There are a number of possible causes. Are the fruits also scarred? If so it could be apple scab which works on the leaf and fruit. This requires treatment with a fungicide rather than insecticide. Another possibility is a bacterial canker which often results in gum leaking from stems. Again this needs treating with a fungicide. If you want to keep the tree it might be worth cutting it back hard this winter to rejuvinate it and giving it a good feed and see if that does any good.