I've pulled up the masonite and tiled floor in my laundry and there's cleary water damage in the floorboards, I just pushed a chisel all the way through the worst part. The adjacent two floorboards are still soft and on top there are white to grey trendils radiating out from the wet area. Unfortunately I'd scraped most of it off without thinking about it. Here's a pic
The water seems to have gone in where the edge of the room wasn't sealed, the worst affected board runs under the wall.
How much of the floor should I cut out, and can/should I treat the adjacent/replacement floor with some kind of fungicide?