Black mould

ashish, it's most of the time dirt but also sometimes fungus. same as in the basement of an house it is at our showers. black mold ( http://www.google.nl/search?hl=nl&sugexp=pfwc&cp=20&gs_id=26&xhr=t&q=black+mold+at+shower&safe=off&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.&biw=1106&bih=665&wrapid=tljp1316856600161038&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi ) is the bad mold (http://en.wikipedia.org/wiki/Black_mold ) on the other hand there also good mold --> Penicillium fungi on oranges or bread (http://en.wikipedia.org/wiki/Penicillin). but believe me ashish , fungi --- mold,no mushroom grows on such area.(in my old rent-appartement i had the same problems- try 90 % alcohol to desinfect the area or get some special product for it) fungi grows almost everywhere !! mushrooms not!!

Fungi do not grow on such area.

Might me dirt...