The oil product Bitumen –a constituent in asphalt- id organic and offers
nutrition to fungus spores.
The asphalt's rough surface gives a good hold for spores. It also collects
dirt and damp –which contains even more nutrients.
As long as the surface contains spores, the fungus recurs. The only
way to prevent growth and affections is to sterilize and disinfect the
asphalt as well as the fungus.