Quick Notes: Mysteries of the Micorrhizae

What is a mycorrhiza? (my-kor-rise’-uh)

A mycorrhiza is a symbiotic relationship between a plant and a network of mycelium in the soil.  Over 90% of plant species depend on mycorrhizal fungi to assist their roots in accessing water and minerals from the environment.  Mycelium extend the reach of a plants’ roots.

 

Why are mycorrhizae important?

Fungi share in the abundance of sugars and other carbon-based phytochemicals produced by a growing plant each day.  They use carbohydrates absorbed from a plants’ roots for their own metabolism while also ‘sharing’ those sugars with other plants. Mycorrhizal fungi break down decomposing plants and animals into their chemical elements, like nitrogen, potassium, and zinc.  They deliver water absorbed from the soil, and these elements necessary for growth, to the plants in their network.

 

What is mycelium? (my-see’-lee-uhm)

Mycelium is the network of slender threads, sometimes too small to see without magnification, which make up the body of a mycorrhizal fungus.  The walls of these delicate tubes are a single cell thick.  They transfer water, phytochemicals, and nutrients between plants.  Sometimes they are visible as a network of pale yellow or off-white strands growing under the surface of leaves or mulch, or even in a new bag of compost.

Feathery filaments which grow from the tips of mycelium cells are known as hyphae.  These hyphae, whose cell walls are made from chitin, the same substance found in the exoskeletons of insects, both release enzymes to ‘digest’ organic matter, and they absorb those elements which are passed back to the plants’ roots.  When hyphae anchor the organism and absorb water, they are known as rhizoids.  Hyphae are strong enough to penetrate and disintegrate rock, which is how plants first gained a foothold on Earth in the distant past.  Mycelia create new soil as they decompose dead organisms and pulverize rock into smaller particles.

 

Where does mycelium come from? How does it reproduce?

Michorrihzal fungi grow from spores.  Spores are produced by fruiting bodies, many of which appear as mushrooms or puffballs.  Some types of mycorrhizal fungi also produce their spores in the mycelium underground.  While the fungus can grow ever larger underground, spores allow its genetic material to spread to another location and begin colonizing new ground.  Fungal spores can travel in the air, in the water, and in the bodies of animals (insects, worms, birds, mammals, fish, etc.) who ate the mushrooms or underground spores. When spores land on a substrate (soil, etc.) that can support them, they begin to grow into hyphae and then eventually into a new network of mycelium.

These fungi can also reproduce asexually when parts of the mycelium are dug up in soil or mulch and moved elsewhere, or when a plant on the network is transplanted.

 

Where does mycelium grow?

Mycelium grows mostly underground, in the top foot or so of the topsoil.  Mycelium can grow deeper to interact with the roots of plants that penetrate several feet under the soil.  Some types of mycelia will grow above the ground within the bodies of plants.

 

What are the two main divisions of mycorrhizal fungi?

Ectomycorrhiza are those mycorrhizal fungi whose hyphae interact only with a root’s out layers, penetrating only between cell walls of the root tip’s cortex.

Endomycorrhiza are those mycorrhizal fungi whose hyphae penetrate cell walls and grow within the cells of a plant’s roots.

There are further distinctions within both groups to describe how these fungi interact with plants.  The arbuscular mycorrhizae, a subgroup of endomycorrhizal fungi, interact with the roots of 80% or more of known plant species and are the fungi that helped plants make the leap to land from the oceans.  Their network of hyphal growth within a plant cell, as seen under magnification, resembles a tree. Known as arbuscules, these bundles of branching hyphae within a root cell are responsible for the exchange of nutrients between the plant and the fungus.

 

What are the ecological benefits of mychorrhizal fungi?

Mychorrhizal fungi:

• Increase the absorption surface of plant roots to seek water from soil distant from the plant
• Increase crop yields
• Seek and transport phosphorus, ammonium, zinc, nitrogen, and other nutrients back to those plants that need them.
• Sequester carbon captured by plants’ leaves within the structure of mycelium in the soil.
• Improve soil texture and its ability to absorb and hold water.
• Provide for communication between plants about environmental stressors via chemical messaging
• Protect plants from certain soil-borne diseases
• Help plants withstand stressors like drought, excessive heat, salinity, and toxic waste build-up in the soil
• Decompose leaf litter, dead wood, insects, and other dead organisms to build rich soil and free up nutrients for plant growth
• Provide structure for healthy soil and protect it from erosion
• Feed insects, birds, reptiles, mammals, and humans

 

What does a mushroom mean if it appears in my yard?

Mushrooms demonstrate that mycorrhizal fungi live in the soil.  It may take decades after mycelium begin to grow in the soil before mushrooms first appear.  Mushrooms sometimes appear very suddenly, with no warning, after a heavy summer rain.  They were likely already there on their mycelium, just beneath the surface, waiting for the right conditions to ‘bloom.’  The cells of these fruiting bodies, spore-filled mushrooms, expand when soil moisture increases during rain. They swell up with moisture quickly, so they appear to suddenly grow out of the lawn.  Most mushrooms appear quickly and may also disintegrate quickly once they have released their spores.

