Answer: Sometimes a tree will simply fail to thrive, and it is better to replace it. A tree that is badly infested with scale can serve as a source of infection for other trees, so it is important to deal with the problem and be careful to clean up and dispose of any plant debris.
It is possible that the tree has never rooted well into the native soil, perhpas due to encircling roots within the planting hole, and is thus stunted. A tree that is doing poorly is weakened and less able to withstand attack from pests and diseases, too.
You might want to also examine the planting area carefully to see if there are any other possible explanations such as root competition from neighboring trees, shade from a building, a poor drainage, and so on.
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