I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
You are seeing a white mold type of us on the tree it is either a lichen growing on the outer bark, or possibly a decomposer fungi growing on a section of the tree branch that is already dead. I suggest you take a sample and if possible some good photos of the white areas to your County Extension Office for them to look over and assess
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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