Pacific Horticulture | Mycorrhizal Fungi, Nature's Key to Plant Survival and Success

Mycorrhizal Fungi, Nature’s Key to Plant Survival and Success

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Chris Todd

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Mycorrhizal fungal filaments (mycelia) radiate into the soil from mycorrhizae-colonized root tips, extending the capacity of the plant’s root system to absorb water and nutrients<br>Over 460 million years ago, plants and mycorrhizal fungi formed a beneficial relationship below the soil surface that nurtured and protected plants while feeding the fungi and other organisms in the root zone of the plants.1 That relationship exists today for over ninety percent of terrestrial species in natural settings, allowing plants to survive harsh conditions of drought, diseases, temperature extremes, poor soil, and competition.2 They are the dominant microbial life form in undisturbed soils, accounting for sixty to eighty percent of the microbial mass of the soil. Without them, for instance, large oaks, which use hundreds of gallons of water per day in the summer heat, would not thrive or grow to mature size on the dry hills of California.

Although the root-fungus association has been known since classical Greek times, when Theophrastus traced the mycelium of a mushroom back to oak tree roots, it has only been in the last ten years that this relationship has been utilized for agriculture, forestry, golf course management, and ornamental horticulture. Commercial production of mycorrhizal fungi has been available to the green industry for a decade, but only recently in small quantities for home gardeners. This article is intended to educate the home gardeners and landscape contractors, who can make use of this essential resource to bring long term health and beauty to their gardens, while weaning their properties of unsustainable practices dependent upon the use of chemical fertilizers, pesticides, and herbicides.

What are Mycorrhizae, and How Do They Work?

The word "mycorrhizae" comes from the Greek words for fungus and root, and refers to the symbiotic relationship that exists between plant roots and certain fungi. In natural settings, these mycorrhizal fungi are present in the soil in association with plant roots. The fungi colonize by attaching to the surface of the root (ectomycorrhizal) or to the inside of the root cells (endomycorrhizal). Then they send their filaments (called mycelium) into the surrounding soil, effectively extending the plant’s roots and root absorbing capacity ten to 1000 times—far beyond what the plant can do alone.3 Several miles of these ultra-fine filaments can be present in less than a thimbleful of soil. Mycorrhizae supply the water and nutrients needed by the plant for establishment and survival, and, in return, receive from the plant roots sugars and other compounds needed by the fungus. Mycorrhizae are much smaller than roots, so they can easily penetrate into smaller spaces between soil particles, where they release powerful enzymes that dissolve tightly bound minerals like phosphorus, sulfur, iron, and all the major and minor nutrients known to be used by plants. The nutrients are retained by the mycorrhizae and become available for use by the plants.

Mycorrhizae provide many other benefits to plants. The fungal filaments take up and store water, decreasing drought stress during dry periods. The fungal filaments bind soil particles into larger aggregates with organic glues such as humic compounds; the resulting soil structure allows air and water movement into the soil, encouraging root growth and distribution. Mycorrhizal fungi attack disease organisms that enter the root zone in several ways. Some produce antibiotics that immobilize or kill disease organisms. Others trap root-feeding nematodes and kill them. Some protect roots against attacking insects with a thick "net" of their mycelium. Some protect the plants from disease-causing fungi, such as Fusarium, Phytophthora, and Rhizoctonia. These activities result in improved survival, enhanced top and root growth, and increased production of flowers and fruits, plus protection from disease and an improved soil structure. The plants can better compete against invasion by weed plants and can better survive drought conditions.

A single root tip colonized by Rhizopogon mycorrhizal fungus will branch into a dense, coral-like accumulation of many root tips<br>When Do We Need to Apply or Replace Mycorrhizae in Our Soil?

Most of our man-made environments were built using practices that destroy the soil conditions supportive of beneficial soil organisms. In many cases of landscape projects in disturbed urban environments, it has been documented that the new plants had not formed mycorrhizal relationships many years after planting, and were surviving only through "intensive care"—fertilizers, pesticides, and lots of water....