Answer: It is not unusual to find a plumeria visibly shriveled after its dry winter storage period, but the rot is discouraging.
Based on your description it sounds like a severe case of black tip fungus. This causes the growing tip to turn black and stops future growth and it usually only occurs in the spring. The cure is to use a fungicide early, as soon as the problem is noticed, and to cut the plant back to good wood using a sterile knife.
It may be too late, but since this is such a special plant, you may be able to save it by cleaning away all of the affected parts, bare-rooting the plant, treating it with a fungicide according to label directions, and repotting it. Obviously, this will only work if the roots are still viable.
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