Although there aren't many photos, this site has some useful info on banana affectations via fungal, bacterial or viral pathogens. I suggest scrolling down to "leaf spot" section as a start. Here's the link:
Diseases of Banana and Plantain (Musa spp.)
You may have to dig a bit, but the first thing I would suggest is quarantining your affected plants from the healthy ones to avoid spreading whatever is causing this. Perhaps sanitizing the area where they have been will help prevent spreading the unknown.