Ask Mr. Smarty Plants

Fraxinus texensis (Texas ash) shows on this USDA Plant Profile as growing natively, if not in Guadalupe County, at least very close, so we are assuming you have the right climatic conditions and soils for this tree. No, we can't say we have heard of this before, but you seem to link it with the fertilizer. What kind of fertilizer were you spreading? Was it "weed and feed" fertilizer for your lawn? That would be a high nitrogen fertilizer to encourage lots of green blades of grass, with a herbicide designed to kill broadleaf, that is, non-grass weeds in the lawn. The tree is also a broadleaf plant, and it is possible that the damage was done by that material being spread around, either getting down to the trees roots, or being distributed in the air by wind or the method of applying it. 

What to do about it? Well, to begin with-don't do that again. For one thing, many of the "weeds" you want to kill are other grasses than the lawn grass; they won't be killed by that particular herbicide. The second reason is what you may have already experienced, leaf loss on your broadleaf ash tree.  You also no doubt have broadleaf shrubs and garden plants that will not be happy to receive a dose of that broadleaf herbicide.

Ash trees are generally weak trees, growing fast but not living very long and are very susceptible to pests and diseases. At this point, about your best option is to wait and see. Make sure the ash has plenty of water and don't fertilize it. Plants native to the area in which they are being grown usually need no fertilizer and it could shock an already stressed tree into trying to put on new growth when it needs to be concentrating on staying alive. 

From our Native Plant Image Gallery:

Fraxinus texensis

Fraxinus texensis

Fraxinus texensis

Fraxinus texensis