The slime mold fungi always elicit interest from gardeners and landscapers because they are so different from other fungi. A vegetative stage is found in warm, dark, moist places, such as under shrubs on mulch. This slimy, amoebalike stage appears on low-lying objects and may be watery white, gray, cream-to-light-yellow, violet, blue, green, or purple-brown greasy masses as large as 1 to 2 feet in diameter. There is also a reproductive stage that usually pops up in drier locations with more available light, such as on the surface of rocks, tree stumps, and even live, healthy plants. Some of the slime molds have bright colors, while others are tan or brown. They may occur in puffball-like forms, in crusty formations, or flat on the chosen substrate.
Slime molds suddenly appear after heavy rains or after watering plants in warm, muggy weather. Although these organisms cause much concern, they do not take nutrients from the plant material. They feed on decaying organic matter, fungi, and bacteria in the soil and the turfgrass thatch layer. Slime molds are primitive organisms that flow (too slowly to watch) over low-lying objects (such as mulches, sidewalks, driveways) or vegetation (such as turfgrasses, strawberries, flowers, ground covers, weeds, and the base of woody plants).
Most gardeners want to know what to put on these molds to kill them. Chemicals do not provide control. Instead, for abundant molds, break up the unsightly spore masses by vigorous raking, brushing, or hosing down with a stream of water. Mowing the lawn usually removes the spore masses in turfgrasses. Slime molds disappear with hot, dry weather, so we should expect this soon. For more information about slime molds, read Report on Plant Disease (RPD) no. 401, which discusses slime molds in turf.
A related, but more annoying fungus is the artillery fungus (also known as the shotgun fungus). This fungus also thrives in wet mulch and has become a nuisance with the popular use of mulch in planting beds near homes. The fungus is white and forms tiny 1/4-inch puffball-like structures containing spore masses. As these structures dry, the spore mass is shot out of the fungal vessel and as much as 10 to 20 feet away. These spore masses have a very sticky surface and stand out as black spots (pinhead-sized) on sides of homes. You will find it nearly impossible to remove these spore masses from a home without removing the paint. They are particularly nasty on white siding. Try using only bark-based mulch, especially true cypress bark, and avoid the woodbased mulches made from wood chips and ground-up wood pallets. In either case, it is recommended that the mulch be raked or stirred to help it dry out so that it is less desirable to the fungus. Those natural mulches are good for the garden plants and fungi alike.