Question
I have two pin oaks. One of them seems to be sick or have a parasite. For the last two summers, it gets a very stick secretion on the leaves that then blows off and gets all over our cars and anything else that might happen to be down wind. I sometimes see a milky or white powdery residue on the leaves as well, but it's the sticky, sap-like liquid that blows off and causes a huge mess. Any idea what is wrong and if so, if there is a way to treat it? Thanks in advance for any help you can give.

Answer
Sounds like either aphids or a scale insect--both insects will suck the plant juices from the plant and secrete a substance called honeydew which is high in sugar content and sticky. The honeydew will rain down in small droplets and get on anything under the tree. A sooty mold fungus will grow on this sugar making the black. Control the scales and aphids and the honeydew will go away. Treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soil around the tree and the roots will carry the insecticide to the leaves and stems and when the insects suck the plant juices they are killed. It will take a couple of weeks for the insecticide to get to the top leaves depending on the tree size but the treatment will last all season. Here is a web link to this product check with your local nursery for this product.
http://www.bayeradvanced.com/tree-shrub-care/products/12-month-tree-shrub-insect...