Q: For the past two years, the water oak over our deck has been raining sap in the spring. It creates a horrible sticky mess that turns black and dirty. What causes this?

A: I think you have lecanium scale feeding up high in the oak tree. These insects suck sap from leaves and twigs and produce copious amounts of honeydew. The easiest control is to use imidacloprid (Bayer Tree and Shrub, etc) each year in early April (you can still do it now). This systemic insecticide kills scale but it will probably take a couple of years to kill them all.

see

Ornamental Tree/Shrub Fact Sheet