I see this white scab near the base of this dragon fruit plant:
The scab hasn't necessarily grown the way I've seen most white mildew or fungus grow. That part isn't soft or mushy either. The new growth looks nice and fresh. So, not sure if the white portion on the bottom is normal for the base of a dragonfruit or if it indicates that the plant is suffering some disease.
I suspected that it could be fungus and have sprayed some copper fungicide as a preventative measure.