Mulch can be the breeding ground for unsightly but harmless growths.
From time to time mulch can be the breeding ground for a variety of fungal growths. Gardeners are familiar with the slime molds that are yellow, orange or light brown and resemble foam. Shown on the left is another fungus that sometimes appears on mulch — the stinkhorn, or dog stinkhorn mushroom (Mutinus caninus), so called because the slime on the end of its fruiting body gives off an unpleasant odor.
The stinkhorn begins as a roundish structure; when the erect stalk appears, its tip is covered with a dark, slimy mass that contains spores. Flies and other insects are attracted to the putrid smell, light on it and end up disseminating the spores to other areas. The stinkhorn is not harmful to plants or to people, and the smell alone doesn't invite an up-close inspection.
There's not much you can do to rid yourself of the stinkhorn in the mulch bed. You might try raking the mulch to break off the stalk and shorten its life cycle. Stinkhorns tend to appear more frequently in hardwood mulches than in pine bark mulch.
Fungus grows on mulch occasionally as part of the natural decay process. The slime molds tend to appear more often in wet weather and will disappear when conditions no longer favor them or when the mulch breaks down sufficiently.