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Slime Molds and Fungi on Mulches

July 14, 1999

Slime molds are beginning to invade gardens. These growths suddenly appear after heavy rains or after plants have been watered 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. During warm, moist weather, the slimy, amoeba-like stage of the mold appears on low-lying objects, appearing as watery white, gray, cream to light yellow, violet, blue, green, or purple-brown greasy masses as large as 1 to 2 feet in diameter. This stage soon develops into colorful, crusty fruiting bodies filled with masses of dusty spores. 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 bases of woody plants. They always concern gardeners.

Most gardeners want to know how to kill them. Chemicals do not provide control. Instead, for abundant molds, break up the unsightly spore masses by vigorous raking, brushing, or hosing them down with a stream of water. Mowing the lawn usually removes the spore masses in turfgrasses. Slime molds disappear with hot, dry weather. For more information about slime molds in turf, read RPD No. 401.

The artillery fungus, also known as the shotgun fungus, is another fungus that thrives in mulch. It has become a nuisance as the use of mulch in planting beds near homes has become popular. Again, in wet conditions, this fungus grows in the mulch. The fungus is white and forms tiny, 1/4-inch, puffball-like structures that contain spore masses. You probably won’t notice the puffballs. As these structures dry, the spore mass is “shot” out of the fungal vessel as much as 10 to 20 feet away. These masses have a very sticky surface and stand out as pinhead-sized black spots on the sides of homes. It is nearly impossible to remove the spore masses from a house without removing the paint. A colleague in the Ohio Cooperative Extension Service recommends using only bark-based mulch, especially true cypress bark, and avoiding wood-based mulches made from wood chips and ground-up wood pallets. In either case, mulch should be raked or stirred to help it dry out so that it is less desirable to the fungus. Natural mulches are good for garden plants and fungi alike.

Author: Nancy Pataky


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