In that case, Smurf, I'd want to check that the sub-floor ventilation is clear. It's not a big deal to have a bit of fungus above the floor. Under the floor might be more of a concern.
It does suggest water from somewhere. Quite possibly a leaky wastepipe. Worth investigating. You could start getting a bit of "wet rot". If the fruit is a nice pretty shade of pale yellow, then I'd be looking for the much more serious "dry rot"
I agree with the fungicide treatment though. A good dowsing.