   Mycorrhizae: The Friendly Fungus - October 21, 1998 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Imagine a fungus that wraps around the roots of practically every plant in the landscape and uses the plant's energy to sustain it's own cellular metabolic needs. This fungus is everywhere. In fact, they are probably lurking under the soil in your back yard right now. No, this is not some scary story designed to raise the hair on the back of your neck during the appropriate season, it's real...it's mycorrhizae. Mycorrhizae (pronounced mi-cor-ri-zay with long "i"s, short "o", and long "a") have a mutualistic relationship with the green plants that they colonize. In other words, both the fungus and the plant benefit from the association. In general, mycorrhizae assist the plant by expediting water and mineral nutrient acquisition. Tiny fungal strands called mycelia can grow through soil much faster and more easily than plant roots and can therefore exploit a much larger soil volume in search of these resources. In return, the plant provides carbohydrates (sugars) to the mycorrhizal fungus. Almost all plant species, herbaceous and woody, evergreen and deciduous, can form mycorrhizal associations. When ending with "ae", it is in plural form. Mycorrhiza is the singular form of the word. There are two major classifications of mycorrhizae: ectomycorrhizae and endomycorrhizae. As the names imply, ectomycorrhiza refers to a fungus growing outside the root cells. Endomycorrhiza refers to a fungus that penetrates root cells. If this seems complicated, then imagine a very tiny, fungus-laden "sock" that surrounds the tip of a plant root - this characterizes endomycorrhizae. Now imagine a fungus literally colonizing the inside of root cells, looking much like a plastic bag full of water and spaghetti. The plastic bag is the outer membrane of the cell, the water is the cell cytoplasm, and the spaghetti is the fungus - this characterizes endomycorrhiza. Another name for endomycorrhizae is vesicular-arbuscular mycorrhizae which is usually shortened to the acronym VAM. I hope these crude analogies assist in visualizing the differences between the two and don't send you in search of fresh air and more appetizing reading material. Both endo and ectomycorrhizae are difficult and costly to grow in cultures outside of plant roots, but it can be done. Soil inoculants are available for both types. Most crop plants form associations with endomycorrhizae. And the inoculum can be applied to the soil prior to planting. There are certainly biological benefits to inoculating, but the jury is still out on the economic returns of doing so. One of the biological benefits is protection from root rot fungi. In theory, the mycorrhizae fill a niche that could otherwise be occupied by a disease causing pathogen. On the other hand, if crops are doing well and require low inputs of fertilizers, you may well already have a natural population of mycorrhizae. This is an excellent reason NOT to fumigate soils just because we can. There are also many beneficial soil organisms: nitrogen fixing bacteria, predatory nematodes, benign decomposing fungi, etc. So why do mycorrhizae exist? To make life easier for plants...probably not. Because they want to improve agriculture...I doubt it. They probably responded to environmental changes over time that allowed them to make a decent living by coexisting with plants. Hey, that sounds like something familiar. Soil microbiologists spend lifetimes studying these fascinating interrelationships and they have learned that some mycorrhizae border on being parasitic, but provide just enough benefits to the host plant to keep it alive. Other research has shown that as a plant community matures and changes it's species composition, the mycorrhizal species present change also. So, the next time you want to impress a friend, tell them about the benefits of mycorrhizae. Or, better yet, learn more about mycorrhizae and soil biology. The University of Arizona Cooperative Extension has publications on gardening and horticulture. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |