Answer: Unfortunately, based on your description I am not certain what is happening with your banana tree. Bananas are subject to a wide variety of diseases such as Cercospora leafspot fungus which would cause spots, as well as scale and other pests which could cause spots, but spots are very difficult to diagnose long distance especially when there are so many possible culprits -- one list I found included over 50 possible diseases. For a specific diagnosis and recommendations for treatment (if necessary) I would strongly suggest you work with your local county extension. They may appreciate photos of the overall plant as well as closeups of the spots to help them identify the problem. I'm sorry you are having trouble with your Musa basjoo.
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