It is July, the nights have been warm and sticky, and circular brown areas have appeared in the lawn overnight. Someone tells you it is lawn fungus. They are probably right.
That is where most advice stops, and it is exactly where it stops being useful — because “lawn fungus” covers several diseases that want completely different things from you.
Two of them dominate home lawns in warm weather. And the most important thing about them is this: one is fed by nitrogen and the other is cured by it. Reach for a bag of fertilizer without knowing which you have and there is a fifty per cent chance you have just fed the disease.
What each one is doing
Brown patch is caused by Rhizoctonia solani. It wants warm, humid nights — air temperatures roughly 21–29 °C with leaves that stay wet through the night. It develops quickly during prolonged wet periods: extended rain, heavy dew, over-irrigation, or poor drainage, as Penn State describes.
It produces large circular patches — anything from 15 cm to well over a metre.
Critically, brown patch is made worse by nitrogen. A lush, soft, well-fed lawn in warm humid weather is exactly what the fungus is looking for. Over-fertilising in late spring or summer is one of the reliable ways to invite it.
Dollar spot works the other way round. It turns up on lawns that are slightly hungry for nitrogen, and it produces small tan spots about the size of an old silver dollar — hence the name, and hence roughly the size of a large coin.
A lightly fed lawn usually grows straight out of it.
So the two diseases sit at opposite ends of the same lever.
What you can see without any test
| Brown patch | Dollar spot | |
|---|---|---|
| Patch size | 15 cm to over a metre | 3–8 cm, coin-sized |
| Patch count | A few large ones | Many small ones, often merging |
| Edge | Sometimes a darker “smoke ring” | No ring |
| Weather | Warm and humid, hot nights | Warm days, cool nights, heavy dew |
| Nitrogen | Made worse by it | Made better by it |
| The blade | Irregular tan lesions | A clean band straight across, reddish edge |
| Merged appearance | Large irregular blotches | A mottled, freckled look |
The “many small versus few large” distinction is the quickest first pass, but it gets unreliable once dollar spot has been running for a while — enough small spots merging together start to look like one large patch.
Which is why the real answer is on a single blade.
Pick one affected blade and hold it up to the light.
Dollar spot leaves an unmistakable mark: a tan or bleached band running straight across the blade, with a distinct reddish-brown border on each side. Often the band is narrower in the middle, which is where the classic description of an hourglass lesion comes from. It is a band, not a tip burn — there is usually still green blade beyond it.
Brown patch produces irregular tan lesions with a darker margin, but no clean band and no hourglass. The blade looks damaged rather than marked.
The second check has to happen at dawn, before the dew dries. Go out at first light and look at the outer edge of a large patch. Grey-white webbing — mycelium — around the advancing edge points strongly at brown patch. It is gone within an hour of the sun reaching it, which is why almost nobody sees it.
If it is brown patch
Every move here is about breaking the warm-and-wet-overnight condition the fungus needs.
Stop watering in the evening. This is the biggest single lever you have. The fungus needs leaves that stay wet through the night; watering at 7 pm guarantees it. Move to early morning so the blades dry within a couple of hours.
Hold off on nitrogen until it cools down. Feeding now feeds the disease. If the lawn also looks hungry, that is genuinely awkward — and the answer is to wait rather than to compromise. Autumn is soon enough.
Improve airflow. Mow at the correct height, thin overhanging branches, and open up anything that traps humid air over the turf. Still, damp air over a lawn all night is the condition itself.
Do not bag your way out of it. Brown patch overwinters in soil and thatch, and removing clippings does not meaningfully reduce it.
Leave the mowing height up. Cutting short stresses the plant exactly when it is fighting an infection.
If it is dollar spot
This one is usually simpler, and often the treatment is the thing you were already going to do.
Give it a light nitrogen feed. Dollar spot is associated with low fertility, and a modestly fed lawn generally grows it straight out. This is the rare case where the fungus is a symptom of under-feeding rather than over-feeding.
Water deeply in the morning rather than lightly in the evening — same rule as everything else, for the same reason.
Knock the dew off at dawn. Dragging a hose or a length of rope across the lawn early in the morning breaks the surface film of dew. It sounds like folklore and it is a genuine, long-standing turf management practice: less time with wet leaves means less disease.
Do not scalp it. Same as above.
When it is neither
Worth saying plainly, because circular brown patches in summer have several causes that are not fungal at all.
Dog urine makes circular dead spots with a green ring — see dog urine or fertilizer burn. Grubs make irregular patches where the turf lifts like a rug, covered in grubs or chinch bugs. Heat dormancy browns off an entire lawn at once without any patch structure, and dormant or dead covers how to tell whether it is coming back.
The blade test is what separates the fungal cases from all of those: a lesion on an individual blade is a disease. Damage with no blade lesion is something else.
A word about fungicide
Most home lawns do not need it, and reaching for it first is usually a mistake.
Both diseases respond to changing the conditions that produced them, and neither kills the crowns outright — so lawns generally recover on their own once the watering time moves and the nitrogen goes in the right direction. A fungicide applied to a lawn still being watered at night buys a few weeks and nothing more.
If it recurs in the same place every year despite all of the above, that is when a targeted product becomes reasonable.
Do not treat “lawn fungus” — the two common ones want opposite things. Pick one affected blade and look at it: a clean tan band across the blade with a reddish border is dollar spot, and a light feed usually ends it. Irregular lesions with grey-white webbing at the patch edge at dawn is brown patch, and feeding it now would make it worse.
Either way, move the watering to early morning. That one change does more than anything else available to you.