Mycorrhizal Fungi

by Mike Amaranthus
Excerpt from Digger Magazine April 2002 Issue

Continuing Education

The more we learn about life on Earth, the more important seem the mechanics of survival. Brute force, the old idea of “bloody competition,” is no guarantee of survival. In reality, nature works in subtle ways. There is no doubt that competition is everywhere, and attaining resources is important for establishing plants. However, we now realize that in natural systems, organisms work together interdependently. There is no doubt that nature is less a battleground and more a marketplace. Symbiotic organisms that exchange materials and services in a mutually dvantageous living arrangement can be the key to successful growing and planting.

Tiny Secrets and Successful Plantings

Clearing of natural areas and new construction in the urban and suburban landscape represents the extreme of soil and plant disturbance. Getting plants established is often a great challenge. Numerous tight or tenuous links between plants and soil microorganisms are broken. These linkages are nature’s “tiny little secrets” that have allowed plants to survive and thrive in natural environments for millions of years without the use of fertilizers, pesticides and irrigation. Nursery and landscape professionals are gaining increased appreciation of the living soil and more frequently incorporating soil biology and mycorrhizal products into their practices.

What Are Mycorrhizae?

More than 90 percent of plant species form a symbiotic arrangement with beneficial soil fungi called mycorrhizal fungi. The roots are colonized by the soil fungus, which attaches to the roots and extends far into the surrounding soil environment (figure 1). The colonized root is called a mycorrhiza. Mycorrhizal fungi are the dominant microbes in undisturbed soils accounting for 60 percent to 80 percent of the microbial biomass. Mycorrhizae are fundamental to plant establishment, supplying the water and nutrients needed for survival and, in exchange, receiving essential sugars and other compounds supplied by the plant. There are basically two broad groups: those forming ectomycorrhizae, so termed because of the external modification to the root, and those termed arbuscular mycorrhizae (also termed endomycorrhizae), the name coming from the structure formed within the root cells. Unlike the ectomycorrhizae, no external modification of the root accompanies arbuscular mycorrhizae.

Plants forming ectomycorrhizae include the vast majority of the commercially grown tree species in the temperate and northern forests and 70 percent of the tree species planted in the tropics. Trees such as conifers and oaks are dominantly ectomycorrhizal plants. Most of the commercially important horticultural and agricultural plants form arbuscular mycorrhizae. There are few plants that don’t rely on mycorrhizae in their natural environment, and most nonmycorrhizal plants are “weedy species” that can get the upper hand following disturbance events that raise havoc with existing mycorrhizal fungi.

What Do They Do for Plants?

These mycorrhizal fungi increase the surface absorbing area of roots 10 to 1,000 times, thereby greatly improving the ability of the plants to use the soil resource (figure 2). Estimates of amounts of mycorrhizal filaments present in soil associated with plants are astonishing. Several miles of fungal filaments can be present in less than a thimbleful of soil. But mycorrhizal fungi increase nutrient uptake not only by increasing the surface absorbing area of roots. Mycorrhizal fungi release powerful chemicals into the soil that dissolve hard-to-capture nutrients, such as phosphorous, iron and other “tightly bound” soil nutrients. This extraction process is particularly important in plant nutrition and explains why nonmycorrhizal plants require high levels of fertility to maintain their health. Mycorrhizal fungi form an intricate web that captures and assimilates nutrients, conserving the nutrient capital in soils. In nonmycorrhizal conditions, much fertility is unavailable to plants or lost from the soil system.

What Other Functions Do Mycorrhizal Fungi Perform?

Mycorrhizal fungi are involved with a wide variety of activities that benefit plant establishment and growth. The same extensive network of fungal filaments important to nutrient uptake is also important in water uptake and storage. In nonirrigated conditions, mycorrhizal plants are under far less drought stress compared to nonmycorrhizal plants. Suppression of diseases and pathogens are additional benefits for a mycorrhizal plant. Mycorrhizal fungi attack pathogen or disease organisms entering the root zone. For example, excretions of specific antibiotics produced by mycorrhizal fungi immobilize and kill disease organisms. Some mycorrhizal fungi protect plants from Phytophthora, Fusarium and Rhizoctonia. Mycorrhizal fungi also improve soil structure. Mycorrhizal filaments produce humic compounds and organic “glues” (extracellular olysaccharides) that bind soils into aggregates and improve soil porosity. Soil porosity and soil structure positively influence the growth of plants by promoting aeration, water movement into soil, root growth and distribution. Many practical benefits can be expected from using mycorrhizal fungi in common practices. These include improved survival, growth, more rooting (figure 3a, 3b), flowering and fruiting, protection against disease, improved soil structure and resistance to invasion by nonmycorrhizal or exotic plant species.

