by George Buehler

I have been using mycorrhiza (pronounced my-ko-risea) for a number of years after it was recommended at a workshop I attended at Brussel's nursery. I originally started using it only on conifers but decided to use it on all transplants a few years ago. I recently was searching for articles for this newsletter and ran across a 1992 article entitled "The Case for Mycorrhizae in Bonsai Cultivation" written by Dr. Terry Davis. After reading the 13 year old article, I decided to do a little research to see if any of the information had been updated for bonsai and to try to find out why it worked. It appears that there was a fl urry of articles on mycorrhiza between 1990 and 1996. Not a lot has been written since that period. Since I was not a botanist, a pathologist, nor a microbiologist, it was hard for me to understand exactly what it was or why it worked. Hopefully, the short essay below will help you understand mycorrhiza or at least see the benefits of using it on your bonsai.

The word mycorrhiza (singular; mycorrhizae – plural) is derived from the Greek meaning fungus-roots or simply roots. The term was first applied to the fungus-tree associations described in 1885 by the German forest pathologist A. B. Frank¹. The word itself refers to any of several types of associations between plant roots and soil fungi. There are several types of soil fungi that form mycorrhizal associations with almost all types of plant material. It was first thought that each variety of tree – that has a mycorrhizal relationship – has its own characteristic type of fungi². It has been shown that each group of plants seems to prefer a single kind of mycorrhiza, although more recent investigations show that most plants form these associations with more than one type of fungi. Mycorrhiza offers several benefits to the host plant, including faster growth, improved nutrition, greater drought resistance and protection from pathogens. In general garden agriculture, they also seem to help modify the structure of the soil itself.

Before we go on, when most people think of a fungus, there is often a negative association. Fungal diseases are a major plant pest. But fungi of the category mycorrhiza aren't the harmful pest that can destroy your bonsai.

Plant responses to colonization by mycorrhizal fungi can range from dramatic growth production to growth depression. The factors affecting the response include the mycorrhizal dependency of the host plant, the nutrient status of the soil, and the inoculum potential of the mycorrhizal fungi.

If the soil is fertile and moist, yet well drained, the plant will grow at its maximum rate, and there will be little need for mycorrhiza. However, trees planted in marginal conditions would probably not survive without mycorrhizae. Since the ‘soil' we put our bonsai in is definitely less than ideal and the conditions we put them in (i.e. daily drenching and drying, exposure to temperature extremes, no internal fertility, etc.), we have exactly the conditions in which mycorrhiza thrive.

TYPES OF MYCORRHIZA

There are two major types of mycorrhizal fungi based on the anatomy of their association with the host roots: Ectomycorrhiza (EM) and Endomycorrhiza (VAM).

Ectomycorrhiza fungi produced large quantities of hyphae (one of the threads that make up the cells of fungi) on the root and in soil. These hyphae produce a netlike structure (called a Hartig net) between root cortical cells. Many EM also have a sheath of fungal tissue that may completely cover the absorbing fine feeder roots. The sheath increases the surface area of absorbing roots and often affects fineroot morphology. Often these sheaths will combine to form rhizomorphs that may be visible to the naked eye. There are over 4,000 fungal species that are known to form ectomycorrhizae. In garden agriculture, many of these fungi produce mushrooms and puffballs under the plants (we have probably seen these on forest fl oors but didn't realize what they were). As previously stated, most mycorrhizal fungi are plant specific. However, one ectomycorrhiza type fungi uses more than 46 tree species belonging to at least eight genera as a host³.

Endomycorrhiza is the general term where the fungus grows within cortical cells (the cell layer external to the vascular tissue and internal to the corky tissue of green plants – in other words under the "skin" of the roots). In this association, neither the fungal cell wall nor the host cell membrane is breached. As the fungus grows, the host cell membrane envelops the fungus, creating a new compartment called an arbuscle. This apoplastic space prevents direct contact between the plant and fungus cytoplasm and allows for efficient transfer of nutrients between the fungus and plant.

The arbuscles take carbohydrates from the cells and grow as they do. Once they have completely filled the cell, they break down, releasing their nutrients to the host and the fungus proceeds to colonize another cell.

