The photograph shows dry rot spores discovered in the basement of a Victorian building.
The conditions for it’s growth were ideal: a damp atmosphere and very restricted ventilation (the original cellar grating had been blocked up by the Local Authority very kindly when they repaved the street some time ago.
Dry Rot is much more serious than Wet Rot. It spreads across timbers and into masonry and brickwork whereas Wet Rot sticks to timber.
It’s eradication is a job for a specialist contractor and can be very expensive to deal with thoroughly. Make sure you use an experienced and reputable contractor, and that you obtain a comprehensive (and preferably insurance backed) guarantee for all the work carried out.