Definitely too late to prune now. Silver leaf is a fungal disease which enters through damaged bark, as in pruning cuts. It is more prevalent from September on. Pruning is not usually necessary on established trees, but tell that to mine. It is normally done between June and the end of August. Always paint over the cuts with wound paint (a kind of wax). Usually all you need to do is to remove any dead branches, then any that cross and rub, then any going in the wrong direction and finally thin out the crown to let air and light get into the middle of the tree. When cutting prune back to a leaf joint. It is just like pruning a big rose.
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