Mycelium (pl.: mycelia)[a] is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. [1] Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads.[2] Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates into a monokaryotic mycelium,[1] which cannot reproduce sexually; when two compatible monokaryotic mycelia join and form a dikaryotic mycelium, that mycelium may form fruiting bodies such as mushrooms.[3] A mycelium may be minute, forming a colony that is too small to see, or may grow to span thousands of acres as in Armillaria. Armillaria can be a destructive forest pathogen. It causes "white rot" root disease (see Plant pathology section) of forests, which distinguishes it from Tricholoma, a mycorrhizal (non-parasitic) genus. Because Armillaria is a facultative saprophyte, it also feeds on dead plant material, allowing it to kill its host, unlike parasites that must moderate their growth to avoid host death.[3]