14
HONEY MUSHROOMS
The Race for the World’s Largest Fungus

You cannot find a mushroom without leaving the house.

TRADITIONAL RUSSIAN SAYING

I n April 1992, an unexpected headline shone from the pages of the New York Times: “Twin Crowns for 30-Acre Fungus: World’s Biggest, Oldest Organism.”¹ The story that followed was based on an article published that day in the journal Nature, detailing the results of four years of research by a team of three biologists studying a plot of forest near Crystal Falls, Michigan. The researchers, Myron Smith and James Anderson of the University of Toronto and Johann Bruhn from Michigan Technological University, claimed to have discovered a mushroom-producing fungus of enormous proportions.² Before long, this single colony of the common honey mushroom, Armillaria bulbosa, growing beneath the surface of the forest soil, garnered attention from around the world.

Although they initially set out to determine the genetic diversity of fungi growing within a given area of forest, and specifically to determine how to differentiate between individual colonies of a species of fungus in the forest, the team discovered that all the samples of Armillaria in an initial plot of 120 × 60 meters belonged to the same individual mycelial network. The team extended its sample area along a transect and still did not reach the limits of the genetic individual, referred to as a genet. Over the following years, they determined that the mycelial network of this single genet extended over a thirty-acre area. After estimating—conservatively by most accounts—the annual growth rate of the mycelium, the team determined that the fungus was between 1,500 and 10,000 years old, making it the oldest known living creature on Earth. They estimated the combined weight of all the fungal growth by weighing the mass of the honey mushroom mycelium contained in a measured area of soil and then extrapolating to the amount of soil in the surveyed area. They determined that the fungal mass was about 100 tons, approximately the same weight as a blue whale.

The biologists were completely unprepared for the barrage of media attention following publication of their results, as calls from media and glory-seekers came from around the continent and from across the world. According to Wisconsin mycologist Tom Volk, one Japanese businessman wanted to fund a boardwalk exhibit and charge people fees to view the giant fungus and CNN called to report that a jet was en route to the site and requested that one of the researchers be on the ground for aerial photographs of the mammoth fungus!³

Of course, what anyone standing in the midst of the world’s largest fungus would have seen was . . . a forest. With trees, shrubs, grasses, and flowers, it would have looked indistinguishable from any other thirty acres of forest in the area. If they happened to be there the previous September or October after a good soaking rain, they might have seen honey mushrooms fruiting singly and in scattered clusters across the forest floor—a sight nowhere near as impressive as a blue whale. Thrill-seeking customers touring a honey mushroom boardwalk exhibit would likely have been just that: bored.

Almost immediately following publication of the findings, rival claims to the title of the world’s oldest and largest fungus came flooding in. Almost twenty years earlier, two forest pathologists, Kenelm Russell and Terry Shaw, working in southwestern Washington studied a genet of a related honey mushroom, Armillaria ostoyae, which they reported as covering perhaps 1,500 acres. Russell and Shaw, who completed work on their humongous fungus in the 1970s, based their claim on a study of incompatible mating types, working from the premise that strands of mycelium from different individuals will not combine when growing together on agar medium, whereas strands from the same individual will grow together into one unit under the same conditions. The two collected sexually compatible samples of the fungus over an area encompassing a huge swath of forest—almost one and a half square miles. Some scientists reviewing the competing claims noted that Russell and Shaw had excellent aerial photos to support their claim but lacked the convincing genetic evidence of the Wisconsin study.⁴ Even today, the race for the biggest fungus continues. In 2000, another team of forest researchers reported the discovery of an individual fungus growing in the mountains of Oregon that dwarfs both prior claims. In a 2008 updated report on the area, the largest genet of A. ostoyae is reported to cover more than three and a half square miles or 2,385 acres and to be between 1,900 and 8,650 years old.⁵

These big fungal genets have raised even bigger questions about what defines an “individual,” especially in a fungus that spreads by mycelial growth through the forest soil when those almost-invisible colonies of mycelium have no defined shape or boundary. In other words, where does the growth of one individual end and a new individual begin? In the case of the Michigan honey mushroom, the researchers acknowledged that there were interruptions in the mycelial network of the fungus; it was not a continuous interconnected web. Though the islands of mycelium proved to be identical, they challenged the normal definition of an individual, which is characterized by a defining and limiting boundary. In humans, the limiting boundary is our skin. In a tomato plant, it encompasses all the roots, stems, leaves, flowers, and fruit. But if an organism doesn’t have skin or a stem, and in fact, doesn’t even have a determined growth pattern or shape, where does singularity end and plurality begin? It’s obvious where an elephant begins and ends and, if there were two genetically identical elephants, we would call them clones or identical twins, but they would certainly be considered separate individuals. The distinction is much less clear in the case of a fungus that grows outward in all directions as it seeks out available food and moisture, but might, over time, become isolated islands of growth in a larger sea of the forest ecosystem. If the space between these genetically identical or sexually compatible islands of growth is widely spaced with no known contiguous connection, are they still one individual? The questions raised by the thirty-acre fungus are monumental indeed and will likely provide rich fodder for debate for some time to come.

