Gray Leaf Spot and Take-All Root Rot in St. Augustine
How to recognize and manage gray leaf spot and take-all root rot in St. Augustine grass, and how they differ from large patch.
The short version
- St. Augustine has three main diseases: large patch (cool and wet), gray leaf spot (hot and humid), and take-all root rot (spring and after wet periods).
- Most disease starts with water: too much, at the wrong time of day, or soil that won't drain.
- Fix the watering and the nitrogen first. A fungicide on top of a watering problem is money spent twice.
Quick comparison
| Disease | When | What you see |
|---|---|---|
| Large patch | Fall and spring, cool wet weather | Circles or rings of brown grass, often with an orange or yellow edge. Leaves pull off the runner easily. Full guide |
| Gray leaf spot | Summer, hot and humid | Small oval spots on the blades, gray or tan in the middle with a darker brown border. Lawn looks scorched or thin from a distance. |
| Take-all root rot | Spring green-up, or any time after long wet stretches or stress | Yellowing and thinning in irregular patches that fertilizer doesn't fix. Roots are short, dark, and rotted, and runners lift up easily. |
Gray leaf spot
Gray leaf spot is a summer leaf disease. It thrives on hot, humid nights and grass that stays wet for long stretches. It's especially common on newly laid sod and on lawns that got a heavy dose of nitrogen in summer.
How to identify it
Look closely at individual blades. You'll see small oval or diamond-shaped spots, gray to tan in the center with a darker border. In humid weather the spots can look fuzzy. Badly affected areas look thin and scorched, almost like drought.
What helps
- Back off summer nitrogen. Lush, fast growth is what this disease feeds on. Stick to the planned feedings and skip extra summer boosts.
- Water in the early morning only, deeply and infrequently, so blades dry quickly. See how to water St. Augustine.
- Keep mowing at your normal height with a sharp blade. Torn leaves are easier to infect.
- Fungicide for bad cases. Use one labeled for gray leaf spot on St. Augustine and follow the label.
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Take-all root rot
Take-all root rot is a soil fungus that attacks roots, runners, and the base of the plant. It tends to show up as the lawn comes out of winter, or any time the grass is stressed by heavy rain, poor drainage, shade, drought, or alkaline soil. Because it lives in the roots, it's slow to diagnose and slow to fix.
How to identify it
- Irregular yellow areas that thin out and turn brown, from a foot across to much larger.
- Fertilizer and water don't help.
- Pull up a runner from the edge: the roots are short, dark, and rotted, and it lifts with almost no resistance.
What helps
- Fix drainage and cut back on water. Wet, compacted soil is where take-all thrives. Core aerating in late winter helps.
- Go easy on nitrogen. Avoid heavy quick-release feedings.
- Topdress with sphagnum peat moss. Texas A&M AgriLife recommends it as a slow way to bring alkaline soil pH down, often mixed with compost. Expect results over seasons, not weeks.
- Don't scalp. The grass needs every bit of leaf to support a damaged root system.
- Fungicides are preventive. They work best applied in spring and fall before symptoms appear in areas with a history of the disease. They won't revive dead grass.
For the extension research behind this, see Texas A&M AgriLife's take-all root rot fact sheet.
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