Hello, I just recently noticed my black oak tree leaves are starting to turn...
Hello, I just recently noticed my black oak tree leaves are starting to turn hard and shrivel up, but they do not fall off. This is not localized to one or a few branches, it appears all over the tree as far as I can tell. Generally speaking the tree looks healthy.
Your tree's leaves have vein pocket gall. It is caused when an insect called a midge lays eggs, those eggs hatch into larvae, and the larvae feed on the tissue along the midrib and veins of the leaf. Control is difficult and usually not necessary. This is not going to kill your tree.