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What are those spots on my siding?

By Mary Beth Breckenridge Published: July 25, 2014

Maybe you've noticed little black spots appearing on your home's siding, your car or other surfaces around your house. That's the unfortunate result of shotgun fungus (also called artillery fungus), which often grows on decaying wood mulch and shoots tarry spores that are almost impossible to remove.

It's the time of year when those spots start making their unwelcome appearance, the Ohio State University Extension reports in the latest issue of its Buckeye Yard and Garden Line e-newsletter.

No fungicides are known to be effective in controlling the fungus, but the extension says you can reduce the problem with these practices:

  • Choose a wood mulch that has more than 85 percent bark. Bark doesn’t decay as fast as wood.
  • Use pine bark nuggets or cypress mulch. Both resist rot.
  • Cover existing mulch with pine needles or fresh mulch each year to suppress development of the artillery fungus. But be careful not to let mulch pile up higher than 3 or 4 inches. 

Plant pathologist Donald Davis at Penn State University has a website devoted to artillery fungus with lots of good information, including methods for removing the spots. In addition, Ohio State has a fact sheet that addresses shotgun fungus and other problematic molds and fungi associated with mulch and compost.

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