Rhizomorphs above and belowground: finally we see the humongous fungus
Robert A. Blanchette and Cristina E. Toapanta-Alban
University of Minnesota, St. Paul, MN 55108; email:
robertb@umn.edu
Tey can be found belowground or above—where they are produced aerially along the forest floor, in the understory, or up in the canopy of some forests. In temperate regions, they remain unseen and you have to dig into the ground or remove the bark of infected trees to see small segments of them. Armillaria, commonly called the Honey Mushroom, is one fungus that produces rhizomorphs as it moves from one substrate to another. Te rhizomorphs
R
hizomorphs are thick strands of mycelium that are fused together and often protected by melanin.
seek out new hosts, attach to them, and cause infection that may kill woody plants. Round, black, shoestring-like subterranean rhizomorphs or flat subcortical rhizomorphs under the bark are produced. When a tree is killed by the fungus,
new rhizomorphs extend out through the soil as they search for new substrates to colonize. Trees are killed, new rhizomorphs develop, and the fungal colony gets bigger. If you can imagine this process taking place year after year in a large forested area with unlimited access to trees, the colony can be huge and very old. In fact, the fungus may never die. Armillaria has often been referred to as the “humongous fungus.” Studies in Michigan have found colonies of the same Armillaria clone to cover 37 acres and in Oregon one root rot center
has been determined to inhabit 2,384 acres and estimated to be 1,900 to 8,600 years old. Te rhizomorphs are well adapted to this unusual mode of growth through the soil with their thick layer of melanin coating the mycelial strands. Tis provides a physical and chemical protective barrier for the fungus as it ramifies through the ground in search of new hosts. In addition, our past research has shown that fungal melanin absorbs high concentrations of metal ions from surrounding soil producing an armor- like protective covering for the fungus (this work completed when Professor David Rizzo, mycologist at UC Davis, was a PhD student at the University of Minnesota;
https://doi.org/10.1139/b92- 190). All this is happening underground and out of sight. It is impossible to see the complete
Armillaria is a root rot pathogen producing rhizomorphs that attach to roots, penetrate them and kill the root and stem (left photo, courtesey of Martin MacKenzie, US Forest Service). Armillaria produces a white mycelial fan under the bark that is bioluminescent at night and shoestring-like subterranean rhizomorphs. One of these is pointed out by the knife blade. In larger trees such as this dead oak (right photo), flat subcortical rhizomorphs are produced under the bark .
28 FUNGI Volume 15:1 Winter 2022
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