Answer: I'd check first for insect problems. Avocado Brown Mite can be controlled by applications of powdered sulfur. Six-spotted Mite is very harmful; even a small population can cause massive leaf shedding. A miticide may be required. Root rot is another possibility. There's not much you can do to save a tree if root rot is present. You may want to hire a certified arborist to assess the trees. If control is possible you can contract with him/her to spray the tree. Wish I had more information to share. Best wishes with your trees!
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