Fungi at work

Although we can’t see it, we live in a microbial soup. I just learned that there may be a Milky Way of viruses that floats on air currents around the globe and that trillions fall to earth every day. We know our guts are inhabited by billions of bacteria that we can’t get along without. A teaspoonful of soil contains up to a billion bacteria.

    Some of the most important players in this microscopic world are fungi. This summer has produced a particularly handsome crop of mushrooms and other fungal growths in woods and shady gardens in my area.  Walking woodland paths this month, I can’t resist stopping to snap photos of new, colorful fungal growths that seem to pop up almost every day.

Wet, rotting wood displaying a fungal bonanza

    The mushrooms I’m admiring are just the tip of an underground iceberg, the fruiting bodies of vast networks of thread-like fungal hyphae in the soil below. Compost couldn’t happen without the work of soil fungi. Bacteria, fungi, nematodes, insects and protozoa all work together to break down the organic material we throw on the compost pile into nice black compost.

Microscopic view of fungal cells

    I was amazed to learn a few years ago that experts can customize the balance of fungal and bacterial content in compost by choosing the right mix of ingredients. This is when they make compost on the scale of tons, moving giant mounds of material around with backhoes. 

At large-scale composting operations, piles are turned by machines

High-carbon, tough woody materials are what soil fungi like, so our yard’s endless supply of oak leaves predisposes the compost I make to have high fungal activity. I once had my compost tested for its biological content. The report came back showing excellent fungal presence.

    That’s just as well, because shrubs, trees and perennials—what my garden is mostly made up of—particularly like soil and compost with lots of fungi. By mulching with fall leaves and wood chips, I give the fungi even more to chew on.

Shredded leaf mulch

    Not only are these organisms hard at work in the soil. They actually create an underground architecture in the rhizosphere, the top few inches of soil where there’s the most oxygen and the most biological activity takes place. Roots and soil fungi have evolved to work together, exchanging nutrients. 

A stump in my yard is hosting this impressive mushroom bouquet

     This justifies the lazy woman’s approach to soil amendment: layering compost and good stuff such as composted manure on top of the soil and letting the soil organisms do the work of mixing them in. Tilling the soil, or even turning it with a spade, breaks up those underground networks that fungi have labored to create. That makes it harder, not easier, for plants’ roots to obtain the nutrients they need.

Let soil organisms do the digging

    As the beans and tomatoes in the vegetable garden finish producing and start to wind down, it’s almost time for me to play my part, spreading some compost on top of the bed. By spring, those fungi will have made the soil ready for another season’s planting. I appreciate their beautiful above-ground creations, and I’m even more grateful for the work they’re doing underground.

Fungus among us

Thank goodness some rain fell this fall, although we’re still in a drought. Apparently mushrooms were biding their time, waiting for a bit of moisture before making their move. Once it rained, they popped up around the yard.

    These mushrooms are the fruiting structures of a group of fungi called basidiomycetes. The mushrooms are small parts of these organisms, which include extensive underground networks of hyphae, slender filaments that take up water and carbohydrates. A single individual fungus can send up fruiting bodies over an area as big as a baseball diamond.

    Invisible to me, even after welcome rain, are two huge groups, the saprophytic and mycorrhizal fungi. Saprophytic fungi are decomposers, breaking down dead organic material, including wood, into nutrients plants can use. 

Mycorrhizal fungi join in a symbiotic relationship with plant roots. Thinner and more extensive than root hairs, their hyphae seek out water and nutrients from far and wide. They share these with plants in return for simple sugars.

Fungal hyphae

    Some mycorrhizal fungi excrete a sticky polysaccharide called glomalin that glues soil particles together in small clumps called macroaggregates. These clumps improve growing conditions for roots by creating pore spaces in the soil for water and air. As if that weren’t enough, fungi in the soil also sequester carbon, attack plant pathogens, and cultivate helpful bacteria.

    Luckily the best way to foster these useful partners is to stay out of their way. If I tilled my soil with a rototiller, I’d be breaking up their networks. If I sprinkled chemical fertilizer, I’d turn off their nutrient-collecting efforts. Fortunately there’s no call to do either in my garden. If I need to plant something, I get along fine by digging an appropriately-sized hole. Compost and mulch are providing enough nutrition for my plants.

