Describe the damage and where it is. The causes that fit come back cheapest check first — and on a lawn that often means the mower: a dull blade, a low deck, a spill where you fill it. Then the pests, then the diseases, each with the check that tells it apart and what to do.
Lawn · brown or straw-colored patches · roundish spots or patches · summer
Check first: dog urine7 causes on our list fit what you described. They are in the order you should check them — the cheapest check first, and what is not a disease before what is. None of it is a diagnosis; the check is.
This lists what the extension services say fits what you describe, in the order to check it. It is not a diagnosis — when the checks do not settle it, a plant clinic will, for a small fee. No product here is named for a rate or a mix: the label on the product is the only place for that.
Causes and sources checked 2026-09-25.
Is it disease weather where you are?
Your ZIP stays on this device. Only its rough area — the middle of every ZIP that shares its first three digits — goes to the National Weather Service, which sends back the next few nights. Nothing goes to us.
Photo diary — is it spreading?
Photos are shrunk and kept in this browser, on this device. Nothing is uploaded. Take one today and another in a week: the two appear side by side.
Send a sample to your state’s plant clinic
When the checks do not settle it, the land-grant university clinics will — usually for a modest fee. Pick your state.
How to take it and pack it
Quick answer: brown spots in a lawn are more often the mower, the weather or the dog than a disease. A dull blade browns every tip within days; a low deck scalps the bumps; a gas spill kills a patch the shape of the spill. Rule those out first — they cost nothing to check — then look for grubs (lift a square foot of turf) and only then for fungus. The University of Maryland’s own diagnostic tables are built the same way, and so is this tool.
How to use it
Pick what you see, where it is and when you noticed it. The causes that fit everything you said come back in the order you should check them: the mower first, then the site (water, spills, soil, the dog), then pests, then diseases. Each one has the single check that tells it apart from its look-alikes, and what to do about it in the order the extension services use — change how it is grown first, then by hand, then biological help, and a product only last. Not sure about an answer? Say so; “not sure” rules nothing out.
Then use the three boxes underneath. The weather check tells you whether the next five nights match the conditions Maryland Extension publishes for brown patch, Pythium, dollar spot and red thread. The photo diary keeps today’s photo on your phone and puts it beside next week’s, because “is it spreading?” is the question every guide asks next. And if none of it settles the matter, the lab finder gives you your state’s plant clinic and how to cut and pack a sample.
Start with the mower — honestly
This is a mower site, so I will say it plainly: the machine causes a lot of what people call lawn disease. Maryland Extension’s list of lawn problems includes three that are pure mower. Dull blades tear the tips instead of cutting them, and the torn ends dry out pale brown — the whole lawn goes grey-brown a day after mowing. Mowing too low scalps the high spots and leaves them bare and stressed. And spilled gas or a leaking engine kills a patch exactly where you fill up or park. The fixes are cheap: sharpen the blade (Maryland’s guide says after every 10 hours of mowing ideally, at least twice a year), raise the deck, fill up on the driveway. Our blade sharpening debate covers the first one in detail.
There is a fourth that most people never think of: the same mowing path every week. A riding mower’s wheels pack the soil into ruts, and the grass along them thins. Maryland’s suggestion is simple — shift the path a few inches each time.
Then the site, then the pests
Dry weather is the next suspect: walk across the lawn and look back. If your footprints are still there a few minutes later, it is thirsty. An established fescue or bluegrass lawn can be allowed to go brown and dormant — Maryland advises that over frequent light watering, which grows shallow roots — and it comes back with the rain. Round, bleached spots with a dark green ring around them are the dog; see how to stop dog urine killing the grass.
Grubs are the pest to rule out before any fungus. The test takes five minutes: cut three sides of a square foot of turf at the edge of a dead patch and fold it back. If it lifts like a carpet with no roots holding it, count the white C-shaped grubs. Maryland puts the line at 5 to 10 per square foot; a few scattered ones do no harm. Chinch bugs and sod webworms have their own checks — a can of water, and a look for silk tunnels — and both are in the tool.
The diseases, and where the experts disagree
Brown patch is the common one in tall fescue lawns in summer: tan marks with dark brown borders on the blades, and white cobwebby threads in the morning dew. Maryland gives its weather as overnight temperatures above 68°F with humidity at 95% or more; NC State says it can start once nights pass 60°F. On fungicides the two services genuinely differ. Maryland does not recommend them for home lawns at all, and says the ones sold to the public are not very effective against brown patch. NC State describes a preventive program for tall fescue, started when nights stay above 60°F. The tool shows you both; neither of them suggests spraying after the damage is done. What both agree on: water early in the morning, never in the evening, and go easy on summer nitrogen. Most tall fescue recovers with autumn rain.
Dollar spot is the one with the hourglass: the mark on a single blade is pinched in the middle, which no other lawn disease does. A late-spring feeding helps the grass outgrow it. Red thread and rust are both signs of an underfed lawn and fade once nitrogen is adequate. Slime mold looks alarming and is harmless — wipe a blade and it is green underneath.
About products
The tool never gives a rate, a mix, or tells you a product is “safe for pets and kids”. It names the kind of product and says the same thing every time: the plant and the problem must both be on the label, and you follow the label word for word. The EPA’s own summary is four words long: “the label is the law.”
What it does not do
It does not identify grass species, and it does not look at photos — the diary keeps them for you, on your phone, and nothing is uploaded. It covers the common lawn problems in Maryland Extension’s tables, which fit most cool-season lawns and many warm-season ones; a problem it cannot place is exactly the kind a clinic should see. For trees, shrubs, vegetables and houseplants, use the Plant Doctor instead.