Ask Mr. Smarty Plants

There are molds (e.g., sooty mold) that blacken leaves on hollies such as Ilex vomitoria (Yaupon holly) as well as other landscape plants.  The mold forms on secretions of plant sucking insects such as aphids and mealybugs.  If it is sooty mold, though, you probably can recognize it fairly easily and it will wash off.   My strong conviction, however, is that this is a result of the extreme heat and drought that Texas is undergoing.  You can visit the U. S. Drought Monitor map to see that most of Texas is in "extreme" or "exceptional" drought conditions.  I don't know how much and how you are watering the tree, but when you do water the soil needs to get a thorough soaking—the equivalent of an inch or more of rain.  (The Texas Forest Service recommends 1-4 inches of water every 10 days.  Use a rain gauge or some sort of container under the sprinkler pattern to measure it.) This will insure the water seeps deep into the soil and it will reach the plants roots there and keep them from growing toward the surface.   Also, the roots generally grow out to at least the same distance as the spread of the tree—the drip line—so watering with a soaker hose at the drip line is an efficient way to get the water to the roots.  Be aware that it takes longer to deliver 1 or 2 inches of water using the soaker rather than a sprinkler.  Mulch spread around the tree can also help to hold in the moisture.  Here is an article, Mulching Trees and Shrubs, from North Carolina State University with good tips on mulching and here is an article from our website, Helping Plants Handle Summer Heat.

Of course, overwatering can be harmful to trees, too, with many of the same symptoms of yellowing and dying leaves.  If the soil you describe as "black gummy" is always wet and gummy then you probably are overwatering.  If your soil is always damp more than 4 or 5" down (use a screwdriver or similar sharp instrument to push into the ground), then you are watering too much.   Here is a an article, Watering Trees and Shrubs, with more information from the University of Arizona College of Agriculture & Life Sciences Cooperative Extension Program.