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A Gardener’s Primer to Mycorrhizae: Understanding How They Work and Learning How to Protect Them

A Gardener’s Primer to Mycorrhizae: Understanding How They Work and Learning How to Protect Them

FS269E
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Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, Washington State University
Mycorrhizae are symbiotic associations between many plants and the beneficial fungi that colonize their roots. Gardeners are often unaware of these relationships and may inadvertently injure or kill the beneficial fungi through common gardening activities. This publication helps home gardeners understand the benefits of mycorrhizae and explains how to enhance their presence in landscapes and gardens.
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Overview

Mycorrhizae are symbiotic associations between many plants and the beneficial fungi that colonize their roots. Gardeners are often unaware of these relationships and may inadvertently injure or kill the beneficial fungi through common gardening activities. This publication will help home gardeners understand the benefits of mycorrhizae and explain how to enhance their presence in landscapes and gardens.

Introduction

Mycorrhizae are associations between some fungal species and the roots of many host plant species (Figure 1). The word mycorrhizae reflects this partnership:

myco = fungus

rhizae = roots

These are primitive associations which developed hundreds of millions of years ago when vascular plants emerged on land. Originally, mycorrhizal relationships were thought to be unusual oddities. We now know that they are the rule, rather than the exception, especially in woody plants.

Mycorrhizal fungi are divided into two categories: those whose root-like hyphae surround and occasionally penetrate root tissues (ectomycorrhizae) and those whose hyphae always enter the root cells (endomycorrhizae). Ectomycorrhizae colonize the roots of many woody plant species and form an extensive hyphal network throughout mulch and topsoil layers. Because ectomycorrhizae are commonly found on tree and shrub roots and are the easiest for gardeners to see, this publication will use them as general examples.

The Benefits of Mycorrhizal Relationships

The relationship between plants and mycorrhizal fungi is mutually beneficial. Plants are photosynthetic and provide sugars, B vitamins, and other important chemicals to their fungal partners. Fungal hyphae are long and thin and can better explore the soil for water and nutrients compared to plant

Figure 1. A mycorrhizal partnership between a fungus and a plant root.
Figure 1. A mycorrhizal partnership between a fungus and a plant root.

roots. Mycorrhizae are particularly adept in extracting phosphate from the soil. Phosphate is often immobile in soils, and mycorrhizae are able to solubilize phosphate in their immediate environment (Badawi 2010). Phosphate and other nutrients and water are shared with the plant through the mycorrhizal relationship.

Increased water and nutrient uptake allow plants to establish faster, grow bigger, and survive longer than plants without mycorrhizae. Healthier plants are more resistant to environmental stress, pests, and disease. This is especially evident with root pathogens. Mycorrhizal plants are more resistant to diseases such as Verticillium (Garmendia et al. 2004; Whipps 2004) and pests, including nematodes (Affokpon et al. 2011; Verma and Nandal 2006).

In comparative studies, mycorrhizal plants had increased tolerance to drought (Auge 2004; Walker et al. 2003), salt, and heavy metals such as zinc and lead (Ma et al. 2006). Mycorrhizae can help prevent uptake of these toxic minerals from soil (Meharg 2003) and inhibit their movement from the roots to the shoots (Chen et al. 2005).

Mycorrhizae provide economic (Al-Karaki 2002) and environmental benefits as well. Because mycorrhizae increase uptake of essential nutrients (Ma et al. 2006), there is less need for fertilizers (Hamel and Strullu 2006; Sharma and Alok 2004). Mycorrhizal networks are also credited with reducing excess soil nutrients from seeping into aquatic ecosystems (Hamel and Strullu 2006; Liu et al. 2004).

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