BENEFITS OF MYCORRHIZAE

To understand the benefits of mycorrhizal fungus, we need to investigate how plants take up nutrients. Organisms differ greatly in their abilities to take up different forms of nutrients, and it is an axiom of biology that the lower forms, such as fungi, generally have more generalized capabilities than the more specialized higher forms⁴. Plant root hairs can only adsorb nutrients as nitrate ions. When we use chemical fertilizers, the NPK is usually in a complex form (i.e. ammonium nitrate). If you use organic fertilizers, these are even more complex forms of NPK. These complex forms must be broken into a form that is usable by the plant (is this why organic fertilizers work over a longer period of time – it is more difficult to break these extremely complex forms down to nitrate ions). This is where the fungus (or certain types of bacteria) comes in. In the case of bacteria, these organisms are very fussy in that they only operate in a certain temperature and pH range; and once out of these ranges, they shut down. Fungi are generally more tolerant of both temperature and pH and will survive in the extremes our bonsai experience in their small pots. In addition, the trees' uptake of the nutrients also has its own temperature and pH optima. If we depended only on bacteria to metabolize the NPK complex so that it is converted into a usable form by the plant, we would be waiting quite awhile since the bacteria itself uses the nitrogen complex in its own metabolism and little would be left for the plant. This is where fungi does it tasks so well. It converts the nitrogen (or phosphorous and potassium) complex into a form usable by the plant but doesn't use that converted complex itself. Being a ‘living' organism, it must use something for its nutrition and this is where the symbiosis (association) with the plant comes into play.

The tree produces certain types of carbohydrates as part of the process of photosynthesis. To overly simplify the process, but to make it easy to understand: the nutrients that the fungi needs is produced by the tree and vice versa. The exchange of these nutrients occurs in the Hartig net (for ectomycorrhizae) or in the arbuscles (for endomycorrhizae). Because the tree is constantly depleting the nutrients it needs, and producing more of the nutrients than can be used by the fungi, the fungi can rapidly reproduce.

A nutrient depletion zone develops when nutrients are removed from the soil more rapidly than they can be replaced by diffusion. For an ion such as phosphate, a sharp and narrow depletion zone develops close to the root. Hyphae readily bridge this depletion zone and grow into soil with an adequate supply of nutrients. Uptake of micronutrients such as zinc and copper are also improved by mycorrhizae.

Mycorrhizal fungi's most important job is to extract toughto- absorb nutrients like phosphorus from the soil. The more fungal strands in the soil, the more moisture and phosphorus the plant roots are able to absorb. Phosphorus is as essential to a plant as an engine is to a car. It keeps the plant running; without it, growth grinds to a halt. This is why phosphorusdefi cient plants are inevitably stunted. Mycorrhizal fungi have the ability to collect phosphorus from the soil, making it available to the plant, leading to lush, vigorous growth. Plants treated with both phosphorous and mycorrhizal fungi have higher growth rates than plants treated with only phosphorous and substantially better growth rates than those that were not treated.

As the fungi reproduce they greatly increase the roots' efficiency to adsorb nutrients because of the vastly increased absorptive surface area (they have relatively narrow diameter when compared to roots). A comparison of the surface area of non-mycorrhizal roots and fungi-treated roots show a significantly higher surface area for the fungi treated, due to the millions of hyphae present in the treated roots. As the hyphae extend and multiply, they are able to draw on more distant or inaccessible supplies of nutrients than if the roots did not have the hyphae present. Furthermore, narrow hyphae can grow into small soil pores that are inaccessible to roots or even root hairs (in the pumice, volcanic rock, etc. we use in our bonsai soil).

All this increased efficiency of the plant to obtain nutrients (and show quicker growth) is probably the best know benefit of mycorrhizal inoculation. However, it has been shown that there are several other benefits of the mycorrhizal fungus. Among these are altered root physiology, protection against root pathogens, alleviation of stress and disease, and in garden horticulture the detoxification of soils and modification of soil structure.

It has been shown that ectomycorrhizae produce growth hormones which are responsible for the changes in both metabolism and growth of roots. These fungi enhance the production of roots, increase the number of root tips, and aid in the rooting of cuttings.

Mycorrhizal fungi have long been known to help defend roots against soil-borne disease. There are probably several mechanisms involved, including a strengthening of the plant through improved phosphorus nutrition. Both kinds of mycorrhiza appear to promote beneficial bacteria which may be directly responsible for protection against root pathogens.

Stresses influence the plants' susceptibility to different types of diseases. Because the fungi increase the uptake of nutrients (especially micronutrients) and physically block the movement of some pathogens, the plant becomes more resistant to a number of stresses.

