Hi Simply,
It looks like Black Sigatoka. According to the other members the best course of action is a Copper Sulfate foliar fungicide. You should be able to get some at HD or local garden center. I'll post a link of an earlier discussion.
Blk Sigtka - I officially hate FL
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Sigatoka is spread by spores carried in the wind and leaf on leaf contact. You do not have to remove the entire leaf, however. Cut off the pieces that are the most damaged (especially those touching other leaves). Don't worry about harming the plant, once Sigatoka has compromised 30-40% of the leaf, the photosynthetic membranes can nolonger feed the plant. I would remove the entire leaf you have pictured, and if the fungus is not that bad on other leaves just cut off the infected section. It is done very easily by slicing the leaf blade along the midrib, but not through the midrib.
Sigatoka does not kill the plant, but if left untreated it will kill the leaves and make it unable for the plant absorb sunlight. The sooner you treat the better because Sabas are fast growers and it should be able to outgrow the spread of Sigatoka once its treated.
Good luck with your bananas this season, I'm sure they will be fine in a couple months.
Chris P.