Ask Mr. Smarty Plants

According to this USDA Plant Profile Map, Fraxinus velutina (velvet ash) does not grow natively in your area of Comal and Guadalupe Counties, USDA Plant Hardiness Zone 8a, so your problem could be with soil or climate. However, if the tree has grown there for 10 years, it's hard to understand why it would just now be showing these symptoms.  Up until August 1, we have had a relatively mild and damp summer, so heat stress doesn't seem to be indicated. Therefore, you need to ask yourself what in the environment has changed to cause that tree to have leaf tips browning?

We will do a little research into the Arizona Ash and see if we can find what might be causing a symptom you have not seen before. None of the sites we looked at mentioned any disease that caused that sort of leaf browning, but there were several mentions that this tree depended on an underground water source or was "riparian" meaning it likes river bottoms or other locales where there is a constant supply of water for its roots. We have had some rain in Central Texas, but perhaps for other reasons underground water has diminished, especially now that the "wet spell" is over and the heat is digging in. One source recommended deep irrigations of the tree two or three times a month during extremely dry spells. This would mean getting a hose down in the dirt around the roots, turning it on to a slow dribble and letting it run until water appears on the soil surface. The leaves browning at the top of the tree would seem to be indicative of this, as all the moisture for the tree comes up from the roots. The leaves at the very top, the end of the line, as it were, would be most susceptible to moisture deprivation. 

Another possibility, although more remote, is that of ash borers. See this article from Colorado State University Extension on Ash Borers.  This article points out that borers are more apt to attack trees weakened from drought or other reasons. However, first see if there are any signs of the beetles before you try pesticide. For more information on the possibility of ash borers in your vicinity, or other diseases that have appeared in the area, contact the Texas Cooperative Extension Offices for Comal County or Guadalupe County. 

Pictures of Fraxinus velutina (velvet ash) from Google.