It's a bit far away to tell, but I've never seen any witches broom attack my bamboo ever, and we've got it on some of our other wild weeds around here, especially the wild roses and that honeysuckle bush stuff. Is it the only culm affected? Do you notice any other anomalies? Is one of your neighbors spraying round up or some other toxic organophosphate? That might explain the growth of a Super Bamboo.

That would be cool if you got some sort of genetic mutation with that.
Kudzu is the expert, maybe he'll be able to interject some wisdom on this one.
I'm so curious! Other than some sort of radiation, I can't imagine what else it could be. A close up picture would also help. :)

I have actually seen this once before on some yellow groove. dug some of it to see what happened. There were 7 or 8 culms that were very skinny like that but when they started to run and make new culms they were normal. no idea why they did what they did.

My Golden Fishpole Bamboo has been getting this on and off for many years. It seems to happen when rains get scarce after the culm reaches mature height, then when rains do come, they produce the growth like in your picture. I used to think it was caused by herbicides, since some were used in the area of the bamboo, but I noticed that it happens when the leaves finally grow after a dry spell.