To summarize the benefits to bonsai of a mycorrhizal association:

• Increased nutrition – When we repot a tree and root prune, the tree has limited access to the available nutrients. When the mycorrhizal fungi are added to the pot, they grow more rapidly than the remaining roots and cause the plant to utilize more of the available nutrients.
• Alleviation of Stress and Disease – Bonsai are always under stress due to the non-organic inert soil we use, the temperature and moisture swings we expose them to, the constant trimming/plucking we give them, and the use of synthetic fertilizers. Bonsai are therefore more susceptible to damage by disease and stress related diseases than equivalent field grown trees. With the increased uptake of nutrients, the tree is at least able to cope with some of these stresses.
• Protection against root pathogens – Since our bonsai have limited roots, especially after transplanting, any infections by root pathogens could do irreparable harm. Based on a number of studies, mycorrhizae help defend the plant from some of these pathogens by increasing the plant vitality with increased nutritional uptake.
• Increased root growth – The mycorrhiza produce growth hormones that infl uence the production of roots. In addition, since the mycorrhiza aid in the uptake of essential nutrients, the plants' ability to produce roots is increased.

Mycorrhiza Inoculation

The question is how the fungi get into our bonsai soil. There are several ways to inoculate your soil. In fact, if you don't remove all the old soil, there are probably enough spores left that no additional inoculation is necessary. I find this not worth the risk and add additional material.

1. When you remove your tree from its pot at transplant time, look at the old soil for a whitish material around the roots. If it is there (the mycorrhiza fungi), collect some of that soil and mix it with the new soil you use to reset the tree. Try to get this old soil close to the remaining roots of your tree after you root prune.
2. As you root prune, collect the pruned away root tips. Chop up these tips; and after the tree is wired into place, sprinkle the chopped up tips around the remaining roots, working them into the root mass.
3. As you remove the old soil from around the roots, put the old soil into a bucket. Cover the old soil with fresh water and mix. Allow the old soil to settle and pour off the water. Save this water and use it to water the tree when you have finished adding all the new soil needed.
4. After you have removed the soil but before you do any root pruning, gently wash the roots, collecting the water. Use this water on the tree when you have finished repotting.
5. Many bonsai suppliers sell a dry mix of both ecto and endomycorrhiza at a reasonable cost. When the tree is transplanted, you add some of this dry mix to the soil in the pot, mix it around with the new soil, and then continue in the normal manner with the transplanting. You also might want to estimate the amount of new soil you will need to transplant a tree, put that soil in a bucket or bag and add the dry mycorrhizae, mixing well. You could possibly make up enough of this mix to pot up several trees. However, ensuring a uniform mixture will be difficult if you use too much new soil. One word of caution when using this dry purchased mycorrhiza fungus: it does have a shelf life. Check the bag to see what the expiration date is. If you will be transplanting only one or two trees this year, you want the longest expiration date (or find another bonsai grower and share it with him). If you order over the internet, you may want to arrange with a buddy to share the bag. One bag will inoculate four to ten trees depending on the size of the pot.

I mainly use the last method for several reasons. When we are transplanting, one of the goals is to replenish the old soil. Adding back some of the old soil, as in #1, is defeating the purpose. Of course, you should only add a small amount of the old soil so this may not be important.

If you use method 2 through 4 you will have a better chance of reintroducing the useful fungi, but you also run the risk of reintroducing any harmful pathogens that were in the old pot. In addition, you can use the water collected in 3 or 4 to water some other trees.

If the tree has shown any signs of problems during the last growing season, under no circumstance use methods 1 to 4. You will be reinfecting the tree again. It is better to use only method 5.

I always add the new dry fungi when transplanting. If I am transplanting a pine or juniper, I sometimes gently wash the roots and use this on my transplants. I try to be very cautious, since I definitely don't want to spread problems to other trees. Many of the articles from Japanese magazines or books on transplanting always say that they leave a small amount of soil during transplanting. This is probably done for two reasons. The first is to leave some of the beneficial mycorrhiza and the second is to not completely disturb the roots.

Summary

Everything that I can find says that the bonsai enthusiast should use mycorrhiza since it benefits the growth of the tree. Almost every picture that showed the use of mycorrhiza treated plants versus non treated plants always show the increased growth of the treated plants. Using new mycorrhiza or inoculating plants from previously transplanted trees is a simple step in the repotting process that can help the survival of your little trees.

Reference:

¹Sylvia, David, et al. Principles and Applications of Soil Microbiology. New York: Prentice Hill. 1998
²Koreshoff, Deborah. Bonsai Its Art, Science, History and Philosophy. Brisbane: Boolarong Publications. 1997
³Mycorrhiza web site
⁴Davis, Terry. "The Case for Mycorrhizae in Bonsai Cultivation." Bonsai Clubs International, March/April 1992, pp. 14-15.