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Nature's Planting Tool

Posted on: Wednesday, 13 July 2005, 03:00 CDT

Researchers discuss the benefits of and uses for mycorrhizae in grower operations.

A favorite research topic for the last decade has been the symbiosis between a plant root and beneficial fungus, called mycorrhiza (Figure 1). Mycorrhiza is the name of a widespread symbiotic (mutually beneficial) root relationship. The fungus helps the plant extract nutrients from the soil, and the plant repays the "kindness" with sugars produced by photosynthesis.

The plant/mycorrhizae team is perhaps nature's most successful long-term relationship. It allowed plants to move from their aquatic origins, more than 460 million years ago, to the land, where they proliferate today. Mycorrhizae expand plant roots and can amplify nutrient and water extraction from rock itself.

Figure 2. Arbuscule within the root cell of endomycorrhizal plants are centers of material exchange between plant and fungus.

Figure 3. Electron micrograph of ectomycorrhiza shows mycorrhizal fungal filaments radiating from conifer root.

The relationship is such a winner that more than 90 percent of the world's plants form mycorrhizae in their native habitats. It truly acts as nature's planting tool because of its vital role to plant health below the soil surface.

Figure 1. Mycorrhizal roots of pine.

Types Of Mycorrhizae

The most common kind of mycorrhizae is commonly referred to as "endomycorrhizae" or "endo." The scholarly know its formal name of "arbuscular mycorrhizae (AM)" because it forms an arbuscule structure inside the root cell (Figure 2).

The next most common kind, commonly known as "ecto" (EM) combines a mushroom, puffball or truffle type of fruiting fungus, forming a sock-like covering that is evident on the exterior of the absorbing roots. Tiny threads or filaments radiate into the surrounding soil to absorb nutrients and water (Figure 3).

AM host plant species include most nonforest plants of economic importance, and most natives used in habitat restoration. EM host species are mostly forest trees: pines, firs, spruces, oaks and some tropical tree species. Visit www.mycorrhizae.com for a list of plants and the type of mycorrhizae they form.

Are Your Plant Species Mycorrhizal?

In nature most are, and most form the AM symbiosis. So, can we borrow from what mycorrhizae do in nature to benefit the nursery and landscape?

Native prairies and forests function for decades and centuries without artificial inputs. In the successful native system, fertility is used over and over again and nutrients aren't lost to runoff, groundwater or by erosion. This can be of great benefit to nurseries where conservation and avoidance of soil and water pollution are becoming important issues.

What Are The Benefits?

What does the mycorrhizal symbiosis do for the plant? One thing for sure is that mycorrhizal plants are far better than non- mycorrhizal plants at mining nutrients from the soil. The obvious result of better nutrition is better plant growth and improved utilization of nursery fertilizer regimens. There are hundreds of published examples showing mycorrhizal plants growing significantly larger than non-mycorrhizal controls (Figure 4).

This "growth response" is only the most obvious benefit. Other benefits, while appearing unrelated to nutrition, may be caused by the improvement in phosphorus uptake. For example, plants with plenty of phosphorus are more resistant to water stress than plants low on phosphorus. Thus, mycorrhizal plants have better tolerance of drought (Figure 5).

Some plant species are highly dependent upon mycorrhizae and thus have been difficult to work with during and after the nursery stage. This is primarily due to the lack of symbionts onsite or their suppression by horticultural practices.

Figure 4. Mycorrhizal inoculation examples demonstrate improved performance of hosta.

Intentional introduction of the missing beneficial microorganisms invariably makes the difficult plant species easier to grow and leads to improved rooting growth rate. Other benefits, not apparently growth related, include the ability of mycorrhizal fungi to actively deter the activities of pathogens in the root zone and improve the plant's tolerance for salts.

Another indirect but extremely important effect is soil aggregation. Mycorrhizal filaments in the soil surrounding the roots bind the soil together and contribute importantly to soil structure. Soil structure influences the ease of water and root penetration, soil drainage and aeration. Soils with good soil structure are said to be well-aggregated. Well-aggregated conditions occur when individual soil particles are joined together in stable clusters that improve the infiltration and porosity of the soil.

Figure 5. Mycorrhizal inoculated rose (right) and not inoculated rose (left) subjected to systematic levels of plant moisture stress.

Nature Vs. Nursery

Generally, you can duplicate or exceed the mycorrhizal growth response with high inputs of chemical fertilizer and pesticides, but you cannot easily reproduce the combination of benefits such as improved outplanting performance, feeder root production, drought tolerance and improved soil structure. In addition, the use of mycorrhizal inoculum can result in significant cost savings by decreasing the need for water, fertilizer and pesticides.

