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.