Truffles, mushrooms and mycorrhizas
© Truffles and Mushrooms (Consulting) Ltd
The vast majority of terrestrial plants harbour beneficial fungi on and in their roots. These combined structures formed by fungus and root are called mycorrhizas (literally fungus-root).
Mycorrhizal fungi have been around for at least half a billion years and so it is hardly surprising that most are now so specialised that they cannot survive unless in contact with their host plants. Many plants have also become equally dependent on mycorrhizal fungi. By extending the absorptive area of the root system, mycorrhizal fungi have their principal beneficial effect - increasing plant uptake of nutrients, in particular phosphorus. Without mycorrhizas plants can become stunted and yellow primarily because of a lack of phosphorus. Mycorrhizas are, therefore, of major significance in forestry and agriculture as well as in natural ecosystems.
Mycorrhizas can be divided into two broad groups: endomycorrhizas where the fungus is mainly inside the outer cells of roots and ectomycorrhizas where the fungus is predominantly on the outside of the root. With the exception of a few species such as the brassicas most agricultural and horticultural species form arbuscular mycorrhizas, a type of endomycorrhiza. Some forestry trees also form this type of mycorrhiza but the vast majority of the dominant forest trees of the Northern Hemisphere form ectomycorrhizas.
These swollen, yellowish root tips from one of Truffles and Mushrooms (Consulting) Limited’s client’s nurseries, are Périgord black truffle mycorrhizas.
Mycorrhizas in Forestry
For more information on mycorrhizas and their importance in forestry and background to the work that was done by Truffles and Mushrooms (Consulting) Ltd, Chris Perley and Associates, and Symbiotic Systems NZ Ltd through MAF’s Sustainable Farming Fund click here:
Each species of plant is not restricted to just one mycorrhizal fungus and it is not unusual to find half a dozen different ectomycorrhizal fungi competing for space on the roots of a suitable host. For example, Douglas fir can form mycorrhizas with hundreds of different mycorrhizal fungi.
Worldwide there is a move towards raising plantation forest plants in containers (www.bccab.com, www.transplantsystems.co.nz) and the soilless potting mixes that are used in these containers are almost always devoid of mycorrhizal fungi. While mycorrhizal fungal spores might blow into a greenhouse through vents and doors this cannot be relied upon to ensure adequate mycorrhizal infection. Some nurserymen inoculate with mycorrhizal fungi but others simply compensate for a lack of mycorrhizal fungi by applying large amounts of nutrients, in particular phosphorus, to ensure that their plants look healthy. Nurserymen may also assume that any mycorrhizal fungus on a root system is satisfactory when instead the fungi could well be ineffective "weed" species such as Thelephora-a species that will probably have little or no benefit to the plant after outplanting. For example, Douglas fir without ectomycorrhizas and outplanted into upland pastures in southern parts of New Zealand can become stunted and yellow because the soils in these areas can be completely devoid of ectomycorrhizal fungi as well as being deficient in plant available phosphorus.
Examples of poor growth due to a lack of arbuscular mycorrhizal fungi are less common. When they are identified it is usually because plants have been grown in soil that is naturally deficient in arbuscular mycorrhizal fungi such as eroded soils, where subsoils have been exposed, mine spoils, soils that have been heated to a point where some or all of the mycorrhizal fungi are killed, fumigated soil, soil less media, and/or media to which high concentrations of nutrients have been applied before outplanting.