The nursery environment can differ drastically from the natural habitats of plants and mycorrhizal fungi. Factors affecting mycorrhizal colonization under nursery conditions include inoculum type, nutrient regime and fungicide applications. Mycorrhizal colonization under nursery conditions is best understood through a conceptual model that takes into consideration differences between the natural characteristics of phosphorus in the environment and those found in the nursery environment. Under natural conditions, phosphorus (P) occurs in patchy locations in relatively insoluble forms. For a plant to access phosphorous, it must produce an abundance of fine roots and contact the resource via chance root interception, or form mycorrhizae.

At many nurseries, fertilizer is applied in a soluble inorganic form, which completely saturates the container media. Under P saturated conditions, the plant is not P deficient and effectively avoids mycorrhizal colonization. Replicating a patchy, slowly available P source under nursery conditions results in increased colonization. This environment can be created by using fertilizer with:

1) low solubility such as rock phosphates.

2) time release forms of P, such as controlled-release native formulations.

3) reduced amounts (30 to 50 percent) of inorganic chemical forms of P, such as ammonium phosphate.

Certain fungicide treatments can kill mycorrhizal fungi, just as they kill pathogens. While some fungicides reduce or eliminate mycorrhizal fungi, other research indicates that certain types of fungicides do not adversely affect mycorrhizae.

In addition, mycorrhizal inoculants can generally be used four to six weeks before applying fungicides, allowing the mycorrhizae to become firmly established within the plant root. Mycorrhizal inoculum can also be added following the use of a fungicide. Follow the manufacturers' guidelines for the time in which the fungicide "clears" the soil media.

Improved Outplanting Performance

Nurseries often produce plants utilizing high levels of water and nutrients and without mycorrhizae. This regimen can discourage the plant from producing the extensive root system it will need for successful transplantation. The results are plants poorly adapted to the eventual outplanted condition that must be weaned from intensive care systems and begin to fend for themselves.

Application of mycorrhizal inoculum before or during transplantation can encourage plant establishment and set the plant on track to fend for itself. In the outplanted environment, mycorrhizal inoculation at the nursery will often lead to great improvements in survival, growth rate, plant species diversity, soil structure and other beneficial ecosystem aspects.

There are numerous clues to whether or not a field site has native mycorrhizae present. A site most likely has native mycorrhizal fungi if it is occupied by healthy upland native vegetation that is several years of age or more, lacks serious weed growth and the native vegetation is of the same type or closely related to the vegetation to be planted.

Mycorrhizal populations are particularly low and many times nonexistent, in both urban and suburban soils that have been altered by clearing, compaction, erosion, topsoil removal and use of certain pesticides and chemical fertilizers. In today's manmade environments, plants can be greatly stressed and the relationship between fungus and root is critical. The site probably does not have native mycorrhizal fungi if it is unvegetated, has been recently graded, tilled or badly eroded.

How To Use Mycorrhizal Inoculum

Fortunately, recent advancements in mycorrhizal research and application have made nursery usage of mycorrhiza easy and inexpensive. Mycorrhizal inoculum can be applied in a powder, granular or liquid form. The goal is to get the mycorrhizal inoculant down into the soil profile, where there is access to the root system.

Mycorrhizal inoculant can be sprinkled onto roots during transplanting, worked into seed beds, blended into soilless and p\ropagation mixes or watered in via existing irrigation systems. High quality, concentrated mycorrhizal inoculum containing diverse species is the best choice. Because of the wide variety of plants and conditions characterizing most nursery environments, a diverse species mix of mycorrhizae assures the best response.

Many nurseries are already benefiting from the use of mycorrhizal inoculum and usage continues to increase dramatically. Many nurseries now address pest and nutrition problems through a careful combination of chemical and biological means. New mycorrhizal products and knowledge have resulted in increased production, transplant survival and lower longterm maintenance costs. Nurseries are discovering that biological agents can more than make up for the decrease in chemical applications.

Got Mycorrhizae?

Here are some instances in which mycorrhizal inoculation for improved field performance makes sense:

1) Your plants are mycorrhizal host species (see www.mycorrrhizae.com).

2} Your plants will go into soil with inadequate native inoculum.

3) Your plants will not receive regular landscape-type maintenance.

4] The soil has been excessively disturbed, compacted, tilled etc.

Here are some instances in which mycorrhizal inoculation is probably not necessary:

1) Your plants are non-host species.

2) The soil of the planting site contains abundant native inoculum.

3] Your plants will be maintained in perpetuity by regular irrigation, fertilization and weeding.

About the authors: Mike Amaranthus, Ph.D., is chief scientist and John Hunt is technical services manager for Mycorrhizal Applications Inc.

Copyright Meister Media Worldwide Jul 2005

Source: Greenhouse Grower

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