When people see their woody plants (shrubs and trees) decline, they often want a "magical" treatment to make them lush, green and healthy again. But no magic exists. This publication should help you understand some common causes of woody plant stress, how stress leads to decline and what measures, if any, can make the tree healthy again. Remember, however, that all trees and shrubs like every living organism, will eventually die.
Hill, Deborah B. and Fountain, William M., "Woody Plants Under Stress" (1988). Agriculture and Natural Resources Publications. Paper 36.