Mushroom generation

Can anyone here help me in mushroom generations? I'm trying to code mushroom biomes but I have no idea over Mushroom (object) generation. Here's my code. The problem is that the mushroom stem is made of quartz pillars. I would like it to be made out of 100:15. Help?

PHP:

namespace pocketmine\level\generator\object;

use pocketmine\block\Block;
use pocketmine\block\RedMushroomBlock;
use pocketmine\level\ChunkManager;
use pocketmine\utils\Random;

class MushroomTree extends Tree{

    public function __construct(){
        $this->trunkBlock = Block::QUARTZ_BLOCK;
        $this->leafBlock = Block::RED_MUSHROOM_BLOCK;
        $this->type = RedMushroomBlock::RED;
    }

    public function placeObject(ChunkManager $level, $x, $y, $z, Random $random){
        $this->treeHeight = $random->nextBoundedInt(3) + 4;
        parent::placeObject($level, $x, $y, $z, $random);
    }
}