It's late October in north Florida and a St.

Diagnosis: Large Patch (Rhizoctonia solani). This is the classic cool-season presentation in St. Augustine. The pathogen activates when soil temps drop to 70–75°F (late Oct–Nov in N. FL) and the grass is wet. Low, shaded spots hold moisture longest, so rings appear there first. The "yellow halo" is the active infection margin; the brown center is dead leaf tissue.

Immediate Cultural Fixes (Do these first):

1. Stop irrigating. Turn the system off. Water only if the grass shows drought stress (folded blades). Large Patch requires 48+ hours of leaf wetness; irrigation feeds it.
2. Mow high (3.5–4"). Never scalp. Remove clippings from active rings to reduce inoculum load.
3. Zero Nitrogen. Do not apply any nitrogen fertilizer until soil temps are consistently >75°F next spring (usually April). N feeds the fungus. If you must fertilize, use potassium only (0-0-16 or similar) per UF/IFAS SL-21.
4. Improve drainage/aeration. Core aerate low spots if soil is compacted.

Fungicide (If cultural fails or history is severe):

• Active Ingredients: Azoxystrobin (Heritage G, Scotts DiseaseEx), Propiconazole, or a combo (Headway G, Armada).
• Timing: Curative now (late Oct) to stop spread. Preventative next year: First app when 2-inch soil temp hits 75°F falling (usually late Sept/early Oct), second 28 days later.
• Application: Granular is easier for homeowners; water in lightly (⅛") to move to crown/stolon zone. Follow label rates exactly.

Recovery: St. Augustine spreads via stolons. The rings will fill in next May/June if you don't fertilize now. Weak, N-pushed growth in winter dies back anyway.

Code/Standard Ref: UF/IFAS EDIS PP-233 (Large Patch); FL Statute 482.156 (fertilizer blackout periods may apply locally—check county ordinance).

Pro Call: If rings exceed 50% of lawn, or if you need liquid fungicide apps requiring a spray license, hire a Licensed Pest Control Operator (Lawn & Ornamental category).

Based on your description—late October in north Florida, St. Augustine lawn, yellow-to-brown rings a few feet across in low, shaded areas—this is most likely large patch disease (caused by Rhizoctonia solani), also known as brown patch in warm-season grasses.

Why it fits:

• Timing: Large patch is active in fall and spring when temperatures are 60–80°F and humidity is high—perfect for late October in north Florida.
• Symptoms: Circular patches (2–10+ feet) start yellow, turn orange/brown, often with a "smoke ring" of active infection at the outer edge. Grass pulls easily from runners at the base.
• Location: Thrives in shaded, poorly drained, low-lying areas with excess moisture and thatch buildup—exactly where you’re seeing it.
• Grass type: St. Augustine is highly susceptible.

What to do:

1. Stop nitrogen fertilizer—it fuels the disease. Avoid N until spring green-up.
2. Improve drainage & airflow: Aerate compacted soil, reduce irrigation (water only if needed, early morning), and prune overhanging limbs to increase sun/airflow.
3. Fungicide (if needed): For active spread, apply a fungicide labeled for large patch on St. Augustine (e.g., containing azoxystrobin, flutolanil, or propiconazole). Follow label rates—typically 2 applications 14–28 days apart. Note: Fungicides suppress but don’t cure; cultural fixes are key.
4. Rake out dead thatch in affected areas to improve recovery.
5. Monitor: Recovery may be slow; grass often rebounds in spring if conditions improve.

When to call a pro:

If patches keep expanding despite care, or you’re unsure of the diagnosis (e.g., rule out take-all root rot or nematodes), consult a UF/IFAS Extension agent or licensed lawn care professional. Avoid over-treating—misdiagnosis wastes money and can harm the lawn.

Source: UF/IFAS Extension (EP331, Large Patch Disease in St. Augustinegrass).