What are mycorrhizal fungi?

Mycorrhizas are beneficial fungi growing in association with plant roots, and exist by taking sugars from plants ‘in exchange’ for moisture and nutrients gathered from the soil by the fungal strands. The mycorrhizas greatly increase the absorptive area of a plant, acting as extensions to the root system.

Phosphorus is often in very short supply in natural soils. When phosphorus is present in insoluble forms it would require a vast root system for a plant to meet its phosphorus requirements unaided. It is therefore thought that mycorrhizas are crucial in gathering this element in uncultivated soils. Phosphorus-rich fertilisers are widely used in cultivated ground and not only reduce the need for this activity but are thought to actually suppress the mycorrhizas. For this reason it is best not to use phosphorous rich fertilisers in conjunction with mycorrhizal fungi.

Neither fungi nor plants could survive in many uncultivated situations without this mutually beneficial arrangement.

Mycorrhizas also seem to confer protection against root diseases.

Appearance

Toadstools, especially ones that follow root runs and appear below trees, can be a sign of ectomycorrhiza (mycorrhiza living on the outside of a plant), as are the fungal coating and odd branching structure of affected tree and shrub roots.

Endomycorrhizas (mycorrhiza living within a plant) are difficult to detect without laboratory facilities.

Cause

Mycorrhizas are not a cause for concern. The fine fungal strands that make up mycorrhizas associate with roots in two ways:

Ectomycorrhiza (mycorrhiza living on the outside of a plant):

• Often called sheathing fungi
• Found mainly in association with tree roots
• Form a thick mass around the fine feeder roots
• Although roots thicken and branch in ways that appear unnatural, they are not harmed and, in fact, are helped by the fungal strands
• The pattern of root branching differs between (and is characteristic of) each fungus-plant association; pines, for example, have Y-shaped branches and beech branches are at right angles
• The fungal sheath sends out branches between the surface cells of the roots in order to exchange nutrients from the soil for sugars from the plants
• The network of branches that penetrates the roots is called a Hartig Net
• Sheathing fungi often send up fruiting bodies. These include the many toadstools commonly seen around trees that often alarm gardeners who mistake them for honey fungus

Endomycorrhiza (mycorrhiza living within a plant):

• Often called vesicular-arbuscular (VA) mycorrhiza
• Found mainly in association with herbaceous plants
• Form strands (hyphae) within roots growing between the living cells
• These hyphae extend outside the cell to form an extensive network that absorbs water and nutrients
• Strands form small storage sacs (vesicles) every so often
• Fine specialised structures called arbuscules branch and enter living cells to exchange nutrients

There are other more specialised mycorrhizas, including ones where the fungus supplies organic compounds to plants, orchids for example, that break down soil organic matter.

Mycorrhizal fungi in the garden

Mycorrhizal fungi are often seen in gardens but may be less effective on frequently cultivated soils that have been heavily fertilised and manured. Use of fungicides can also inhibit mycorrhizal fungi.   It is good gardening practice to use the lowest feasible amounts of manure, fertiliser, fungicides, and cultivation. This will not only save gardeners time and money, but also helps towards protecting the environment.