Minecraft wiki entry for Nether_survival, associated text: When a fungi is placed on its corresponding nylium and then fertilized with bone meal, it can turn into a huge fungi. Warped (blue) fungi can turn only into a huge warped fungi if placed on warped nylium (blue nylium), and vice versa with crimson. There is no special setup, but you need to destroy the nether wart blocks (fastest with a hoe), because it needs space to be able to generate a huge fungi. Also, after it grows into a huge fungus, the nylium below it decays into netherrack, and you need to apply bone meal to it, which turns it back into nylium if there is adjacent nylium next to the netherrack.