Slime molds

Fuligo septica, Physarum & Mucilago spp.

also called Dog vomit fungus, Fuligo

A bright yellow foam on the mulch or a gray powder on the grass. It is not a fungus, it is not a disease, and it is not hurting anything.

Fungus Disfiguring Airborne sporesSoil-borne

What you'll see

A bright yellow, orange or white foamy mass on bark mulch, appearing overnight after rain and looking exactly as unpleasant as its common name suggests.
Within two or three days the mass dries to a crusty gray-brown pad that puffs dark spores when poked.
On lawns, a gray, blue-gray or purple powder coating the grass blades in a patch a hand-span across.
The grass beneath the coating perfectly green and undamaged when you rub it clean.
The whole thing gone within a week, and back after the next wet spell.

Why it happens

Not a fungus and not a plant pathogen. A slime mold is a colony of single-celled organisms that creep together across damp organic matter feeding on bacteria and decaying material, then gather into one mass to produce spores — which is the stage you notice. It uses your mulch and your grass blades as a surface to climb, nothing more.

Warm, wet weather over damp organic matter: fresh bark mulch, a compost heap, a lawn with heavy thatch, or shady beds after a run of rainy days. It appears within a day of a downpour and disappears as soon as things dry.

Treating it

Nothing needs treating. If it bothers you, break it up with a rake or a jet of water and it will dry out and go.
Scoop the mass off the mulch and put it in the compost heap, where it will carry on doing what it does.
Mow the lawn or brush the grass blades to knock the gray coating off.
Let mulch dry between waterings and turn it over occasionally.
Do not spray a fungicide. It will not work, because this is not a fungus, and there is nothing to cure.

Keeping it away

Turn and fluff bark mulch occasionally, and keep it to a five-centimeter layer rather than piling it.
Reduce thatch in a lawn that gets it repeatedly.
Improve drainage and light in a shady bed that stays permanently damp.
Water in the morning so surfaces dry through the day.
Accept it as a sign of a healthy, biologically active mulch — because that is what it is.

Who gets it

Nothing is attacked. It appears on mulch, lawns, compost, low bedding plants and anything else damp that it can climb.