While the fungus can grow ever larger underground, spores allow its genetic material to spread to another location and begin colonizing new ground.  Spores may travel by wind or water, or they may be spread by animals.  Since mushrooms provide important food for insects, birds, and many other types of animals, including fish; spores may be deposited miles away from the parent plant in an animal’s excrement.  Some fungi don’t even produce mushrooms or puffballs, and so their spores may remain underground, sometimes spread by earthworms and other invertebrates.

 

What type of weather causes mushrooms to appear?

Mushrooms ‘bloom’ and rise through the soil and from mulch when there is sufficient water in their substrate, during warm weather.  They are most common during late summer and autumn.

 

Are all types of fungi good for my garden?

No, fungi can be bad or good, depending on your point of view; hurtful or helpful depending on your interests. Fungi, in the form of yeasts, inspired agriculture and permanent communities when early humans learned to transform grains into both beer and bread.  Fungi is a catalyst for change, for transforming one thing into another.  They break down organic molecules to transform them, freeing up elements to re-combine for other uses.

There are several different types of fungus, each with its own niche within the ecosystem.  Some are parasites, and some are decomposers which consume organic material already killed and decomposed by other fungal species.  The fungal genera gardeners depend upon to help support plants are symbiotic mycelium species which work cooperatively with plants for the good of the entire plant community.  A seedling plant supported by mycelium grows faster and stronger than one grown in sterile potting mix.  Those mycelia give it resistance to certain microbial infections and feed essential nutrients to young plants that a commercial fertilizer cannot replace.

Some fungi attack plants, however.  Chestnut blight; fungal infections that suddenly kill oak trees; and the ‘honey mushrooms’ which sometimes colonize and kill some species of living conifer and hardwood trees; are some of the many fungi which can cause problems in forests and in our gardens.

 

How can I introduce helpful strains of mycelium to my garden?

Since fungal spores are widespread, those needed may already be present in the soil, ready to work with newly installed plants.  A lot depends on the quality of your soil, how it has been handled in the past, and whether the native topsoil was recently scraped away for new construction.  Fungal spores will blow in on the wind, fall in the rain, be left behind in animal excrement, and may already be present in the soil of new plants you acquire.

Adding organic matter like compost, wood chips, and other plant-based mulches always helps to introduce beneficial fungi because spores are likely present in these products.  A handful or two of soil with established mycelium taken from around established shrubs and trees will also introduce enough mycelium to start a new network around a newly planted tree or shrub.

There are many commercial products designed to introduce beneficial fungi to support crops and landscaping plants.  Some are blended with fertilizers.  When blending a ‘seed starting mix,’ consider adding a commercial inoculant so seedlings have beneficial fungi ready to partner with them as they germinate.  This will produce a stronger seedling with a better root system that will experience less transplant shock when it is moved out to the garden.

 

Which common gardening practices destroy beneficial mycelium, and can negatively affect plant growth?

When one of our plants is struggling, most of us want to know which chemicals to apply to solve the problem.  Now we are beginning to understand that most pesticides (insecticides, herbicides, fungicides) and strong chemical fertilizers kill mycorrhizal fungi in the soil.  When we apply fungicides to address black spot or powdery mildew, that fungicide will also kill beneficial fungus which may be supporting the plant’s roots.  Keep in mind that insecticides and herbicides can also kill the symbiotic fungi our plants depend upon for their health.

Tilling and double digging before planting disrupts mycelium networks in the soil.  ‘No-till’ practices protect the life of the soil which supports plants’ roots.

 

Are fungi plants or animals?

Beneficial mycelia belong to the Fungal Kingdom and aren’t plants or animals. They share the most characteristics with animals, but fungi work most closely in support of plants.  Lichens are organisms that are made up of a fungi partner and a plant partner, most often an algal species. Certain fungal species can be parasites of, and work to decompose, both plants and animals.

Most of us know little or nothing about the vast Kingdom of Fungi, and how they support our well-being while they also support most of the plants growing in our gardens.  This is a relatively new field.  To work in cooperation with the vast fungal networks already at work in our gardens, and to encourage new ones, we need to learn more about them.  We can learn to recognize which are beneficial as we also learn to recognize those that may be harmful to the plants we grow.

 

Read the whole article:  Mysteries of the Myccorhizae

 

For More Information:

Elliot, Todd F. and Steven L. Stephenson. Mushrooms of the Southeast. Timber Press. 2018.

McKnight, Karl B., Joseph R. Rohrer, Kirsten McKnight Ward, Kent H. McKnight. Peterson Field Guide to the Mushrooms of North America, Second Edition.  Mariner Books. 2021.

Phillips, Michael. Mycorrhizal Planet:  How Symbiotic Fungi Work with Roots to Support Plant Health and Build Soil Fertility. Chelsea Green Publishing. 2017.

Sheldrake, Rupert. Entangled Life: How Fungi Make Our Worlds, Change Our Minds & Shape Our Futures. Random House. 2020.

Smith, Sally E. and David J. Read. Mycorrhizal Symbiosis 3rd Edition. Academic Press. 2008.

Stamets, Paul. Mycelium Running: How Mushrooms Can Help Save the World.  Ten Speed Press. 2005.

Zonca, Vincent. Lichens: Toward a Minimal Resistance 1st Edition. Polity. 2022.