My musa basjoo in shade gets the same problem. I think it might just be an overwatering problem or maybe a lack of sun. I cut the brown spots and Im hoping less water fixes the problem, but Im not sure if thats a definite fix. My other Musa basjoo gets more sun and has no signs of a brown spot (but it sure hates the wind!)
Good luck!
-Alex

This is to tropicalzone7. I have read here that musa can't be over watered. The more the better. ? I water both my musa and my red abyssenian every day and they seem to love it. I have all my bannanas in full sun. As far as brown 'spots', what I have are browning leaf edges, and the brown spreads. I don't want to loose these pups, and I'm worried that if they are sick, my 'mom' may catch it.