The Underground Market: Mycorrhizae for Arborists
Brandon M. Gallagher Watson, Communication Director, Rainbow Treecare Scientific Advancements
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 phosphorus (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
Rainbow is proud to carry the DIEHARD line of products from Horticulture Alliance (HA). We reviewed every product available on the market and partnered with HA as we found their products offerings were the easiest to use and backed by research. 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 an ISA Certified Arborist and the Creative Director of Rainbow Tree Company. Rainbow consists of a full-service tree care company, a lawn care and pest control division, and Rainbow Treecare Scientific Advancements, a research & development branch dedicated to developing tools and protocols for science-based tree care.