Answer: I can't say for sure what the problem is, but I can make a few educated guesses. Your Andromeda (Pieris japonica) is succeptible to a soil fungus, Phytophthora, that causes dieback. There are several species of nematode that attack the roots as well. This could explain why you haven't seen any clear cause for the problem. There's no stopping Phytophthera once it infests the plant, but nematodes can be controlled.
Lace Bugs, which are masters of camoflage, may also be involved. They are such a problem in the northeast that Pieris in general don't do well. They are tiny, fly-like bugs with transparent wings (hence their name). If you don't find them, then the problem is probably root-related, but the only way to find out for sure is to have a root sample examined by a pathologist. Contact your local agricultural extension office (ph# 617/891-0650) and find out where you can send a sample. There may be a fee for the service, so be sure to call the pathology lab for details. Best of luck!
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