Bay trees are not prone to any particular diseases - when the last one died, did you wash and sterilize the container and use fresh compost when you planted the new one? Is there any evidence of beetles, resinous oozing or anything else in the damaged area? Otherwise, is it possible that damage occurs to the trunk at that point from something banging into it regularly (is the pot placed somewhere where people/objects pass it frequently, like by the front door?) Irregular watering can cause splits to appear in the bark and that may conceivably allow organisms to penetrate the plant.