Phalaenopsis Root fungus

[neilmac]

I'm new to orchids, bought several recently from supermarkets; all appear to be in good health but one has white fluffy fungus on top of the growing medium (bark) which vanishes when it is dry but reappears with watering. Some of the upper roots also have this fungus on them and are withered and brown, some are green but with some fungus on them.
I have decided to isolate it, repot and trim the bad roots but should I use a fungicide and if so which one-The only one which I could find is for trees and roses-is it suitable?

Any advice please, and how likely is it to spread to my other plants (lack of space means that isolating means moving three feet away unfortuately)?