We had oak wilt and may still have it? It seemed to affect black oak trees more than the others. The largest oldest trees on our property are black oaks. I looked it up several years ago. It's a vascular disease - sort of like what Fusarium Wilt does to tomato plants. I just looked it up again, and this site explains a lot. I haven't read the whole site yet
http://texasforestservice.tamu.edu/OakWiltFAQS/