Question
I live in West Houston and I have a Red Oak that has black residue on the leaves and branches. Seems to be more prevalent on the older foliage. Any ideas?

Answer
Sounds like soot mold. The real cause is insects that suck the plant juices from the leaves and twigs. More than likely either aphids or scales. The leaves should be stick. The insects suck the juices and excrete a substance called honeydew and the fungi grows on the honeydew. Control the aphids or scales and you will control the soot mold. the soot mold will not harm the plant. Since it is late in the summer and soon the leaves will be falling I think I would do nothing at the time.  The conditions in your area have been right for aphids and soot mold (hot wet weather) Next year the populations may not even exist since the insect varies in population levels. Next spring if you begin to see sticky wet leaves then you might consider controlling the aphids. At that time if the tree is large I would treat with a systemic insecticide.
Some insecticides can be applied to the soil and taken up by the roots of the plants. These are called systemic insecticides. The most recent, Imidacloprid, is sold under the trade name Bayer Advanced Garden Tree & Shrub Killer Concentrate. (Merit is the trade name of imidacloprid used by professional tree care companies.) It is applied as a drench over the root zone.
There are several insecticides effective for aphid control when sprayed on plants. Perhaps most effective are those with systemic activity that allows them to move through the plant. Acephate (Isotox, Orthene) is the most widely available systemic insecticide.