It looks like black leaf streak a.k.a "Black Sigatoka." Here is a link with pictures for more information.
Banana Black Leaf Streak Disease in Hawaii
The other members said that it is a manageable disease and have recommended a "Copper Sulfate or CuSO4 Nutritional."
Here is a link to my earlier thread:
Blk Sigtka - I officially hate FL
Chris in Fl.