What are Mycorrhizal Fungi?

Mycorrhiza, which means “fungus-root,” is defined as a beneficial, or symbiotic relationship between a fungus and the roots of its host plant. This relationship is a natural infection of a plant’s root system in which the plant supplies the fungus with sugars and carbon and receives water and/or nutrients in return. This type of relationship has been around since plants began growing on land about 400 to 500 million years ago. There are several thousand different species of mycorrhiza fungi.

Types of Mycorrhizal Fungi

Mycorrhizae are classified into two types, based on the location of the fungal hyphae in relation to the root tissues of the plant with endomycorrhiza producing hyphae inside the roots and ectomycorrhiza-producing hyphae outside the roots. These are further classified into Arbuscular (AM) endomycorrhizas, Ericoid endomycorrhizas, Arbutoid endomycorrhizas (subgroup of Ericoid), Monotropoid endomycorrhizas (subgroup of Ericoid), orchidaceous endomycorrhizas and ectendomycorrhizas. Arbuscular mycorrhizae (AM) are the most common type of endomycorrhizal (EM) fungi, whose hyphae extend into the cell membrane of the cortex root cells and form vesicles. These vesicles are structures that help the plant-fungi association exchange water and nutrients. Ectomycorrhizae (ECM) form a thick mantle of hyphae (mycelium) surrounding the root and root tip, extending into the spaces between the cortical cells.

Benefits

Endomycorrhizal fungi benefit not only a large number of desert plants, but a majority of the plants in the world (Table 1). Ectomycorrhizal fungi, which account for about 3 percent of mycorrhizhae, are more advanced and benefit mainly woody and tree species (Table 2). In total, mycorrhizal fungi benefit 80 to 90 percent of all plant species. Plants that do not respond to mycorrhizae include azalea, beet, blueberry, broccoli, Brussels sprouts, cabbage/kale, carnation, cauliflower, collards, cranberry, heath, huckleberry, mustard, protea, rhododendron, sedge and spinach.

Table 1. Partial list of plants that benefit from endomycorrhizae.

Table 2. Partial list of plants that benefit from the use of ectomycorrhizal fungi.

The main benefit mycorrhizal fungi provide is access to large amount of water and nutrients (particularly nitrogen, phosphorus, zinc, manganese and copper). This is because the hyphae increase the root surface area of absorption from soil. The mycorrhizal hyphae are smaller in diameter compared to plant roots and can reach areas unavailable to the roots. Other reported benefits of the mycorrhiza include:

• Increased pathogen resistance
• Increased drought and salinity stress tolerance
• Higher transplanting success
• Increased crop yield with enhanced flowering
• Increased water and nutrient uptake
• Improved soil structure

Use, Products and Cost

Mycorrhizae are designed for many uses, including vineyards/orchards, nurseries, commercial growers, landscapes, homeowners or for land reclamation projects. The use of mycorrhizal fungi is also popular in organic production. It is important to note that mycorrhizae can be found in most soils naturally, so it might not be necessary to purchase mycorrhizae. Most soilless media does not contain mycorrhizae, so they could be incorporated if growing in containers.

Mycorrhizal fungi can be found as granular, powder or in concentrated solution. Products vary in type, number and spore counts of fungi used as well as cost, which can range from a few dollars to several hundred dollars, depending on the product and amount needed. Mycorrhizal fungi can be purchased at garden centers, nurseries or online from companies like Plant Success, Bio Organics, Soil Moist or ARBICO Organics.

Application

Application of mycorrhizal fungi in production can be conducted as direct infection of cuttings or plugs during transplanting, incorporating into the media or the soil or applied through the irrigation. Application rates vary by product and application area, but rates can be as little as 1 teaspoon or 50 milliliter, if using a liquid solution. Most commercial mycorrhizal fungi products do not require any reapplication; however, others recommend additional applications after several weeks. The inoculant can reproduce with ideal circumstances, such as adding mulch and compost. Avoid over-watering and exces-sive fertilization applications. However, irrigation, harvesting and crop rotation may influence the root-fungi combination. Some fungi can colonize new roots within a week, while others may take as long as a month.

Precautions

• Product storage temperature should not exceed 140 F or be colder than 40 F.
• Heavy phosphorus, nitrogen and zinc applications will inhibit mycorrhizal infection.
• Most products have a shelf life, which can vary from months to several years.
• Fungicides should be avoided, since mycorrhizae fungi are a type of fungi.

Bruce Dunn

Associate Professor, Floriculture

Richard Leckie
Undergraduate Student

Hardeep Singh
Graduate Student