Organism of the Day: Armillaria mellea

Yesterday I presented the Organism of the Day to my Biology class, and selected one of my favorites, Armillaria mellea. First, the complete classification.

Armillaria mellea (Vahl.:Fr.) Kummer (common names include: oak root fungus, honey mushroom)

Armillaria

Physalacriaceae (it was Tricholomataceae when I first learned it, but we all knew Tricholomataceae was a dumping ground for white-spored mushrooms)

Agaricales

Agaricomycetes

Basidiomycota

Fungi

Eukarya

Why should you care about A. mellea? Well, first it's an important plant pathogen, infecting hundreds of plant species, though mostly noticeable on woody species. Even though it's called oak root fungus, this is clearly a misnomer. It can also survive on dead plant material, as a saprobe.

It does produce mushrooms, being an agaric, but these are analogous to the apples on the tree, they are just a way of getting around. The mushrooms are fairly typical, with a pileus, a stipe, and a partial veil. Like most common mushrooms, the fertile part or hymenium takes the form of gills, or lamellae. The spores produced rain down and you can see deposits of them on the caps of some of the other mushrooms. Notice that it’s white, the color is an important diagnostic feature as well.It also produces structures called mycelial fans, under the bark of trees that it’s infecting, which is an important structure for feeding the organism.

But another interesting feature is the bioluminescence. This is present in many species of mushrooms (at least 70 species) and other organisms, many animals. The glow of Armillaria mellea has been observed since ancient times, and has the common name foxfire. There’s a town called Foxfire in North Carolina, and Mark Twain in mentions the boys using it in Huckleberry Finn. In some fungi, the mushrooms glow, but in Armillaria, it’s the mycelium and rhizomorphs.

Perhaps the neatest thing about Armillaria is this item. In 1992 it was reported that a single clone of Armillaria gallica was estimated to cover an area of 15 hectares (or about 37 acres), weigh over 10,000 kg (about as much as a blue whale), and be over 1500 years old. And you would have never known it, because it was underground and under bark, and much of it was made up of microscopic threads.

The researchs who discovered the humongous fungus baited for the fungus using poplar sticks, buried under ground, and tested for somatic incompatibility. Basically, if the fungal isolates were genetically distinct, they would repel each other. If they fused, the isolates recognized each other part of the same whole. Comparison of genes further demonstrated that it’s all one big thing. The folks in Michigan are proud of their humongous fungus and celebrate it every year.

But the story doesn’t end there. Later that year, another humungous fungus was claimed near Glenwood, Washington, that was supposed to be 600 hectares, 40 times bigger than the one in the upper peninsula. Though the evidence wasn’t as strong, the question arose. How big can these things get?

In 2003, a paper was published demonstrating an even larger Armillaria clone, in the Blue Mountains of northeastern Oregon. This one was found to be even bigger than the purported thallus of Glenwood, 965 hectares, or 2385 acres. Though the estimates of the age are variable, it's thought that the Oregon Armillaria clone (actually A. ostoyae) could be as old as 8650 years. Can you imagine? An individual organism that might predate the Egyptian pyramids?

Does this seem like deja vu? I just realized that I blogged about this before, and not too long ago. But not as a prestigious Organism of the Day. That part is new. I still think it's one of the coolest stories in mycology. I also think Tom Volk tells the story better, but there it is.