Question

largest organism in the planet?

Answer 1

Don't know if it's THE largest but one of the biggest ones is Armillaria solidipes
http://fungalgenomes.org/blog/2007/06/the-worlds-largest-organism/

Answer 2

A giant fungus of the species Armillaria solidipes (honey mushrooms) in the Malheur National Forest in Oregon was found to span 8.9 km2 (2,200 acres),[4] which would make it the largest organism

Answer 3

Armillaria Ostoyae,a humongous fungus or the tree killing fungus of about 8000years old.