WHAT IS MYCORRHIZAE?
A Beneficial Garden Relationship
...and a term used for benficial fungi.

Under natural conditions, many plants, especially California native plants,  live in close association with soil microorganisms. The roots of almost all higher plants are known to form "mutualistic symbioses" with fungi. This is termed "mycorrhizae" which  means fungus-root. This term is derived from the Greek, "mykes"  which means mushroom or fungus and "rhiza" which mean root. The relationship is mutually beneficial. It forms colonies on the roots of a host plant. The fungus spreads threadlike feeding structures called hyphae into the soil. This type of colonization allows the host plant to develop a much larger root system, sometimes 100 times in length with up to 2000 times more surface area. The increased the root surface area provides more room for water and nutrients to be taken in. The fungus serves as a bridge between the host plant and available nutrients. The hyphae help to break down organic material thus helping to release nutrients to the plant. Some fungi release chemicals to inhibit other organisms that might otherwise harm the plant. They develop a soil community that supports friendly bacteria, nematodes, springtails, earthworms, etc., but inhibits herbivores and pathogens. The fungus is considered a plant, but it has no chlorophyll so can not produce its own food.  In return for their hard work, the fungi obtain a steady source of food (sugar) from the host plant.

This relationship developed because of need. Under conditions which are optimum, when the plant gets appropriate water and regular fertilizer, the plant had no need for the fungus and it will often disappear. For this reason, where there is adequate nutrition and moisture, non-mycorrhizal plants occur. These include most cultivated garden annuals and perennials. But in nature, conditions are not always optimum. Over past 400 million years or more, many plants have developed such that they depend upon this relationship. California natives are a prime example of plants that need this fungal relationship to survive. A coast live oak, for example, will be more pest and disease ridden and have a significantly shortened life span, if there IS plentiful water and fertilizer but no mycorrhizae.

 There are several different types of fungi that take part in mycorrhizae. Two major groups are recognized.

Ectomycorrhizae (EM)

• Fungi that forms a covering around plant roots which protects the roots and acts as a reservoir for the major nutrients (N-P-K) as well as calcium.
• Found on most stress-tolerant plants, such as willow, eucalyptus,  birch, and oaks,and are common to the forests of the Pacific northwest.
• There are thousands of this type.
• They tend to be "host-specific".
• Acid soils with good drainage favor ectomycorrhizae.
• These fungi usually produce mushrooms above ground which produce spores.

Endomycorrhizae 

• There are three main types.
  One is associated mainly with orchids. A second works in unison with the family Ericaceae which includes heather, manzanitas and rhododendron.
• The most important type in this discussions are the " vesicular-arbuscular mycorrhizae" (VAM) which actually penetrate inside the roots of the plants and make a significant contribution to the effectiveness of the root.
• They cannot be seen without a microscope.
• They are clumped in the soil and not spread by the wind.
• There are only 100-200 know species of VAM
• They are often found where there is low organic matter and low phosphorus in the soil, e.g. deserts, grasslands, tropical forests
• The same species of VA may continue, interconnected, for miles. (That is why digging around some plants is so disruptive. It breaks and kills the mycorrhizae life support system.)
• They are important to grasses, sages, buckwheat and ceanothus.
• VAM is not particularly host-specific and will work on a wide range of plants.

Soil pH and type dictates mycorrhizal type and associated plant type. Plant material that is native to an area with a soil pH more than 2 units different than yours should not be used. pH is logarithmic. A pH of 5 is 100 times more acid than pH of 7. A soil fungus that grows in soil pH of 5 will not generally survive a soil pH of 7.5. Amending the soil with organic matter tend to increase the pH to "neutral" over time. Again, a soil fungus that grows in soil pH of 5 will not generally thrive a soil with a neutral pH of 6.5 or 7.

Soils that have been moved or compacted, heavily amended or planted, or soils with pronounced fertilizer and pH imbalances are not good homes for mycorrhizae. In the average home garden, urban settings and commercial development, mycorrhizal fungi are often not present in adequate quantities. This is especially true of commercial potting mixes and fumigated soils (in which transplant trees are grown). They often sterile and may contain no mycorrhizal fungi at all. Plants may be "inoculated" with commercially produced mycorrhizae. In natural soils, mycorrhizae are abundant and readily available to the plants that grow naturally there.

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