Lawn Disease, Fungus, Grubs and Armyworms
From the lawn and property care guide.
Lawn and property care guide · North Alabama
Why a lawn goes brown, yellow, thin or patchy
The patterns that tell drought from disease from insect damage from mower damage, and the point at which guessing costs more than a visit.
Where to start
The pattern tells you more than the color does
Almost everybody starts with what color the lawn has gone. The more useful question is what shape the damage is, because shape is the thing the causes do not share.
Water problems follow the terrain. They show first on slopes, along drives, over shallow soil and in the sunniest corner, because those are the places that dry out first. The boundaries are soft and they follow the ground rather than anything else.
Disease tends to spread in circles, rings or irregular blotches that get bigger week on week, and it often has a distinct edge where dying leaf meets healthy leaf. Insect damage frequently advances as a front across the lawn, and where roots have been eaten the turf lifts away like a loose rug.
Anything mechanical or chemical leaves geometry. Straight lines, a fan shape where a spreader was turned, wheel tracks, a strip along a fence: no disease and no insect makes a straight line. If the damage has corners, look at what was driven or spread over it.
The usual suspects
What each cause tends to look like
These overlap, and more than one is often happening at once. Treat this as a list of what to check rather than as a diagnosis.
| What it looks like | Often points at |
|---|---|
| Even browning across the whole lawn in winter | Normal dormancy in a warm-season grass |
| Dull blue-grey color, folded blades, footprints that stay | The lawn is short of water |
| Browns first on slopes, along drives, over shallow soil | Drought, or an irrigation zone that is short |
| Circles or rings that get bigger week on week | A turf disease. Worth a look rather than a guess |
| Damage advancing as a front, birds working the area hard | Insect activity, armyworms among them |
| Turf lifts away like a loose rug | Roots have gone. Check underneath for grubs |
| Small round spots with a darker green ring around them | Dog urine |
| Straight streaks or a fan shape | Something spread or driven - fertilizer burn or mower damage |
| Frayed, whitish, torn tips across the whole lawn | A dull mower blade |
| Thin under trees, or on the north side of the house | Shade, and a tree taking the water first |
| Yellow in the low ground, soft underfoot days after rain | Drainage. Roots cannot breathe |
Several of these look identical in a photograph. That is not a failure of the photograph; it is why an inspection exists.
Yellow specifically
Yellow usually means nutrition, water or roots
Brown is generally the end of a process. Yellow is usually somewhere in the middle of one, and it is worth acting on while it still is.
A nutrient shortage will yellow a lawn, but so will an iron problem tied to soil pH, and those want completely different work. Feeding a lawn that has a pH problem changes very little, which is the argument for a soil test before anything goes on the ground.
Waterlogging is the other common cause and it catches people out because it looks like hunger. Saturated soil has no air in it, roots cannot breathe, and the plant yellows exactly as though it were starved. If the yellow area is also the low area, and it is still soft underfoot three days after rain, feeding it will not help.
And a lawn coming out of dormancy unevenly in spring will be patchy yellow-green for a few weeks quite normally. That one resolves itself.
Thin and patchy
A thin patch is a symptom of the site, not of the grass
Filling a bare spot without finding out why it is bare produces the same bare spot again next year. That is the single most repeated expense in home lawn care.
The shortlist is short: shade, compaction, drainage, traffic, scalping, and pet damage. Every one of those is a different job, and every one of them will defeat reseeding if it is left in place.
Shade is the one people are most reluctant to accept. Below about four hours of direct sun a day, no grass grown here will hold reliably, and repeatedly seeding it is money spent on the same result. Mulch, pine straw or decorative rock with shade-tolerant planting is the honest answer, and it looks deliberate rather than defeated.
Compaction and drainage are covered in the soil and site guide. Once the cause is dealt with, a running grass such as Bermuda will often close a small gap on its own if you help it with soil contact and water, and larger areas want sod, plugs or seed depending on the species.
- Work out why the area is thin before filling it
- Check the mowing height and the mower blade first - both are free to fix
- Look at what is above it, not just what is under it
- Compare it with the rest of the lawn. Is it thin, or is the whole lawn thin and this bit worse?
- If it has come back in the same place two years running, it is the site rather than bad luck
When to stop guessing
Four situations where a visit is the right call
Guessing wrong on a lawn problem usually costs a season, because most of the useful work has a window and the window closes.
Estimates are free, so having somebody stand on the lawn costs nothing but an appointment. If you have a photograph, send it through the contact page with when it started, whether it is spreading, and what the watering and mowing have been like - that history is often worth more than the picture.
- It is spreading. Something advancing week on week will not wait for a considered opinion
- It has come back in the same place more than once. That is the site telling you something
- More than one cause would explain it. Treating the wrong one is worse than doing nothing
- The next step would be a treatment. Nothing should be applied to a lawn on a guess
Answers
Questions about brown or thin lawn guide
Why is my grass turning brown even though I water it?
Water is only one of several causes. Disease, insect damage, compaction, a dull mower blade, fertilizer burn, shallow soil and dog urine all produce brown. The pattern of the damage is the useful clue.
Why are circular brown patches spreading across my yard?
Circles and rings that spread outward are the classic look of a turf disease, and large patch is the one that most often produces them here in cool wet spring and fall weather. It is worth having looked at rather than guessed.
Why is my lawn yellow?
Usually nutrition, water or roots. A nutrient shortage, an iron problem tied to soil pH, and a waterlogged root zone all produce yellow, and they want different work. Where it is yellow matters more than that it is.
How do I tell disease from drought?
Drought follows the terrain and browns the high and dry areas first, with soft boundaries. Disease tends to spread in circles or blotches with a distinct edge, and it gets bigger week on week.
Why do the bare spots keep coming back?
Because the cause is still there. Shade, compaction, drainage, traffic, scalping and pet damage will all defeat reseeding if they are not dealt with first.
Can I send a photo?
Yes, through the contact page. Include when it started, whether it is spreading, and what the watering and mowing have been like.
Read next
Related reading
Drought, Heat Stress and Lawn Dormancy
From the lawn and property care guide.
Shade, Drainage, Compaction, Aeration and Thatch
From the lawn and property care guide.
Turf Management
A service HRH Lawn Care handles.
HRH Lawn Care Services
Not sure what your property needs?
Call and ask. We would rather come and look at it than guess over the phone, and it does not cost you anything either way.