TAXONOMY

Honey mushrooms comprise a small group of closely related species of forest fungi. Not too long ago, most were lumped into Armillaria mellea, but taxonomists today generally recognize them as a complex made up of at least six species and perhaps up to fourteen, based on chemical, ecological, and morphological differences.⁶ In the Northeast, honey mushrooms growing on conifers are usually A. ostoyae and on hardwoods, either A. mellea or A. gallica. Most amateur mycologists or casual mushroom foragers will have a difficult time differentiating between these closely related species. To identify a honey mushroom, the best bet is first to determine whether the host tree is conifer or hardwood deciduous.

DESCRIPTION

(The following is a general description for all honey mushrooms and may not closely fit some varieties. The species in this complex are often difficult to distinguish using simple field characteristics alone. Consult a good field guide to your area for more detail.)

Honey mushrooms are tan to yellow-brown to brown mushrooms fruiting singularly or, more typically, in clusters on wood or on the ground from the buried wood of roots. (See #15 in the color insert.) The caps are generally 2–4 inches across and convex, becoming flat with maturity and with a region in the cap center with a number of dark, coarse hair-like scales. In the emerging button, the entire cap may appear covered with these scales. The gills are light tan, and attached to the stalk or slightly decurrent. The stalk is colored like the cap, generally twice as long as the cap is wide and may taper at the base. Each has a thick, fleshy whitish ring and often is irregularly scaly. The spore print is white.

LOOK-ALIKES

One mushrooming myth says there are no poisonous mushrooms that grow on wood and, therefore, any that you find on wood are safe to eat. Wrong! This myth, like many, contains a germ of truth. None of the polypore bracket fungi, which often fruit on wood, are poisonous. Although it’s more accurate, even this generality should never form the basis of a decision to eat a new mushroom. Never eat a mushroom unless you are 100 percent certain of the identification and the edibility of the species. When in doubt, throw it out!

There are several poisonous wood-rotting mushrooms with gills, none more notable and toxic than the deadly Galerina, Galerina autumnalis, and its cousins, which also grow singularly or in small clusters on wood. Unlike the honey mushrooms, it is smaller and produces a brown spore print. As its name implies, the deadly Galerina is a potentially deadly toxic species, containing the same cyclopeptide toxins that are found in death cap amanitas. Galerina mushrooms typically fruit in the same cool wet autumn weather that honey mushrooms favor.

Another famous and notable clustered wood-rotter is the jack o’lantern, Omphalotus olearius, which usually grows in dense clusters on the ground at the base of trees infected with its mycelium. (#8) Jack o’lanterns cause severe gastric upset in anyone mistaking them for edibles. A last potential problem mushroom is the brown-spored big laughing gym, Gymnopilus spectabilus, which grows on wood during the same autumn period and is generally quite bitter and also hallucinogenic.

A number of smaller mushrooms superficially resemble honey mushrooms but generally fruit singularly rather than in clusters. Be certain your collection contains the hallmark identification features of clustered growth, a white spore print, the presence of small dark scaly hairs on the cap, a thick fleshy ring, and attached or slightly decurrent gills.

ECOLOGY

The various species of honey mushrooms function as saprobes and parasites in the forest ecosystem. As saprobes, they are known as a “white rot” fungus because they consume the brown lignins in wood as a food source leaving only the white cellulose. In the process, they recycle the nutrients bound up in the wood. As parasites, honey mushrooms have earned a reputation as virulent pathogens, attacking, weakening, and killing a wide variety of tree species. Generations of foresters and land owners have regarded Armillaria root rot with a mixture of awe, fear, and loathing for the damage it causes to forest, orchard, and landscape plantings. As the mycelial network becomes established, it forms thick, dark-skinned rhizomorphs of densely compacted hyphae. These hyphae, which look like bootlaces or shoestrings, grow through the soil and search out the roots of new trees to invade, making the fungus capable of rapidly finding and attacking weakened trees over a broad area. The rhizomorphs, extending from an area of established infection, also act as effective conduits of moisture and nutrients to regions of an expanding mycelial network as the fungus moves into new territory.

