Despite appearances this slime mold is growing on the mulch beneath this sedum, not on the plant itself.
Slime mold on a succulent
Close-up of fruiting bodies of slime mold on spirea leaf (Spiraea)
Slime molds are primitive organisms with an identity crisis. They share many characteristics with fungi and are still placed in the Kingdom Fungi by some authorities, but others now place them in the Kingdom Protista (Protoctista), a division of Myxomycota.
Slime molds obtain nutrients from bacteria, fungi, and decaying organic matter. There are many species in nature, but those most commonly encountered by gardeners are found either as inhabitants of landscape mulches or turfgrasses.
Slime molds in mulches typically appear in summer after rainfall and are frequently observed when they form large, colorful colonies on the surface of mulch around trees, shrubs, and perennials. Size is variable, ranging from several inches to 2 feet or more in diameter, with an irregular shape. Color is also variable, but the most noticeable form appears as a bright yellow, slimy mass when fresh. At this stage it is frequently referred to as the “dog vomit fungus”, and is plasmodial in nature, able to “flow” slowly across the mulch a distance of several feet, ingesting dead matter as it goes. They may appear to “grow” on plants during this flow stage but are not plant parasites and may only harm small plants by covering and shading them.
When conditions for growth are no longer favorable, movement stops, the mass changes color, and dries up. Spores are then produced which are carried away by wind to new locations.
Integrated Pest Management Strategies
1. Wait it out. Slime molds will eventually disappear on their own, but their unsightly appearance begs for more rapid removal.
2. A forceful spray of water from a garden hose will wash them away.
3. They can also be raked or turned under in mulch or lifted using a shovel or garden fork and composted or discarded in trash.
4. Mow. When they appear on turfgrasses, they can be mowed.