Offering a home for soil fungi

    It turns out that by mulching with fall leaves and wood chips, I was unknowingly creating a happy environment for soil fungi. I learned about this when I researched the reasons for balancing types of ingredients in the compost pile. Before this I’d thought compost recipes were unnecessarily fussy.

    Compost professionals can titrate ingredients and conditions in their compost piles to determine the proportion of bacteria and fungi in the finished product. By managing the balance of “brown” high-carbon and “green” high-nitrogen compost ingredients, they can cause bacteria or fungi to predominate. Bacteria thrive on simple sugars from green plant material, and fungi prefer high-carbon woody material. 

    Compost high in fungi is best for use around trees, shrubs and perennials, whereas annuals and vegetables prefer more bacteria in the mix. This is because of what soil organisms do with nitrogen from the air. Fungi convert atmospheric nitrogen to ammonium that long-lived plants prefer, whereas bacteria make nitrate that fast-growing short-lived plants particularly need.

    Making compost by recipe probably isn’t in my future, but it’s nice to know that my high-carbon mulches are friendly to soil fungi.

Mushroom superheroes

This week I went to Amherst, Massachusetts to attend the annual conference of the Ecological Landscape Alliance, this year titled, “Sustaining the Living Landscape.” One of the speakers pushing the boundaries of conventional horticulture was Tradd Cotter, who gave the keynote address, “Mycoremediation: Healing Compromised Eco-systems with Fungi.”

    Cotter came across as an obsessed genius and kind of a wild man. He introduced himself by explaining that he had been accepted to train as an Air Force fighter pilot when he decided instead to go into mycology. At Mushroom Mountain, his mushroom farm and research facility in South Carolina, he has developed fungi that can break down toxic chemicals in soil and water.

    As I understood it, the fungal mycelium (the vegetative stage that gathers nutrients, usually out of sight, before creating the mushroom that is its fruiting body) can be “trained” to feed on a particular substrate. 

Oyster mushroom mycelium growing on coffee grounds-photo by Tobi Kellner

Cotter isolates fungal cultures on Petri dishes with substances he wants them to “eat.” When it has no other food, in a few generations the fungus modifies itself genetically to extract nutrition from that substance, which could be a chemical, a microorganism (such as coliform bacteria in sewage), a particular plant, or even an unwanted insect. It breaks down the noxious entity into building blocks that can be useful, for example for soil improvement.

    While fungi in general are opportunistic, some have more potential to develop this capacity to attack specific chemicals and organisms. Interestingly, one of the species Cotter finds most useful is the oyster mushroom, Pleurotus ostreatus, a familiar ingredient in Asian cuisines. 

Oyster mushrooms on a sugar maple-photo by David Spahr

    Cotter put out a call for people to send him problem insects they find that are infected with fungus. He showed a slide of a fire ant covered with white filament. The creepy part: a mushroom eventually grows out of the insect’s brain! When he receives these local ecotypes, or genetically distinct geographical populations, of fungi, he can grow them in his lab and reproduce them for use in the field. This offers a way to reduce use of toxic pesticides.

    I must say that the image of that mushroom growing out of the ant’s head gave me pause. I’ve never been stung by fire ants, but I understand it’s quite unpleasant, so I can understand why a killer fungus would be appealing. The red imported fire ant, Solenopsis invicta, comes from South America and is thought to have been introduced to the US accidentally. With climate change, we can expect it may continue migrating north from humid southern states.

Red imported fire ant-Photograph by David Almquist, University of Florida.

    An audience member suggested we could use a fungus to attack winter moth, Operophtera brumata, a nonnative insect damaging New England trees. 

Birch leaves chewed by winter moth caterpillars

That’s not so different from the work that’s being done releasing insect predators for winter moth. Before those are released, they’re comprehensively tested to make sure they’ll only eat the nonnative insect and won’t go on to disrupt the ecosystem. I’d want the targeted mushrooms to be just as fully screened.