I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
Based on your description it seems that your Mandevilla may have been suffering from soggy wet soil conditions. This prevents the roots from receiving adequate oxygen for periods of time and thus leads to their decline and often to root rot diseases. Hopefully the conditions have improved and the plant recovered be now but if left too long such conditions will be fatal to a plant.
Thanks for the question. Best wishes for a wonderful gardening year in 2007. Please stop in again soon!
Q&A Library Searching Tips