This happens at least once a year. The leaves get first spots that eventually take over the leaf and then I cut it off because its gets yellow and ugly. Then it starts on the next leaf and pretty soon I have a stick plant.
It is a musa basjoo. It lives in a pot - outside in the summer, inside in the winter. The spots happen regardless of season. I wash the plant almost every day because last year it did have spider mites. None at this time. Humidity is 30-40% and it sits in a sunny window.
http://i514.photobucket.com/albums/t...a/L1020790.jpg