Minecraft wiki entry for Mycelium, associated text: A mycelium block can spread to any dirt block within one space above, one sideways, or three down. The mycelium needs light level 9+ above it, while the dirt needs light level 4+ above it, and must not be covered by any light-impeding block or any opaque block. Mycelium reverts to dirt when covered by one of the light-impeding blocks above and the light level at that block is below 4. The death and spread behaviors are checked when a random tick lands on the block.