Ask most people what the world's biggest organism is, and you'll likely hear one of two answers  blue whales or redwood trees. However, extensive mapping by a dedicated team of forestry scientists led by Catherine Parks of the USFS in Oregon has identified a sprawling fungal network as a single organism covering over 1.5 thousand football fields worth of land in the Blue Mountains of northeastern Oregon. This huge Armillaria ostoyae was discovered in 1998 as the scientists explored the boundaries of a fungal network killing thousands of conifer trees with a plague known as Armillaria root disease, although the discovery was published in a 2003 paper.. They determined it to be between 2,400 and 8,650 years old  if its age is closer to the latter, then it is one of the world's oldest known living organisms.

A. ostoyae grows along tree roots like several related strains of fungi, but has dramatically extended its range through a unique technique of sending out flat, noodle-like rhizomorphs to bridge the space between the trees it feeds upon. Scientists utilize a number of genetic tests to confirm that fungus samples are from the same organism; for example, if samples fuse together when placed together in petri dishes, then they are from the same genetic individual. University of WisconsinLa Crosse biology professor Tom Volk defines a single organism as a set of genetically identical cells communicating together with a common purpose. Several other massive fungi have been discovered, including a 37 acre Armillaria gallica (1.5% the size of our A. ostoyae) near Crystal Falls, MI in 1992, and later that year a much bigger, 1,500 acre A. ostoyae (62% the size) in southwestern Washington.

"Armillaria ostoyae" by Charles de Martigny on Flickr courtesy of Creative Commons Licensing.