Nether Growth Spores can only be planted on Netherrack in the Nether. It will start as a small layer of infestious fungi that will grow eventually grow to be half a block tall. Over time the Nether Growth will spread to any Netherrack within one block of the spore, but cannot spread across larger gaps.
The important property of this fungus is that mobs can no longer spawn where it has infested, making it perfect for controlling the spawns of Zombie Pigmen, Ghasts and other Nether inhabitants.
This Growth is also hostile and will begin to eat any living creature that steps into it, throwing them into the air to help weaken them and eating their flesh once they die.
Trying to stop the infestation is difficult, if any parts of the growth are destroyed it will explode into more spores.