Mulches can be a valuable addition to any landscape. Used in a well managed garden, they can enhance beauty, suppress weeds, and keep moisture near the plants. The big question in mulches is can they harbor disease-causing fungus that can be dangerous to plants? The answer is mixed. It is possible to transfer diseases such as pine wilt, but only when infested mulch is incorporated into the soil, placed against a wounded tree, or isn't composted. Verticillium wilt isn't known to infect trees when infested mulch is used, but may infect vegetable plants when used as a garden mulch. The transfer of fungi can be limited by composting, one of the most effective and easiest methods to ensure disease free mulch. Composting in small piles can be enough to destroy plant pathogens. Compost piles should be kept as large as possible and must be turned two to three times. They also must be kept moist to ensure good microbial activity and heat, so it may be necessary to wet the pile from time to time.
Some annoying yet harmless fungi can also be found in mulch. The shotgun fungus, has spores that can be discharged up to 15 feet in the air. The bird's nest fungus also propels spores, but at a lesser distance. These spores are very difficult to remove and must be soaked with soapy water. Stinkhorn fungi and slime molds can also colonize mulches. These molds are harmless, and do not fire spores, but some people find their presence unattractive, although stinkhorn fungi can be attractive to flies. These fungi can easily be raked, picked, or hosed off. If the slime molds are left alone, they will dry up and disappear on their own.
Additional information on mulches can be found in SUL 12, Using Mulches in Managed Landscapes . This bulletin is available from your local Extension office or from the Iowa State University Extension Distribution Center, 515-294-5247.
This article originally appeared in the June 14, 2002 issue, p. 81.