what has caused the leaves of 3 branches on the same limb of one of my apple trees to turn completely brown

Question

the leaves were not distorted or eaten by aphids, they just went completely brown & crispy

Answer 1

I would first suggest removing all the affected branches since those leaves have stopped photosynthesizing. Here's an article that may help you to identify and treat the tree: https://www.gardeningknowhow.com/edible/fruits/apples/apple-chlorosis-treatment.htm

There is also a possibility that canker has set in or root rot is beginning or, perhaps, a high wind has harmed those particular branches.

https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-root-rot.htm

https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-with-canker.htm

If these articles don't help you identify the issue, you may want to take a sample to your local extension service.

You can get that info here: https://www.gardeningknowhow.com/extension-search?zip=38559&radius=10&services_search_action=search&submit=Search