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Honey Mushroom - Armillaria mellea

Good evening, friends, and Happy New Year.


As I teased last week, this week's mushroom is Armillaria mellea, commonly known as the honey mushroom. This mushroom was found in the ramble on 10/8/2021. This is an instance where we can't definitively confirm the ID of this mushroom just from photos and macroscopic evidence, but A. mellea is the most common Armillaria species in the area so we're gonna roll with it. Even Tom Volk's key to North American Armillaria species wasn't too helpful of a companion in my search for the absolute. Until the 1970s all honey mushrooms were grouped under Armillaria mellea, but then DNA sequencing allowed scientists to parse out separate species within this group. If we've learned anything to this point, mycology is not binary and the science is rapidly evolving and expanding.



Fun Facts

The fungus is bioluminescent which means it glows in the dark. Interestingly enough, the chemicals that cause the fungus to emit light at night are only present in the mycelium and not in the mushrooms themselves. Further, the glowing mycelium isn't strong enough to observe with the naked eye in a natural forest setting. One would need to bring the mycelium into a completely dark room sans windows to observe this phenomenon.


The largest organism ever found is a species of honey mushroom, Armillaria ostoyae, located in Oregon's Malheur National Forest. Known as the Humongous Fungus, this single organism stretches over 3.5 square miles (~2,400 acres) and is somewhere between 2,000 to 8,000 years old. It's older than Jesus and he just celebrated another birthday last week. One hypothesis as to why Armillaria species are able to grow for multiple miles over many millennia is the ability of the fungus to produce rhizomorphs (seen in the photo below).


Armillaria mellea rhizomorphs

Rhizomorphs

I was able to take the new camera phone for a spin this week and went looking for Armillaria rhizomorphs, also known as mycelial cords. If you have traipsed through woods or parks while paying close attention to deadwood then there's a good chance you've encountered these black, waxy bands. The majority of Armillaria species, including A. mellea, produce these heavily melanized mycelial cords. This is an uncommon instance where we can see the "body" of the fungus whereas we normally only see the mushroom - the reproductive organ. The mycelial cords allow the fungus to move through the soil quickly and colonize new trees/substrates. The cords are black due to a heavy melanin presence which helps the fungus remain undetected in the soil and evade the natural chemical defenses of healthy trees.


The picture below is interesting because the decayed wood looks like a hair comb and the armillaria cords are dangling out of the space between the wood bristles. My hunch here is that fungi and microbes have consumed the more susceptible, easily digestible, parts of the annual growth rings while the remaining "bristles" are the plant cell walls from each year. The plant cell walls are composed of lignin and cellulose which make them more difficult to digest but the honey fungus is still actively working to break them down and strip the proverbial meat off this bone.


Armillaria cords

Ecology

A. mellea is both parasitic and saprobic. Its parasitism makes it pathogenic which means it causes disease in its shrub/tree hosts. However, trees or shrubs that are afflicted by the fungus don't die immediately and some can live for decades after the initial infection. A lot of it depends on environmental factors like water availability and temperature (i.e. drought conditions). A. mellea is found in North America, Europe, and Asia. It typically fruits (produces mushrooms) in the fall in Northeastern North America, but the mushrooms can pop up year-round in the southeast. One key characteristic for A. mellea is that the mushrooms grow in clumps, in mycological terms this is known as cespitose, typically around the base of living trees or dead stumps.


Here's to a fungal and fruitful 2022,

Aubrey


References:

1) Kuo, M. (2017, May). Armillaria mellea. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/armillaria_mellea.html

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