Answer: It isn't too likely that one overwatering incident caused the decline of your daphne, but daphne's need very little water so perhaps it has been getting far too much over a long period of time. They are also sensitive to herbicides and can be harmed by overspray or even by chemicals leaching into the rootzone. There are some fungal diseases that can attack the leaves, roots and crowns of the plants, as well as a canker disease. Crown rot can take the life of a plant in pretty short order and daphne's growing in too much shade seem to be most susceptible. Root rot is caused by a fungus (Phytophthora), as is stem rot. Based upon your description of sudden dieback, I'd suspect root or crown rot. In either case there is no cure and the best course of action is to remove the plant. If you want to grow another daphne, choose a different garden spot to avoid infecting the roots of the new plant with old fungal spores that remain in the soil.
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