Looks fungal, like dry rot. Best way to treat it is to cut back the affected area to about 1m from the affected area THEN BURN THE ROTTED TIMBER. Make sure you check the joists beneath the floor. Next treat the infected area with a dry rot treatment. Ensure that all air bricks or ventilation is clear of obstructions. Finally, replace the cut out timbers with new stuff, preferably pressure treated
Having Zoomed in on your image, looks as if you have a serious issue, Dry Rot?
D/Rot turns timber a russet / red colour, the timber breaks up in a cubical form, generally you can push a finger through the affected area of timber. paint wrinkles and the timber fails.
As "JobAndKnock" above, careful inspection, remove all infected timber about 1. m back from rotted timber, strip out and have a GOOD look at all / any timber within at least the 1.0.m area surrounding this problem area.
is this a ground floor or first floor? If ground floor get under the floor and look for any spread?
There are proprietary fluids that can be sprayed on to walls and surfaces once eradication has been undertaken.
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