Such rots are random and usually don't result in loss of the plant. It could be growing on the surface of the plant only or it could be a rot of the stem itelf. There is nothing to do about a rot except to remove the top part of the plant, cutting just below the rotted area. However I am not at all convinced that this is necessary yet. I would start by wiping the black stuff off as best you can. You might try spraying the area with a general purpose fungicide containing benomyl or bayleton. Then give it some time to see if the top parts start to wilt and die back. If so, that is the time to cut off the rotted area.
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