Answer: Brugmansia can be overwintered under several different sets of conditions depending on the storage capabilities available to you. The American Brugmansia and Datura Society offers detailed temperature-based instructions at their web site. (Scroll down to the English translation.)
I would add the caution that you need to check it very carefully prior to bringing it indoors -- these are very prone to whitefly and other pests that seem to replicate all too fast indoors during the winter.
These plants normally have a fairly bare lower stem so what you are seeing may be normal if the plant has put on a lot of size this summer. However some insects such as spider mites can cause discoloration and leaf drop (you will find fine webbing on the undersides of the leaves and along the stem) and can be treated with commercially formulated insecticidal soap per the label instructions (this also works on whitefly).
And, these plants are potentially subject to many diseases -- particularly a number of wilts that are also commonly associated with tomatoes and also tobacco (for this reason smokers should not handle your plant).
It is very difficult to make this sort of diagnosis long distance. You may want to consult with an experienced nurseryman to see if the foliage loss is disease related. I'm sorry I can't be more specific for you. Good luck wiht your plant!
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