Welcome to my nightmare. I had never even heard of sooty mold until a few days ago. I started noticing a lot of black spots on my pants and rocks and anything else directly under my 2 Robinia trees. The entire area became very unsightly and really had me looking into what was going on. A little time spent on the inter-net and the entire ugly truth was looking me in the face.
My beautiful Robinia trees were stressed and developed a real bad case of aphids. The aphids release a sticky substance called honey dew which falls and lands on everything under the tree. Mold spores floating by attach themselves to the honey dew and then feed off it and reproduce at an alarming rate. The mold is black and will rub off on your fingers just like soot. Hence the name, sooty mold.
Apparently a very common problem probably brought on by our unusual wet, cool spring. To break this vicious cycle you must get rid of the aphids, and on 2 large trees all that is practical would be to spray the trees. As we garden organically I refuse to spray and prefer to not grow plants that cause these type of problems. I also discovered that Robinia are prone to this problem. For me the only solution was to get rid of the trees as they will no doubt continue this cyle every season.
In a yard that has little shade in a very hot, dry climate, and where trees are difficult to grow, cutting down these 2 trees was a sad day for us. However, down they came and as well I cut down all the plants growing underneath which were coated in an ugly black, sticky film. I now have 2 very bare areas sitting in the scorching sun.
These are my trees in bloom this spring. One was purple and the other was white.
I actually had a shady corner to sit in which is rare for our garden.
Once the aphids attacked the tree it quickly was full of yellowed out leaves and looked very bad.
Just before the last cut of the trunk. See how black the bark has become. It is covered in sooty mold. Also notice how black the rocks have gotten and the black covered leaves on the Pyracantha in the background.
On the up side sooty mold does not harm the plants it grows on. It will eventually wear off and the plants below will live another day to tell their story. For this season however, these 2 areas are going to look a bit sad and I now have to figure out how to repair them.