An infected tree shows signs of distress within a year or two as the root function is compromised and the tree suffers from lack of nutrients and water, but sometimes trees can live relatively unaffected until they come under stress and the fungus overcomes the tree’s weakened natural defenses. On heavily infected trees or trees killed by Armillaria root rot, you can see a network pattern of the black bootlace rhizomorphs growing between bark and heartwood. The stump of a tree killed by root rot continues to house living mycelium for many years, serving as a site of inoculation for a new generation of trees planted in an area infected with honey mushrooms. For this reason, careful foresters mechanically remove infected stumps prior to replanting forests in areas where an aggressive honey mushroom infection has been active.⁷ Though honey mushrooms have a reputation as aggressive tree pathogens, some species live primarily as saprobes in the soil or become parasitic only when the trees are dying or under stress due to drought, insect infestation, logging, or other factors.

Another fascinating and eye-catching feature of honey mushrooms is occasionally seen by nocturnal visitors to the rain-moist summer forests. Wood colonized by honey mushroom mycelia occasionally exhibits bioluminescence, a faint ethereal light that glows in the dark to the fright or delight of unwary nighttime sojourners. (For more information, Chapter 16 on bioluminescence.)

EDIBILITY

The late Dr. Richard Homola, a former professor of mycology at the University of Maine, was an avid collector and photographer of Maine mushrooms and an enthusiastic fan of honey mushrooms. He told me that he preferred them to most other edibles and collected and preserved a supply of them for winter use. During the autumn months, usually after mid September in Maine, honey mushrooms will respond to a good period of rain by producing incredibly abundant flushes of fruit during a short intense period. This is a mushroom you can collect almost by the truckload in a good fruiting year. For the best eating, collect young firm caps before they become completely open. The tough stalks tend to be fibrous and are better left behind, although Tom Volk recommends peeling away the fibrous skin and eating the pithy inner flesh of the stalk. This seems a bit labor intensive for my style of cooking.

Honeys regularly fruit with abandon. Huge numbers will grow in clusters of young tight buttons interspersed with mature and over-mature individuals and the whole tableau can seem a bit overwhelming. It can also over-excite the greedy part of our psyche and lead to indiscriminant collection as we bring as many mushrooms home as we can carry. Once home and in the kitchen, make certain that as you prepare your collection for cooking or preservation, you again review each individual to be certain you have only honey mushrooms and that each is firm and healthy. Discard any questionable specimens or any that appear old or potentially spoiled. Many cases of mushroom sickening are due to the consumption of old mushrooms infected with bacteria.

CAVEATS

Whatever age or portion of the mushroom that you choose to eat, be certain that you fully cook your honey mushroom meal! Honey mushrooms contain a heat-labile toxin capable of causing mild to moderate gastrointestinal distress in unwary diners who value undercooked veggies. Dine on crisp broccoli or green beans and take a chance on rare beef, but feast only on well-cooked honeys!

I have a friend, a local chef of German descent, to whom I love to bring mushrooms. Following his friendly open smile at seeing the basket, he almost invariably will tell a story of his family’s use of mushrooms as survival food during the lean years in Germany following World War II. Unfortunately, he bought some honey mushrooms from a collector and, either not warned or unaware, lightly cooked them for his family. He awoke in the night to an uncontrollable urge to visit the bathroom and found it a popular room shared through the night with two other family members. All recovered before noon the next day, but learned the hard way to completely cook this species.

In addition to those stricken by undercooked meals, a very small percentage of people are unable to tolerate honey mushrooms and develop mild to moderate gastrointestinal distress after eating them regardless of the cooking. There are many theories regarding the cause of this phenomenon: mushrooms growing on conifers, over-indulgence, and allergic reactions to name a few. Because of this rare reaction, I recommend that the first time you eat honey mushrooms, try a small amount and see how it suits your system. I would not serve honey mushrooms to a crowd of mycologically naive diners without adequate warning. That said, I happily serve it in many forms to my family and generally dry a supply for off-season use in soups and stews where its robust flavor shines.

Walking through a forest picking the occasional cluster of tight young honey mushrooms, I rarely consider the mass of mycelia growing through the soil duff and colonizing the wood of the tree where the mushrooms have fruited. That this interconnected mat of fungal growth might cover several acres and have a combined mass that dwarfs me doesn’t intrude on my complacent collecting as I enjoy the fruits of all that labor. I certainly never consider building a boardwalk to exhibit the sight.