Mycorrhizae: The Straight Story

Greetings plant people! As a retired research plant pathologist, I have conducted research on diseases affecting ornamental and nursery crops for nearly 50 years and studied root diseases with the goal of developing an understanding of the role soil microbes play in plant growth and health.

I help many growers better understand how to improve the quality and health of soil by highlighting the three interactive soil factors: soil chemistry, soil structure, and soil microbiology. These three factors are like a three-legged stool that must be in balance for optimal results in growing plants.

Currently, I am the science guy for two companies and consult with nurseries and other companies on the development of technology to apply beneficial microbes, such as mycorrhizal fungi and associated rhizobacteria, and contribute microbiological technology that will aid in achieving sustainability in agricultural crop production.

My role within these companies is to develop and transfer knowledge to production staff and provide them guidance and methodology so we can continue to produce the highest quality mycorrhizal fungal inoculum and other organic bio-based products for crop production systems. This boils down to biological farming, both philosophically and functionally.

The following article is part one of a three-part series that aims to give the straight story on the biology and application of mycorrhizal fungi. I start with a description of these amazing fungi, and in future articles, I will address application technology and my views on information and misinformation that growers might have been given.

What Are Mycorrhizae?

The term mycorrhizae describe the symbiotic (living together with mutual benefit) relationship between specialized soil fungi and the roots of most plants on Earth. Myco = fungus, rhizo = root. These fungi have been associated with plant roots for 460 million years ever since plants began to grow in soil.

Other soil microbes become associated with the mycorrhizal fungi in the rhizosphere soil—soil associated with and influenced by root exudates. Then, rhizosphere soil becomes mycorrhizosphere soil—soil influenced by both fungus and roots. The mycorrhizal fungi become the interface between soil and plant roots—the fungi colonize the roots internally, and the soil externally. Internally, the fungus becomes the interface where nutrient exchanges occur between the fungus and plant by direct contact of the fungus with the root cells: carbon energy from the plant to the fungus, and soil nutrients from the fungus to the host plant. A plant with mycorrhizae is physiologically altered due to biochemical changes that occur in the plant.

(More on mycorrhizae? Uprooting Potential of Mycorrhizae)

Types and Benefits of Mycorrhizae

There are three main types of mycorrhizae:

• Ectomycorrhizae—for pine, fir, spruce, oak and eucalyptus
• Endomycorrhizae (arbuscular mycorrhizae) —for most crop plants
• Ericoid mycorrhizae—for blueberry, rhododendron, azalea, Pieris and cranberry

Some plants, such as crucifers, the carnation family and sedges, do not form mycorrhizae. It is important to match host plants with the right kind of mycorrhizal fungi. The following is a list of benefits that refer to all three types of mycorrhizae, although some are more relevant to one type than the others:

Improved root development: Any treatment that can enhance the rate and extent of root development will greatly affect the growth and health of the whole plant. The specific situations can be an increased rooting of cuttings, enhanced secondary root development and simply increased root biomass. The benefit then goes further in that the plant has more roots to be colonized by mycorrhizal fungi, which can impart even more benefits to the plant.

Improved transplant success: Many crops are transplanted into the field or larger containers for further growth. Frequently, transplant shock results in the failure of the transplant to survive or grow properly. It is now known that pre-inoculation of small transplants with mycorrhizal fungi can greatly improve transplant success into soils that may have one or more stress factors. The benefit of pre-inoculation with mycorrhizal fungi is often obvious.

(For more on transplanting, check out Super Starts: Healthier Transplants for Happier Harvests.)

Increased yield and quality: When mycorrhizae form early in the plant growth cycle, multiple benefits can greatly affect plant growth and health. A bigger, stronger root system supports a bigger and more robust above-ground plant. Physiologically, the plant has been modified in many ways, some of which may mean the plant will produce more. Depending on the plant, of course, the product may be more and larger onions or potatoes; larger, healthier trees or shrubs; or increased quantities or types of compounds produced within the plant tissues. Some of those chemical products account for increased resistance to plant diseases, others may be health-improving antioxidants or medicinal plant products.

The literature is increasing with examples of such biochemical changes in plants with mycorrhizae as compared to plants without mycorrhizae. For food produce, such as onions or potatoes, the keeping quality of the produce is increased: onions are firmer and denser, and fruits are tastier. So, increased produce quality can be a significant benefit of having fully functional mycorrhizae as early in the production cycle as possible. To accomplish that, plants need to be inoculated in the early stages of that production cycle.

Greater tolerance to soil-borne diseases: Growers of almost all crop plants can experience losses of plants or productivity due to the incidence and severity of root diseases caused by soil-borne fungal or nematode plant pathogens. This leads to the application of microbe-devastating soil fumigants or drench fungicides, many of which greatly suppress mycorrhizal fungi. Many published examples show that inoculation with mycorrhizal fungi can suppress root diseases by mechanisms that are still being explored.

My own studies have indicated that when mycorrhizae form, there is an increase in the numbers of potential antagonists to the pathogens. The selective pressure for those antagonistic bacteria to increase in the soil around and influenced by roots (rhizosphere soil, now called the mycorrhizosphere soil, influenced by both the roots and the mycorrhizal fungal strands) is distinctly greater than rhizosphere soil around roots that are not mycorrhizal. The net result is that plants with mycorrhizae have less root disease than plants without, due to the increased antagonistic potential of the soil to pathogens.

Improved soil structure due to aggregation: When mycorrhizae form, the symbiotic fungus colonizes the roots internally, but then grows out into the soil, creating a huge fungal biomass in the soil. The fungal strands (called hyphae for AM or ericoid mycorrhizal fungi, or hyphae and rhizomorphs [rope-like strands] for ectomycorrhizae) explore a large volume of soil for nutrients and water to support themselves and their host plant partner. Those hyphae produce a sticky material that binds small soil particles or aggregates into larger macro-aggregates that greatly improve aeration in the soil and enhance the potential for water or nutrients to move down into the soil profile. Thus, soil tilth is improved.

Improved fertilizer-use efficiency: Many studies have shown that mycorrhizal fungi greatly expand the absorptive capacity of the root system by means of the soil-penetrating fungal strands that can mine the soil for more distant and soil-bound nutrients like phosphorus (P), copper (Cu) or zinc (Zn). Those essential nutrients would not be available to the plants without the help of mycorrhizal fungi. So, when P is limited, arbuscular mycorrhizal fungi can explore a huge volume of soil, acquire the P and transport it to the root via the hyphal stands.

Improved tolerance to soil drought: When water becomes limited due to drought conditions, plants with mycorrhizae consistently fair better than plants without mycorrhizae. The mechanisms for that effect are at least two-fold: improved absorptive capacity of water from the soil by the fungi, and altered plant physiology that increases the drought tolerance of the plant.

Improved tolerance to soil toxicities (salinity): Salinity can be a plant growth-limiting factor in soil, as a result of salt accumulation from irrigation practices and the application of fertilizers. When the salt level exceeds an EC of 4, most plants will suffer and exhibit a growth reduction and decreased yield and quality of produce. Many studies have shown that plants with mycorrhizae tolerate soil salinity better than plants without mycorrhizae.