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GardenSMART :: Mycorrhizal Fungi and Healthy Plants

Mycorrhizal Fungi and Healthy Plants

By Therese Ciesinski, GardenSMART

When it comes to the soil in our gardens, there are lots of interesting things happening beneath our feet. One is the interaction between chlorophyll-producing plants and fungi. Over millions of years, they've evolved to work together, forming what's known as mycorrhizal relationships. The word comes from the Greek words for "fungus" and "root."

This evolution occurred because plants and fungi each have something the other needs. Unlike chlorophyll-producing plants, which make their own food, fungi must absorb theirs, and can't get everything they need from the soil. Plants make their own food, but they can't access everything they need from the soil, either. Think of it as a beautiful friendship: plants and fungi provide to the other, sustaining both.

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Graphic by Nefronus, Wikimedia Commons

Here's how it works: a mycorrhizal fungus colonizes the cells of a plant's root system, helping plant roots take up water and nutrients such as nitrogen and phosphorus. In exchange, the plant provides sugars and amino acids to the fungus. It's a kind of infection of the plant's root system, but a symbiotic one: both plant and fungus benefit.

Fungi grow via filaments called hyphae. Invisible to the naked eye and even smaller than root hairs, hyphae colonize a root, expanding its surface area and extending into the soil further than the root can.

Mycorrhizal fungi also produce chemicals that neutralize substances in the soil that can harm their host plants, and help protect plants from diseases and insects such as nematodes, for example, by excreting substances toxic to the invader.

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The larch bolete mushroom (Suillus grevillei) has a mycorrhizal relationship with the tree, European larch (Larix). Photograph courtesy USDA Forest Service – Northeastern Area, USDA Forest Service, Bugwood.org.

That mushroom in your garden is the "fruit" of a fungus growing in the soil. Mushrooms exist because of mycorrhizal relationships and are the visual evidence of them. (There are almost 4 million species of fungi on earth but not all have mycorrhizal relationships with plants.)

Some types of mycorrhizae are symbiotic with a single species, which is why certain fungi are only found growing on a particular plant, such as the bolete mushroom on roots of the European larch tree. Others associate with many kinds of plants.

Gardeners may be most familiar with mycorrhizal associations between fungi and trees such oaks and pines, however up to 80% of all plant species depend on this relationship. If it were not for mycorrhizae, humans would not enjoy rice, wheat, blueberries, or orchids.

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Mycorrhizae branching. Photograph courtesy of Robert L. Anderson, USDA Forest Service, Bugwood.org.

At this point you may be wondering: what does this mean for my garden? Well, you can't have healthy plants without healthy roots, and you can't have either if the soil is poor. While healthy soil will have plenty of mycorrhizal fungi, depleted soil lacking in nutrients cannot support either fungal colonies or plants.

There are many types of mycorrhizae inoculant available to purchase, but the best way to ensure these fungi thrive is straightforward: keep the soil healthy, and avoid practices that might kill them, such as overcultivation, synthetic chemicals, and compacted or eroded soil. There's fungus among us, and it's important to keep it that way.

 


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