Where’s the Beef?

The plant-mycorrhizal fungi relationship is the best understood in the field of soil biology. There are more than 48,000 studies in literature on the subject. But there is more important proof. The mycorrhizal relationship with plants is one of nature’s longest and most successful experiments. The earliest fossil record of the roots of land plants contain arbuscular mycorrhizae almost identical to what is found today. Most scientists today believe the plant-mycorrhiza relationship allowed aquatic plants to make the transition to the relatively harsh terrestrial environment some 430 million years ago. In nature, mycorrhizae make plant growth possible, linking the roots of plants to the s urrounding soil. In nature, neither can survive without the other.

Does My Site Have Mycorrhizae?

Soils from natural and undisturbed areas generally contain robust and diverse populations of mycorrhizal fungi. Events that seriously disturb soil can substantially reduce or eliminate these beneficial microbes (figure 5). Research shows that compaction, erosion, grading, topsoil removal, overgrazing and the use of soilless mixes in growing operations often eliminate mycorrhizae completely. The arbuscular mycorrhizae and many of the top-performing ectomycorrhizal fungi do not disperse their spores in the wind and move by growing root-to-root or by consumption by wildlife species. In a disturbed habitat, the effectiveness of the return of mycorrhizae is dependent on the quality and proximity of undisturbed habitats containing suitable fungi and their associated animal vectors (figure 5). Many cases have been documented where plants in disturbed urban and suburban environments have not formed mycorrhizae many years after outplanting and are surviving only through intensive care and maintenance.

Can I Fertilize Instead?

Many fertilizer regimens push top growth at the expense of root development, making plants vulnerable to stressful environments. Frequent, high levels of fertilizer produce an unbalanced and often unsustainable shoot-to-root ratio. Mycorrhizae, on the other hand, feed your plants and stimulate root growth. Unlike mycorrhizae, fertilizer cannot help prevent root disease, improve soil structure or promote other beneficial microbes. Fertilizers can lead to other side effects, such as deterioration of water quality, soil structure and excess soil salinity. The mycorrhizal relationship improves feeder-root production, and a mycorrhizal plant can better utilize added fertilizer

How Do I Use Mycorrhizal Products Most Effectively?

High-quality commercial mycorrhizal inoculum is now available from a variety of sources. Inoculums containing mixtures of species of mycorrhizal fungi often give the best response. Mycorrhizal inoculum comes in granular, powder, liquid and tablet forms. The most important factor is to get the mycorrhizal propagules near the root systems of target plants. Most mycorrhizal propagules will stay dormant and until root activity begins. The chemicals pumped into the soil by active roots cause mycorrhizal propagules to become active and grow. Inoculum can be incorporated into the planting hole at the time of transplanting, watered into porous soils, mixed into soilless mixes or directly dipped on root systems using gels. The form and application of the mycorrhizal inoculum depends upon the needs of the applicator. What is clear is that on disturbed and stressful sites, inoculation is highly effective.

Micro Reality

Growing plants in a nursery and establishing plants on disturbed sites require an understanding of the many soil processes important in facilitating uptake, storage and cycling of nutrients and water by the target plant species. In nature, these activities are largely performed by the “tiny little secrets,” working hard below the soil surface in the living soil. In past decades, clearing of natural areas and disturbances in suburban and urban environments have substantially reduced mycorrhizal populations. Because above- and below-ground plant/soil systems are tightly interdependent, such changes can result in poor plant survival and health and a reliance on intensive and artificial plantcare programs. Hopes for restoring beneficial mycorrhizal fungi and their important relationship with plants has been aided with the development of quality and increasingly inexpensive sources of mycorrhizal inoculum. Nursery and landscape professionals can now make a declaration of interdependence and incorporate mycorrhizal fungi into their programs.

Dr. Mike Amaranthus spent 20 years with Oregon State University and the U.S. Department of Agriculture Forest Service, where he wrote more than 50 research papers on mycorrhizae. He is the recipient of the USDA Highest Honors for scientific achievement and has been featured on several major national and international programs. He is president and chief scientist for Mycorrhizal Applications Inc.