Oh dear, not sure, but I do hope its not the latest affliction from which oaks are being decimated. A word with your Council's parks/tree department may solve the mystery, and also alert them that there may well be a problem.

thats worrying isn't it , is it going to be like 'dutch elm disease ' allover again :o(((

Are they Holm Oaks? They are evergreen and the ones where I live do drop more leaves in spring than autumn.