Ask Mr. Smarty Plants

Mr. Smarty Plants contacted Eric Beckers form the Texas Forest Service about this question.  Here is what he said:

"We're hearing this over and over again and we'll probably hear about it throughout the new year.  After being stressed by the drought many of our oaks were further damaged by Hypoxylon and borers.  As soon as you hear someone say the bark has fallen off, you can bet that their drought stressed tree has hypoxylon cankers (first brown and powdery, and then a silvery crust) and the prognosis is not good."  

Go to the  TAMU AgriLife Extension Plant Pathology page to find three fact sheets about hypoxylon:  Hypoxylon Fact Sheet #1, Hypoxylon Fact Sheet #2, and Hypoxylon Fact Sheet #3. 

Since you live in San Antonio, your trees aren't likely to be affected by oak wilt.  Eric continued:

"If you live in the areas north of a Wimberley to Kyle line then there is the chance that oak wilt could be at play as well.  Trees damaged by oak wilt can also display hypoxylon cankers, so then we look for the pattern of mortality, groups of oaks dying side by side, and the brown vein leaves.  Drought mortality accompanied by hypoxylon cankers usually occurs on one or two oaks over here and another two or three over there—the sick trees are scattered about with healthy ones in between."

However, you should visit the Texas Oak Wilt site to familiarize yourself with the symptoms, prevention and treatment of the disease.

We are very sorry about your trees and hope that you are able to save some of them.  If you are interested in replacing them, you can visit our Texas-South Recommended page to find trees for your area.  Actually, since San Antonio is right on the line between the two regions, the Texas-Central Recommended page would have some tree species appropriate to your area, also.