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Mycorrhizae may still be considered a new idea for arboriculture, but it is nothing new to the trees. Some of the earliest fossils to contain recognizable plant tissue -- going back more than 460 million years ago -- show evidence of a mycorrhizal association. With such a long history, widespread occurrence, and number of product choices, what should an arborist know about the science of mycorrhizae and how to decide if they are a fit for your business?

The Underground Market

By Brandon Gallagher Watson


 


Mycorrhizae may still be considered a new idea for arboriculture, but it is nothing new to the trees. Some of the earliest fossils to contain recognizable plant tissue — going back more than 460 million years ago — show evidence of a mycorrhizal association. It is also estimated that mycorrhizae is present in 92 percent of plant families that have been studied, representing 80 percent of the known plant species. There are also numerous mycorrhizae products available to tree health professionals these days. With such a long history, widespread occurrence, and number of product choices, what should an arborist know about the science of mycorrhizae and how to decide if they are a fit for your business?


 


A fungi-root partnership


The term “mycorrhizae” does not refer to a type or species of fungus, but rather to the interaction between the fungi and the plant’s roots. When it was first described in the late 19th Century by German scientists, the interaction was thought to be parasitic, with the fungi stealing carbohydrates from the plant and giving nothing in return. It was later discovered that this relationship is mutualistic, benefiting both parties. The plant gives the fungus energy in the form of carbohydrates; the fungus, in turn, provides the plant with increased uptake of water and nutrients, such as organic nitrogen (N), sulphur (S), and potassium (P).


In a 1960s study involving onions, scientists found that P-starved onions that had been inoculated with mycorrhizae were able to take in 160 times more P in a 90-hour feeding period than the uninfected plants. The small fibers that make up the mycelium of the fungus, known as hyphae, have a much greater surface area than a plant’s roots. This helps greatly increase the volume of soil that can be mined by the plant and the fungus for minerals. In addition to helping acquire nutrients, mycorrhizae has been shown to help regulate certain soil-borne diseases, play a protective role for plants in soil that are high in metal concentrations, and assist plants in resisting drought. Thus, it is becoming clear that mycorrhizal fungi are doing more in the soil that simply helping trees grow bigger and faster.


 


Types of mycorrhizae


Mycorrhizal associations are generally categorized into two groups, based on whether the fungi actually penetrates the cells of the plant (known as endomycorrhizae, as endo- means “inside”) or the fungus simply colonizes the root zone without actually entering the plant (ectomychorrhizae, as ecto- means “outside”). Knowing this difference is important, because each species of tree can only form one of these types of associations


The first group, endomycorrhizae, represents the vast majority (about 90 percent) of mycorrhizal associations. Some plant families, such as the rhododendron family (Ericaceae), and the orchid family (Orchidaceae) have very specific endomycorrhizal associations and cannot establish in the wild without their fungal counterpart. Other endomycorrhizae form structures inside the cell membranes of the roots, known as arbuscular mycorrhizae (AM). They get their name from the same Latin root of arboricultural, as arbuscular refers to the tree-like shapes these structures form. These little structures have a high surface-to-volume ratio, and this is where the interchange of water, nutrients and carbohydrates takes place. Although all AM fungi are found in the division Glomeromycota, there are many plants species — from roadside weeds to food crops to hardwood forests — that form this type of association.


The second major group is the ectomycorrhizae (EcM). These do not enter the root tissue to form the arbuscular structures; rather they form a sheath-like coating around the root known as a hartig net. The same principle applies here as in AM — the fungi swap water and nutrients for a sugary reward from the plant. Some of the most conspicuous forest mushrooms — such as Amanita, Boletus, Laccaria and others — are the fruiting bodies of EcM fungi. Although EcM only represents about 10 percent of known associations, it includes several important tree families, such the pine family (Pinaceae) and the oak family (Fagaceae). In most cases, a mature tree will have many different EcM species living on its roots at any given time. Some EcM colonize new, fibrous roots and others live exclusively on older roots. The network of mycorrhizal associates on a tree’s root system can be an entire ecosystem in itself. Although not common, there are some cases where a plant species can support both AM and EcM, such as willows, aspens and other members of the Salicaceae family.


