Q: I have two holly bushes in the front of the house that are 7 feet tall. This year they have a black coating on the leaves. The affected areas are about 20 to 30 percent of the bushes. I have been treating them with insecticide buts so far this has not resulted in any improvement.
A: Your hollies have an infestation of one of the many kinds of scale insects. Scale can’t digest all the sap they suck from leaves so they secrete honeydew, which allows sooty mold to grow on leaves.
Spray them thoroughly with horticultural oil in fall and again in March. Thorough coverage under the leaves is imperative. Apply imidacloprid (click for sources) in May.