Here is the thing that surprises people about large patch: it is not a hot weather disease. Big circular brown areas show up in the lawn and everybody assumes the summer heat cooked it. But the fungus behind large patch, Rhizoctonia, does its best work when the weather is mild and wet. Around here, that means our rainy, humid fall and our cool damp spring, not the middle of July.
The conditions that turn it on
Large patch needs two things at the same time:
- Mild temperatures, roughly the 65 to 80 degree range. That is exactly our shoulder seasons.
- Extended leaf wetness. The grass canopy has to stay wet for many hours at a stretch, night after night.
Put those together and you get the North Florida pattern. It flares up in October and November when the nights cool off and the dew sits heavy until mid-morning. It flares again in March and April when it is warming up but still wet. In the peak of summer, when it is 95 degrees and the canopy dries out by nine in the morning, the disease mostly shuts down.
Warm-season turf that is entering or exiting dormancy is also under stress and slower to defend itself, which is another reason the shoulder seasons are the danger zone.
What it looks like
Large patch shows up as roughly circular or horseshoe-shaped patches. They start small and can expand to many feet across, and multiple patches will run into each other. What sets it apart:
- The shape is round-ish. That is your first clue that you are looking at a disease and not an insect. Chinch bug damage is irregular and creeps outward from the hottest, sunniest ground.
- Many patches have an orange or bronze outer ring where the fungus is actively working the outside edge, with more collapsed, thinned grass toward the center.
- The center often greens back up while the ring keeps expanding, so you get a donut.
- It tends to hit low, wet, shady, poorly drained parts of the yard first. Where the water sits, that is where you will find it.
The tug test that confirms it
This is the diagnostic. Go to the edge of a patch, grab a few grass blades, and pull straight up.
With large patch, the blade slides right out with almost no resistance, and when you look at the base of the leaf sheath where it was attached to the runner, you will find a dark, rotted, water-soaked lesion. It looks brown or almost black and greasy. That rotted sheath is the fungus, and it is why the leaf lets go so easily. Healthy grass fights you when you pull on it. Grass with large patch surrenders.
Also notice what is not damaged: the runner or stolon itself is usually still firm, and the roots are still there. That matters for recovery.
Which grasses get it
| Grass | Susceptibility |
|---|---|
| St. Augustine | Highly susceptible. The classic large patch host. |
| Zoysia | Highly susceptible, often the worst of the bunch. |
| Centipede | Gets it too, especially where it is wet and over-fertilized. |
| Bermuda | Least affected of the four, but not immune. |
What you are doing that feeds it
Large patch is largely a management disease. The fungus is already in your soil. Whether it explodes depends on the conditions you give it.
- Late-season nitrogen. This is the big one. Nitrogen applied in the fall pushes soft, tender leaf growth right as the disease-friendly weather arrives, and the plant is slowing down and cannot outgrow the damage. Stop the nitrogen well ahead of the cool weather. Our fertilizing calendar covers the windows. On centipede especially, over-fertilizing causes problems well beyond this disease.
- Evening and night watering. If you water at 7 p.m., the canopy stays wet from then until the dew burns off the next morning. That is fourteen hours of leaf wetness handed straight to the fungus. Water in the pre-dawn or early morning only, so the sun dries the blades within a couple of hours.
- Poor drainage. The low corner that stays soggy after every rain is where large patch lives. Fix the grade, add drainage, and stop trying to grow turf in a bathtub.
- Thatch. A heavy thatch layer holds moisture right at the crown and harbors the fungus.
- Shade with no air movement. Under a dense oak canopy, hemmed in by a fence and shrubs, the grass never dries. Limb up the trees, thin the shrubs, and get some air across that turf.
- Scalping and dull blades. Every torn leaf tip is an entry wound. Mow at the correct height for your species with a sharp blade.
Cultural control first, fungicide second
Fix the water timing, cut the fall nitrogen, get the drainage right, open the area up to air and sun, and knock the thatch down. Do those five things and a lot of lawns never need a fungicide.
If you do use one, understand what it is and is not. Turf fungicides are far more preventive than curative. Once a patch is there, the fungicide's job is to stop it from getting bigger, not to heal the dead grass. So the value is in applying it ahead of the flare-up, going into the fall, on a lawn with a history of it. And if you spray repeatedly, rotate modes of action rather than hitting it with the same chemistry every time. Read the label for rate, interval, and watering-in. Fungicides applied wrong are just money on the ground.
It usually is not fatal, and it usually comes back
Two things to keep in perspective. First, large patch attacks the leaf sheath, not the crown and roots, so the plant is generally still alive. When warm weather comes back and the grass starts running, those patches fill in. Do not tear out sod in March over something that will grow closed in May.
Second, it is site-specific. That fungus is sitting in that soil, in that low, shady corner, and it will show up in the same spots year after year unless you change the conditions. If you see it in the same place every fall, that is the site telling you what to fix. Do not just treat it, drain it.
A big circular brown patch in the yard is worth a real diagnosis before you spend money on it, because the fix for a fungus and the fix for a bug are completely different. Reach out to Williams Total Lawn Care and we will pull a few blades, check the sheaths, look at your irrigation timing, and tell you what is actually going on out there.