Ask Mr. Smarty Plants

First, let's figure out which "red oak" we are talking about. Quercus coccinea (scarlet oak) is the only one actually called "red," but it is not native to Texas. There are six oaks listed in our Native Plant Database as being native to Texas and having "red oak" as one of their common names: Quercus buckleyi (Buckley oak), Quercus falcata (southern red oak), Quercus gravesii (Chisos red oak), Quercus shumardii (Shumard's oak),  Quercus texana. (Texas red oak) and Quercus marilandica (blackjack oak). Of these, Quercus buckleyi (Buckley oak), Quercus shumardii (Shumard's oak), Quercus texana (Texas red oak) and Quercus marilandica (blackjack oak) are the four "red oaks" considered by the Texas Oak Wilt  Information Partnership to be most susceptible to Oak Wilt. This is not to say we think that's your problem, we just want to narrow down the choices. A retailer can call a tree anything he wants to, so we will just pick one and use it as an example. Since this USDA Plant Profile shows it growing squarely in Plano, Collin County, we will use Quercus shumardii (Shumard's oak) as our example. 

From the Dirt Doctor (Howard Garrett) we extracted this information from a site on the Shumard Oak: "Red oaks can’t stand wet feet. Poorly drained soil causes root disease, illness or death. Red oak are highly susceptible to oak wilt. Occasionally red oaks are bothered by scale, borers and other insect pests but that is always a symptom and related to a bigger problem of an environmental nature.  Another cause of problems is mistakenly buying pin oaks or crossbred trees."

It looks like that is the best guess on what is causing the problems on the bottom of your tree. You planted it during a very dry period in Texas, and it is now a very wet period, especially in North Central Texas. An innocuous low spot before is now a puddle. As pointed out above, this problem in the tree's environment can make it that much more susceptible to other diseases and insect problems, most particularly fungi.

If you want to spend the time and effort, you might try transplanting the tree to a better spot. If you are going to do this, you need to do so right away, before it gets any hotter. We ordinarily recommend transplanting woody plants, shrubs and trees, in Fall or late Winter when they are more or less dormant. You can't very well wait that long. We found an excellent website from About.com: Landscaping on Transplanting Trees and Shrubs.  Pay particular attention to their instructions about selecting the spot-a sunny area and, of course, not low where water will stand. Also, as recommended, get the new hole dug and ready for the tree before you dig it out of the ground or disturb it, and get it back in the ground in its new home right away. 

We do not guarantee that the tree will have been worth the effort. It may die, no matter what happens. Or, it might get well and do fine if you don't do a thing, but we don't think so. We suggest that you contact the Texas A&M AgriLife Extension Office for Collin County, and see if they have any other suggestions. 

From our Native Plant Image Gallery:

Quercus buckleyi

Quercus shumardii

Quercus texana

Quercus marilandica