Question
What is the best way to treat/eradicate white (bright white, colour of white
paint - it does not look organic!) Fungus in soil

Answer
Hi Oonagh, Many fungi that grow on decomposing matter in your soil are not harmful to your plant, but some are.  Scrape away everything you can see and dispose of it. Keeping the soil surface loose by scratching it frequently will inhibit growth.  You local garden center will have a soil drench fungicide you can use, but it may not be necessary if you are scratching the soil surface frequently.  Most of the fungus is underground.  The part you see is the reproductive structures.  By scratching the soil, we are trying to prevent the spores from maturing and continuing your problem.  Jim