The World According to Fungi

From slimy to friendly, these invaders live in symbiosis with plants—and us.

By Gordon Grice|Wednesday, June 18, 2008
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cedarrust
cedarrust
The cedar apple rust fungus induces galls which in the spring produce wormlike growths. Spores infect young apple trees, causing lesions on fruits and leaves.

The lingering death of a tree in my backyard has me thinking. The Eastern red cedar looked bad—shaggy, wasted, a dandruff of gray decline mixed in with the healthier bluish-green needles. We had seen the cause already, in the form of little spiky galls hanging here and there like drab Christmas ornaments. Each of these ornaments was smaller than a golf ball and seemingly made of wood, which might make you think it was some healthy part of the tree. I picked one off. Each small spike was a spout: a hole surrounded by a sharp woody projection. The ball resisted pressure only as much as a fruit might. Carrying it to the cement walk, I crushed it under my heel. It wasn’t difficult. Inside, the thing was pulpy and fibrous, its strands of vegetable matter radiating from a tight core. Its wet texture was like that of the new wood you can find under a tree’s bark.

I knew this, from books and such, as cedar-apple rust, a parasite with a provocative life cycle that requires it to jump between juniper and apple hosts. (The Eastern red cedar, despite its name, is actually a juniper.) On the leaves and fruit of apple trees, this parasite manifests itself as leathery spots of discoloration. On junipers it appears as these woody galls. The apple and juniper forms of the parasite are different stages of a single life cycle that involves both sexual and asexual spores.

To look at it, though, I wouldn’t have recognized this sphere as alien to the tree, made as it was of the tree’s own tissues. The tree itself makes the gall, acting on instructions from the fungus, like an animal whose rogue cells produce a tumorous mass.

Rain revealed more. It rained for two or three days—a steady, soaking rain that had my sons whooping barefoot on the lawn. They were all excitement. “The parasite opened up!” they told me. Indeed. All over the 50-foot tree, the galls had effloresced. Through each spiky spout a vivid orange tentacle projected. It seemed as if the tree had collided with a swarm of sea anemones. I bent a bough down so the boys could take a closer look. My oldest son poked at one and proclaimed it slimy. I tried its texture myself: wet gummy worms. Our gentlest touches marred them. I almost expected them to recoil.

We usually think of fungus as an unhealthy thing, a sign of disease. That is a slander, for parasitism is only one of the possibilities of fungi.

In the woods behind my house I find uncountable lichens. These are, as every high school student learns, symbionts, a fungus paired with an alga or a similar life form. They are the surface I touch when I lay my hand on a fallen tree; they are the first flaky layer my handsaw bites through when I cut dead wood. When my sons climb to prospect for higher views, half their footholds are ledges of lichen or simple fungus.

I call them uncountable. This is only partly because they are numerous. The other reason I can’t quantify them is that they lack integrity. The whole leeward side of a box elder tree is crusted with green: Where does one lichen end and another begin?

Fungi are often colonial rather than singular. A million fungal filaments in a patch of soil may be in communication of a sort, all of them sending strands toward a food source when one detects it. There is, of course, no brain, no central command, merely a shared purpose. If we consider one such aggregation an individual, then the largest life forms we know are fungal, stretching for miles within the soil, their total mass rivaling that of the largest animals on earth—blue whales.

If the individuality we tend to think of as fundamental is lacking in the fungi, then so are the species boundaries. Lichens are only one kind of symbiont; the fungi have many. One style, for example, is the mycorrhiza, a combination of fungi with the roots of a plant. The fungi reach where the plant cannot, bringing in minerals and water the plant needs; the plant in turn shares the food it makes from light and water. Though most people are perhaps not familiar with this arrangement, it is the basis of life as we know it. At least 80 percent of plants cohabit in such a manner, and some estimates go as high as 95 percent. The boundaries are not exactly where we are accustomed to draw them because, practically speaking, the average tree or weed or grass is not merely a plant but a combination of plant and fungus.

It is not easy to grasp this or to see it without benefit of exca­vation, dissection, and microscopy. But the signs are visible, if you look. Sometimes in wet weather I will find an arc of mushrooms in my front yard. It is this distribution that reveals hidden relationships, for the focus of the arc is an oak tree. The mushrooms are the genitals of fungi intimate with the tree. It is possible to trace thicker roots, barely concealed in the dirt, to aggregations of mushrooms.

A moody morning after rain. The rusts poured forth their tentacles again, but after a few hours the tentacles had withered to brown scraps. My oldest son and I decided to retrieve one of them for study. He held a slender branch taut while I cut. The shear blades met in the wet wood with a squeak and a snap; the branch sprang upward, leaving its severed extremity in my son’s hands. He plucked the rust off it avidly.

We watered it in a jar and it revived. Overnight, new tendrils of orange slime pushed their way out through the colandered sphere. It did not need rain, then; any drenching would do.

The next stage of our inquiry took place in a white plastic cup. We had learned that the rust spreads orange spores while it is active. Looking over its gelatinous tentacles, I had no doubt our specimen was alive, but we made the experiment anyway. Into the little white cup it went with a fresh supply of water. In the morning, the water showed orange against the cup, spores coloring the water like a weak dose of Tang.

