I relocated from Michigan to West Little Rock in January and my new home has a Japanese maple in the landscaped area in the front yard. About two weeks ago I noticed what looked like a yellow fungus or mold growing on top of the mulch. On closer inspection, I also found a similar fungus surrounding the trunk of the maple. The growth reached approximately 6 inches above the mulch up the trunk and was a dark gray or black in color. I cleaned it from the trunk and bagged some of the residue in case I needed to have it inspected, but I'm concerned that there's something going on in the bed itself that I need to address.
Now that you have cleaned it up, pay attention to the area in the next few weeks to see if it returns. It could be a simple slime mold or even one of the organisms which helps the mulch decay and break down—neither of which is cause for concern. They are both common, especially during periods of high humidity. Do try to keep the mulch away from the trunk of the tree to allow for good air circulation and allowing the bark to dry.
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