Could be many different disease conditions but no matter what it is.....toss it. It is too far gone to do much about and is a very big threat to your collection to keep around. The possible cures are too involved to warrant effort. Good sanitation and good air movement around your plants are the key to keeping these micro fungal and fungal attacks at bay. Wet conditions and lack of air movement are probably the reason this progressed. Sorry. Don't touch the plant and then touch anything in your plant area. Wash your hands after touching this plant.
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