My only suggestion is to cut off the rotting growth and dust the lesion with cinnamon. If you suspect it's spreading through your collection, isolate the affected plants. The plant in the pic looks pretty healthy otherwise, though, so I'm sure it'll make it.
I think the problem was caused by a fungal infection due to water sitting in the leaf axils. If you tend to water late in the day, you might want to do so earlier so that the water on the foliage has time to evaporate.
I hope that helps. Good luck.