: circular patches, cool/moist conditions, dark smoke ring edge
- Drought stress: diffuse, whole-lawn gray-blue tint, responds to irrigation
- Chinch bugs: summer only, expanding from sunny hot spots, soap-flush test positive
Treatment and Recovery
Curative fungicide applications containing azoxystrobin, propiconazole, or thiophanate-methyl labeled for Rhizoctonia in turf can arrest active brown patch. Apply in the evening, when temperatures are lower and dew period is approaching, for best uptake. A second application 14 to 21 days later handles any secondary spread.
Infected grass that still has a living crown will often recover on its own once conditions dry out and temperatures change, though it may take several weeks. Badly damaged areas where the crown was affected will need to be reseeded or sodded. During recovery, hold off on nitrogen fertilizer, since high nitrogen encourages the soft growth that the fungus targets. Switch watering to morning-only schedules to reduce overnight leaf wetness.
Preventing Brown Patch in Future Seasons
The cultural fixes are mostly about moisture management. Watering in the early morning, keeping thatch under half an inch, and avoiding heavy fall nitrogen applications all reduce risk. In lawns with persistent brown patch year after year, preventive fungicide applications in late August and again in March, before conditions become favorable, can break the cycle.
Improving drainage in low spots where water pools also helps, since standing water on the soil surface after rain is one of the main drivers of rapid spread.