Brown Circles in the Lawn: Brown Patch, Dollar Spot and Fungus

Photo: Scot Nelson, CC0 (Wikimedia Commons) — fairy rings on a golf green.

A patch appeared on Tuesday. By the weekend it’s twice the size, and there’s a second one. Circles that grow are the signature of a fungus, and summer — heat, humid nights, wet leaves — is when the lawn gives them a season. The good news is that the fixes are mostly habits, and the lawn usually comes back.

The recipe

Every lawn fungus is already in your soil. It becomes a disease when three things line up: heat (nights that stay above about 65°F), wet leaves for hours (evening watering, heavy dew, poor airflow), and often lush, soft growth from summer nitrogen. Take any one away and most outbreaks stall. That’s why the plan below is more about watering and mowing than about spraying.

The line-up

Brown patch is the common one on tall fescue and perennial ryegrass. Rough circles from a foot to several feet across, tan to brown, sometimes with a darker “smoke ring” at the edge in the morning dew. Humid nights in June to August. Leaf disease — the crowns usually live.

Dollar spot makes small straw-colored spots the size of a silver dollar that merge into ragged patches. Individual blades show tan lesions with a reddish-brown edge, pinched like an hourglass. It loves a hungry lawn and heavy dew; a light feeding is part of the cure.

Summer patch hits Kentucky bluegrass and fine fescues: crescents and rings six to twelve inches across, often with a tuft of green in the middle (the “frog-eye”). It’s a root disease of hot, wet soil, and it kills — those patches get reseeded in fall.

Fairy rings are arcs or full circles of dark green grass, sometimes a dead ring, sometimes a line of mushrooms. A fungus in the soil is breaking down old wood or roots and releasing nitrogen at its growing edge. The photo above is the textbook version on a golf green. Mostly cosmetic; core-aerate the ring and water it well to stop the soil going water-repellent.

Not fungus, but looks like it: a dead circle with a lush dark ring is a dog spot; turf that lifts with no roots holding it is grubs; a blue-gray patch where footprints stay pressed in is drought; a brown stripe over a bump is scalping. A photo can usually tell these apart. It can’t reliably tell one fungus from another — for that, your county extension office will look at a sample.

The four habits that stop it

1. Water in the morning, deeply, rarely. Early morning, so the sun dries the leaves by mid-morning. About an inch a week in one or two goes. Evening watering is the single most reliable way to grow a fungus.
2. Mow dry, sharp and high. Wet grass and a mower spread spores; a dull blade leaves frayed tips the fungus enters through. Raise the cut a notch — the top of your grass’s range — for the rest of the summer.
3. No summer nitrogen on a cool-season lawn. Soft, fast growth in July is fungus food. Cool-season lawns eat in fall. (Dollar spot is the exception — a light feeding helps a hungry lawn outgrow it.)
4. Air and drainage. Where the same corner goes brown every summer, look at what’s keeping it wet: a downspout, a fence blocking the breeze, thatch over half an inch, compacted clay. Aerate it in the fall.

When a fungicide is fair

For a severe outbreak that keeps spreading through the habits above, or on a lawn that has had it three summers running, a fungicide is reasonable — and works far better applied before the patches appear, in the weather that brings them, than after. Get the disease named first (a sample to the extension office is cheap), because the products differ. It’s a last resort, not a first one, and never a substitute for the watering change.

The robot part

A robot mower helps and can hurt, depending on how it’s set. It helps because it cuts a few millimetres at a time — the least stress a blade can take — and keeps the canopy open. It hurts if it’s mowing soaking-wet grass at dawn every day in a brown-patch summer with blades that haven’t been changed since April. Three settings: fresh blades every four to six weeks (sooner in a bad summer), the cut height at the top of your grass’s range, and the running window set to daytime while the humid stretch lasts, so it isn’t dragging spores through dew. The Lawn Care Plan and My Schedule carry all three.

Make it your plan

Map your lawn once and tap “Brown circles” on the diagnosis tiles: the plan raises the summer cut, prints the morning-only watering line, holds the nitrogen until fall for a cool-season lawn, and books the aeration. Or send a photo to the free lawn check-up — it will say whether the circle looks like a fungus, a dog, grubs or drought, and it will say plainly that it can’t name the fungus from a photo. That part is the extension office’s, and worth the sample.