Large Patch Fungus in St. Augustine: Spot, Treat, Prevent
Brown circles in your St. Augustine lawn this fall? It's probably large patch. Learn the signs, why it shows up in cool wet weather, and how to treat and prevent it.
The short version
- Large patch is a fungal disease that hits St. Augustine in fall and spring, when the weather is cool and wet.
- Look for circular brown patches with an orange or bronze ring at the edge.
- Prevent it with a fungicide in early October, before it shows up.
- Water only in the morning, and skip high-nitrogen fertilizer in the fall.
What large patch is
Large patch is caused by a soil fungus called Rhizoctonia solani. It becomes active when soil temperatures drop into roughly the 50s and 60s °F, which is why it shows up as the weather cools in fall and again during spring green-up. Wet conditions make it much worse: heavy rain, poor drainage, night watering, or long stretches of dew.
It attacks the base of the leaves where they attach to the runner. The leaves rot there and die back, while the runners often survive. That's why patches can recover in spring if you stop the spread.
How to identify it
- Circles or rings of yellow to brown grass, from a foot or two across up to many feet, sometimes merging.
- An orange, bronze, or yellow border around an active patch where the fungus is spreading.
- Leaves at the edge pull out easily from the runner with a light tug, and the base of the leaf looks dark and rotted.
- It often starts in low, shady, or wet spots first.
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How to treat it
- Apply a fungicide labeled for large patch. Treat the patch plus a buffer of several feet around it, since the fungus is active beyond the visible edge. Follow the label for rate and reapplication timing. For patches between scheduled rounds I keep BioAdvanced Fungus Control on hand.
- Cut back the water. Water only when the lawn actually needs it, and only in the early morning.
- Hold off on nitrogen. Fresh, soft growth from a high-nitrogen feeding is exactly what large patch thrives on.
- Be patient. Fungicide stops the spread, but the brown area won't green up until the grass grows back, often not until spring.
Mowing through an active patch can drag the fungus around the yard. Mow those areas last, and consider bagging clippings there.
How to prevent it
- Preventative fungicide in early October. In the Turf Method program, the fall application goes down before large patch weather arrives. Prevention is far easier than chasing patches. My October fungicide is Scotts DiseaseEx.
- Water in the morning only and reduce watering as temperatures fall.
- Use a low-nitrogen fall fertilizer instead of a high-nitrogen one.
- Improve drainage in spots that stay soggy, and core aerate in late winter.
- Don't let leaves pile up and mat on the lawn. Mulch or remove them.
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