For a few days my sons were always bringing in bigger, juicier specimens of the rust, poking them with sticks, marveling at them. Then their interest dwindled, and I would find week-old jars of the things on shelves and windowsills, tentacles half dissolved in water, retaining only their color.

It used to be said that a fungus was a sort of defective or degen­erate plant, one that lacked chlorophyll and so could not make its own food. It was thus reduced to feeding off the work of others—grubbing in the soil to devour dead animals, fallen leaves, and damp wood.

Biologists know better now. In the last few years, DNA sequencing has revealed strong evidence for all sorts of things we could hardly have suspected. The method is to look for similar strings of DNA, then to analyze them statistically. It is a probability game. All life on earth comes from a common stock; the more dissimilar two life forms are genetically, the longer it has been since they were one. By applying sophisticated mathematical models, geneticists can estimate how long it takes for certain kinds of differences to arise. Then, by comparing these numbers, they can deduce how closely related different kinds of living things are. For instance, human and chimpanzee diverged much later than human and orangutan. Therefore, we are more closely related to chimps than to orangs.

Much of the genetic data has supported traditional Linnaean taxonomy. For example, the prevailing view has long been that the order containing bats is monophyletic—that is to say, bats are all more closely related to each other than they are to anything else. A minority view held that the two major groups of bats actually had separate origins and that animals such as the squirrellike colugo might be more closely related to one type of bat. But the DNA analysis supports the first view—that all bats belong together in one order, with nothing else included.

Other data have cast the whole Linnaean structure into doubt (which is why there is now a competing branch of systematics, called cladistics, that tries to chart relationships in a much more precise, if cumbersome, way). Some of it is quite counterintuitive. We always knew that whales must have evolved from some sort of land mammal, but who would have suspected they still belong in the order Artiodactyla—the even-toed hoofed animals?

As a result, our vision of life has altered; the kingdoms, the very fundaments of Linnaean biology, have had to be shifted about. The fungi have made up their own kingdom since the late 1800s, but we understand now, better than we used to, at least, what they are. Certain organisms that we called fungi because they were slimy and repulsive and because we didn’t know where else to put them—the slime molds, for example—have been exiled from the kingdom. That is not too hard to take, because few of us encounter slime molds with any regularity.

What is harder to take is that these disreputable organisms are our kinfolk. The fungi are not plants at all; they are closer to the animals—to us. We always suspected it, after all. They don’t move the way we do, but some of them, the most visible ones, grow so large and are so complex it almost seems a sign of animal life: the mushroom big as a human fist found on your lawn the day after a rain, for example. I have a vivid childhood memory: something smooth and white nesting in the grass, the size and shape of a chicken egg, hard-boiled and peeled. It was not an egg, however; my dog evinced no interest in it. Something about it made me reluctant to touch it. It had no smell, but somehow it reminded me of dog feces, or perhaps merely the Platonic form of disgust. The weird notion that it was an eyeball crossed my mind, and I went so far as to turn it over with a twig, looking for an iris. It was this operation that revealed its true nature, for it tore open under the stick and revealed, first, the stringy origami I associated with the insides of some mushrooms. The other thing this tearing revealed was the smell, which had hitherto been undetectable: the smell of rotten flesh.

Or maybe their overnight appearances simply seem like a sinister kind of magic. Toadstools, elves.

It is not only plants that live in symbiosis with fungi. We animals do it too. We do not like to think about this, because we have for so long conceived of microbes as unclean things, as invaders. But of course we have always been symbionts, dependent on the microbes in our guts to digest our food. We have colonies of fungus inside us. A so-called yeast infection is really an imbalance. It is not a problem that the yeasts are in the human body—they always are. It’s a problem that their numbers have, because of some teetering of the pH level, exceeded their usual bounds. It is a natural thing to share our bodies with them, and with all manner of other organisms. As a tree is not simply a tree, we are not simply what we think we are.

Which is not to exonerate them all. Plenty of fungi are pure parasites, and these have invaded almost every form of life. There are specialized fungal parasites of single-celled diatoms and even of other fungi, and some of these cause serious harm. The rust on my red cedar tree was bad for it; it must have been even worse for the apple trees in the neighborhood, for the leaves of the apple are slowly pierced through by the rust, until spore-shooting orange masts sprout from their undersides. The fruit of the apple, too, may be ruined, its hide marred by soft brown patches.

So, too, with the human form. There are fungi to make the feet and the testicles itch, fungi to discolor and deform the fingernails. And there are neighborly fungi content to live within us unobserved, but which will blossom, in the case of a ruined immune system, into devouring sores inside and out. It is a common enough way for people with AIDS to die.

Two years have passed since the last time our red cedar was dazzling in its orange jewelry. The next year we waited in vain for another blooming of that odd fungus life. We had trimmed the galls off where we could reach, hoping to save the tree, but it wasn’t our earthbound efforts that drove them away. We could still see dozens of them higher up, dry and drab, refusing to bloom. This is the way of the rust. It dies on apple trees, but on red cedar it can persist, give its life to the wind, or leave its old self to rot.

Our tree grows shaggier, more patched with vanilla and auburn. Within the greenery that remains I can reach dozens of lifeless branches. A good yank is sure to be rewarded with the crack of dead wood. The whole tree is ugly now, truth to tell. It reminds me of nothing so much as the shaggy head of a neglected old man.

Not that I neglect the tree. I prune; I am doing what I can to save its life.

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