Answer: In general it is not a good idea to fertilize a tree that is stressed without knowing the cause for the stress. Trying to force growth in the face of an unseen problem (such as root damage or damage to the graft area, or some other physical cause, such as insect infestation or interior rot) may cause even more stress on the tree.
If the branches are truly dead, they should be carefully removed. If they are simply defoliated but still alive, then wait and see if they can manage to leaf out again. In the meantime, inspect the tree very carefully for signs of any unusual activity.
Unfortunately these trees are not always so long lived, and the drought may have stressed your tree to the extreme. You might want to consult with a certified arborist and see if they think a special nutrient or micronutrient program would be helpful and what sort of prognosis they see in general for the tree.
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