Don't know, but maybe this will help. Sounds like you can send in samples to get it tested.

http://texasoakwilt.org/oakwilt/oak-wilt-identification/

Thank you lyfia! That is very helpful. I ended up calling a local arborist and emailing them some pics of the trees and leaves. They said it wasn't oak wilt, but that we had planted our trees too deep. What a relief! Their advice was to remove soil to the root flare and leave it exposed to the air.

Thank you again for your response!