Answer: I can't really diagnose the problem without seeing it but there are two possibilities. Oxyporus is a white rot. It decays both lignin and cellulose and leaves the wood mushy and white. Another possibility is lichen, which is a greenish white patch and is common on the trunks and branches of trees and woody perennials. Lichen does not harm the host except that it might hold too much moisture against the bark and cause rot. You can kill lichen with a lime sulfur spray (bordeaux). Keep the spray off of foliage or it will cause damage. Or, you may want to take a close up photo and send it to your local cooperative extension office for positive diagnosis. Contact Cornell University cooperative extension, 1360 Fulton St # 400,
Brooklyn, NY 11216 phone (212) 340-2995?.
Best wishes with your oak tree!
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