I have something similar going on, although not as advanced as the leaf you've posted. The resident experts can chime in, but I think it's either bacterial leaf spot or a fungus. Google it and compare pics of those to what you have there.
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Has it been consistently damp lately? I think that is the root of mine..brief, isolated showers that pass through almost daily, never dumping enough rain to really water anything but just enough to leave everything consistently wet and humid. Perfect condition for those type of infections to set in.
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Don't take my word as gospel, but I would try a few things, first being try to keep the foliage as dry as possible. Just water the dirt, not the plant. So long as the plant isn't totally infected, you can then remove the infected parts when it's dry out. If it really doesn't show improvement, maybe try a copper based fungal spray. Also, avoid touching the infected stuff and then touching your other plants.