What is the black oily coating on the leaves in the woods (south eastern PA)
All of the leaves regardless of plant type have a black oily looking coating in. The more shade, the more black coating. It feels dry to the touch but does get sticky on your fingers a few minutes later. Some of the leaves have a dry black appearance while others look as if they were sprayed by a used motor oil.
Answer
Dear Hiker: What you are seeing is probably a fungus that is growing in the honeydew or secretion of insects that are or were in the trees. Since fungi prefer shade, the shadier the area, the easier it is for the fungus to grow. It is frequently called Black Sooty Mold.