i've not ever seen that, and hope i don't in person.
could be a fungus, could be some kind of borer or something living in the stems.
i was looking at stem rot, looks like it might be fusarium. it can be controlled with fungicide, but i would be very careful picking one out.
honestly, i think i'd get rid of those plants, that soil, and then sterilize all my pots and that area in general with something like physan 20, then just start again.