Certified GKH Gardening Expert
Unfortunately, your photo did not come through. There are things that could be likely. Some things that come to mind are spores from the fern, sooty mold from an infection on the fern, and sap from other trees nearby.
Likely, it is spores, in combination with moisture and soil residue. Other than cleaning up after them, there not be a lot that you can do.
In the case of sooty mold infection, you will need a fungicide, but I cannot say that this is what is going on without seeing the issue in question.
Here is an article to help you with outdoor fern care, in the meantime: