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Yellow Fungus in Grass: Slime Mould, and It Is Harmless

Yellow fungus in grass is almost always dog vomit slime mould, which harms nothing. How to tell it from the 2 lawn diseases that do need action.

By Salma 3 min read Save to Pinterest — opens Pinterest in a new tab
A mass of bright yellow dog vomit slime mould spreading over bark mulch
Photo: The Cosmonaut, CC BY-SA 2.5 ca, via Wikimedia Commons
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Bright yellow froth that appeared on the lawn overnight after warm wet weather is almost certainly dog vomit slime mould, Fuligo septica. It looks alarming, it is not a disease, and it is not harming the grass.

Telling the three apart

Yellow on a lawn
Slime mouldRustDollar spot
WhereOn top of bladesIn the bladeIn the blade
Looks likeYellow froth, turns greyOrange pustules, powderStraw patches 1–3 in
On your shoesNothingOrange dustNothing
Wipes offYes, cleanlySmearsNo
Harms grassNoYes, weakens itYes
FixNothing neededNitrogen, waterNitrogen, water

What slime mould actually is

Not a fungus, despite the name. Slime moulds are amoeboid organisms that spend most of their life as microscopic single cells in soil and thatch, feeding on bacteria and decaying matter.

When conditions are right — warm, humid, usually a day or two after rain — those cells aggregate into the visible mass that appears on the lawn. It climbs onto grass blades not to feed on them but because it needs height to release spores.

It will go bright yellow, then dull, then grey and crusty as it dries and sporulates, over a few days. Then it is gone.

Note

The only harm it does is shading. A dense patch sitting on grass for several days can leave the blades underneath slightly pale, and they recover within a week of the mould being removed or drying out. That is the entire extent of it.

If you want it gone

Any of these work, and none of them are necessary:

  • Rake or brush it — breaks the mass up and it dries
  • Hose it off — disperses it into the thatch
  • Mow — though it does spread spores around, which does not matter
  • Nothing — it disappears by itself within a week

Do not use fungicide. It is not a fungus, fungicides do not target it, and you would be treating a lawn that has nothing wrong with it.

It recurs in the same places because the food source is there: buried wood, an old root, a spot where mulch sat, or heavy thatch. Dethatching reduces it. Given that it does no damage, that is usually effort better spent elsewhere.

The two that do need something

Rust. Fine orange-yellow pustules on the blades, and the giveaway is orange powder on shoes and mower wheels. Rust appears when grass is growing too slowly to outgrow it — low nitrogen, drought stress, heavy shade, or late season. The correction is to get it growing: feed, water properly, and mow regularly to remove infected tissue. Fungicide is rarely justified on a home lawn.

Dollar spot. Small straw-coloured patches one to three inches across, often merging, with distinct pale lesions banded across individual blades. Also strongly associated with low nitrogen, and also corrected mainly by feeding. Watering deeply in the morning rather than lightly in the evening helps, because it thrives on long periods of leaf wetness.

Both are conditions problems that present as disease. A lawn fed and watered properly largely stops getting them.

Common questions

Is yellow slime mould in grass harmful?

No. Fuligo septica — dog vomit slime mould — feeds on decaying organic matter in the thatch and soil, not on living grass. It sits on top of the blades and can shade them enough to cause slight temporary yellowing underneath, but it causes no disease and needs no treatment. It is not toxic to people, though some find the spores irritating, and it is not considered dangerous to pets in the quantities a lawn produces.

How do I get rid of slime mould on my lawn?

Break it up with a rake, a stiff brush or a jet of water, and it disperses. It will often reappear in the same spot after the next warm wet spell because the food source is still there, which is the thatch and buried woody debris. Reducing irrigation and dethatching deal with the conditions, but given it does no harm, doing nothing is a legitimate choice.

What is the difference between slime mould and lawn rust?

Slime mould sits on top of the grass as a bright yellow or orange frothy mass you can lift or wipe off, leaving the blade undamaged. Rust is in the blade: fine orange-yellow pustules that rub off onto your fingers and shoes as powder, and the grass itself turns yellow and thin. If a walk across the lawn leaves orange dust on your shoes, it is rust.

What yellow lawn problems do need treating?

Two mainly. Rust, which indicates grass growing too slowly — usually from low nitrogen, drought stress or heavy shade — and is corrected by feeding and watering properly rather than by fungicide. And dollar spot, which appears as small straw-coloured patches a few inches across with lesions on individual blades, and also responds to nitrogen. Both are conditions problems before they are fungus problems.

Why does slime mould keep coming back in the same place?

Because its food is still there. Slime mould feeds on decaying organic material, so it recurs over buried wood, old tree roots, heavy thatch or a spot where mulch was once piled. Warm humid weather after rain triggers the visible stage. Dealing with the thatch or the buried debris is the only thing that stops the recurrence, and it is rarely worth the effort.

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