With massive webs of probing black tentacles extending for miles below the ground, the Armillaria group of fungi includes some of the largest known organisms on our planet.

An 8,500-year-old specimen of Armillaria ostoyae in Oregon covers 2,385 acres (3.7 square miles) with its mass of rhizomorph tentacles and is estimated to weigh roughly 7,500 to 35,000 tons – a bulk and coverage that makes it a contender for the largest organism in the world.

This incredible mass allows it to join the category of mind-bogglingly large organisms, like the gorgeous grove of interconnected aspen tree clones known as Pando in Utah. And yet, the humongous fungus largely appears to us as bunches of cute, independent mushrooms.

Armillaria is a vampire-like pathogenic fungus that feeds on trees. It can drain the life from 600 types of woody plants and thus decimate vegetation, causing farmers millions of dollars of damage.

(Debora Lyn Porter/University of Utah)

Above: Once inside a tree, branching white filaments grow from the penetrating rhizomorph that suck water and nutrients from the plant flesh.

The ability of this parasitic fungus to get so massive is in part due to its robustness. Armillaria is incredibly resistant to many methods of biocontrols – typical fungicides can even stimulate its growth. It can also survive dormant in the ground for a remarkably long time without any food.

"These mycelia and rhizomorph networks have been found to remain dormant for decades in the environment when live hosts are not available, becoming active again as new hosts return," University of Utah mechanical engineer Debora Lyn Porter and colleagues explain in a newly published paper, in which they investigated what makes the fungus so tough.

Porter and her team used chemical analysis, mechanical testing, and modeling to closely examine A. ostoyae – comparing lab-grown and wild-harvested samples of its tentacle-like rhizomorphs.

They found that only the wild fungus produced rhizomorphs with a shield layer that can protect the more sensitive tendrils within from both chemicals and mechanical forces.

(Porter et al., Journal of the Mechanical Behavior of Biomedical Materials, 2021)

Above: Lab-grown rhizomorph (blue arrows) compared to harvested rhizomorphs (red arrows).

"This outer layer is pretty tough," says mechanical engineer Steven Naleway. "It's kind of like a tough plastic. For the natural world, it is quite strong."

This layer was darkened by melanin – a pigment that is known to provide fungi with various benefits, such as binding calcium ions that help neutralize toxins, like acids from insects. The wild fungal shield also had far smaller pores than seen in the lab-grown rhizomes and had a more consistent structure that left no room for weak spots.

"If you're going to have some kind of human biocontrol, you need to combat this calcium and better penetrate this outer surface," said Naleway.

These properties provide the fungal tentacles with the strength to apply enough pressure, along with the help of enzymes, to break through tough woody roots and steal tree nutrients. And, with enough time, grow into a hulking mass of fungus rivaling the largest living things on Earth.

Their research was published in the Journal of the Mechanical Behavior of Biomedical Materials.