Answer: Top dieback is usually a sign of a borer infestation, but it sounds like you have investigated that and ruled it out. Check the trunk for swellings, cracks, or other symptoms of damage/disease that might interfere with the movement of water to the top of the tree. In any case, if the rest of the plant is OK, you'll need to train a new branch to a stake, so it can become the central leader of the tree. If none of these ideas cover your situation, have a professional consultation. It may also be that the tree roots are outgrowing the space they're in, but that usually causes general dieback of all branches, not just the top.
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