 


Mycorrhizae as an arborist’s tool


So, that’s what they do and what they are — now the question is what to arborists do with them? I would wager that you could poll 50 high-level arborists and get 50 different opinions as to the benefits of, or the lack of evidence for, mycorrhizal products. No one seems to dispute the presence or importance of mycorrhizae in the ecosystem. It has been one of the more widely studied subjects of 20th Century plant science with more than 50,000 published scientific articles in a 50-year time span. That’s an average of 3 published scientific papers per day for over half a century. Everyone has seen images of a small tree next to a larger tree showing the result of a mycorrhizae treatment over an untreated tree. We know plant communities have trouble establishing in sterile soils where mycorrhizae organisms are absent. So, if the science shows this association of fungi and tree roots has a measurable and visible benefit to the trees, why the controversy on the use of commercial mycorrhizal products?


One issue is that the vast majority of commercial mycorrhizal products are not simply fungal spores in a box, they are formulated with many other plant-growth-enhancing materials. These include beneficial bacteria, humic acids, sea kelp, vermiculite, moisture-retaining gels, micro-organisms, yucca extracts, compost media, and other additives. This makes it difficult to say definitively if the mycorrhizae are doing anything or not.


Detractors say there is little scientific research on the use of commercial mycorrhizal products that provide evidence one way or the other. In an Ohio State Extension publication, Dr. Nathan Kleczewski stated, “At present, there is very limited, unbiased scientific evidence demonstrating that mycorrhizal inoculations of urban soils with commercial preparations make plant establishment more successful or that the inoculated plants grow better and remain healthier over time. In fact, the available evidence is very inconsistent.” Other industry scientists have expressed concerns about commercial mycorrhizal products displacing native mycorrhizal species, or on the other extreme, not being able to compete with native species and never colonizing to begin with. Everything from the viability of the spores coming out of the package to their ability to survive northern winters has been questioned. The general consensus of the tree science industry is that commercial mycorrhizal products may be beneficial to trees but further research is needed to definitively say so.


Proponents stand by their use, saying they have been using them with success on all sorts of tree situations — from new transplants to helping a mature tree recover from stress. Arborists who use these treatments regularly say the benefits they and their clients see in the tree are proof enough the products are effective. Who really cares if the benefit was from the fungi, the bacteria, or the sea kelp? The trees look better, and the client is happy. The products are fairly inexpensive (often less than a few dollars per inch) and have many options for application. Many tree care companies commonly tank mix mycorrhizal products with fertilizers and market the services as an enhanced fall feeding program. In fact, despite the lack of unanimous support from industry scientists, most reputable tree care companies are offering clients some sort of commercial mycorrhizal treatment.


Certainly, additional research is desired to clear up many of the gray areas that surround the use of commercial mycorrhizal products. Many see that the jury is still out as to the level of benefit these treatments provide, but no one is saying these treatments are damaging or harmful to trees. Thus, these services can be offered to clients with a clear conscience so long as the benefits are not oversold. Like all tree health treatments, there is no silver bullet or panacea that will cure all tree problems. Commercial mycorrhizal products cannot correct poor site selection, compacted soils, weak roots or other physical conflicts. They can, however, be a component of a tree health care plan for many situations and, as always, is it up to you, the arborist, to recommend what you believe to be the best course for helping your client’s trees.


 


Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).


Charts courtesy of Horticultural Alliance


 


For more Information:


Harley, J.L., Smith, S.E. 1983. Mycorrhizal Symbiosis. Academic Press: London and New York.


Marx, D.H. 1980. Ectomycorrhizal fungus inoculations: A tool for improving forestation practices. In: Tropical Mycorrhiza Research. Oxford University Press: Oxford, England.


Stubblefield, S. P., Taylor, T.N., Trappe, J.M. 1987. Fossil mycorrhizae: a case for symbiosis. Science. 237. 59-60.


Werner, A., Zadworny, M., Idizikowska, K. 2002. Interactions between Laccaria laccata and Trichoderma virens in co-culture and the rhizosphere of Pinus sylvestris grown in vitro. Mycorrhiza. 12. 139-145


Kleczewski , N., Lewandowski,D., and Bonello, P. 2008. Mycorrhizae in Urban Landscapes OSU Ext Fact Sheet. HYG-3305-08. 1-3.


 

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