Last summer I papered my hall and stairs. Recently, however, I noticed some dark patches under an area near the front door. I have stripped some of the paper off to reveal what appears to be a black fungal growth which wipes off easily enough with a damp soapy sponge. Is there anything with which I can prep the wall to make sure the fungus doesn't come back? I'm lucky this time because I have enough paper left over to re-do this bit. However, I don't want to have to do the whole lot if the fungus appears again.
Many thanks
Bahny.