Introduction

Mycorrhizas are fungi which live in harmony amongst plant roots. Both plant and fungus obtain benefits from this association.

There are seven distinct types of mycorrhizas but only four of these are of relevance in commercial and amateur horticulture. These four are orchid mycorrhizas, ericoid mycorrhizas, ectotrophic mycorrhizas and endotrophic mycorrhizas.

Orchid Mycorrhizas
Orchid growers are aware that for successful growing of orchids, mycorrhizal fungi are required. Orchids go through a very long seedling stage and during this time, the plants are unable to photosynthesise. They are totally dependent on the mycorrhizal fungus amongst their roots which supply all the plants' carbohydrate requirements at this time. For most orchids, the fungus supplies carbon and other nutrients throughout their entire life.

Ericoid Mycorrhizas
This type of mycorrhizal fungus only lives amongst heathers. This type of fungus forms both a sheath around the tips of the roots and also penetrates the roots. An exchange of nutrients takes place with the fungus receiving carbohydrates from the plant and the plant receiving mineral nutrients from the fungus. Heathers tend to grow in lean, peaty soils and without the presence of the mycorrhizas, the root systems of the heathers would not be able to cope with taking up nutrients on their own. Another benefit of these ericoid mycorrhizas is that they are very efficient at taking in metal ions which otherwise could be toxic to the plants. Examples are aluminium and iron which are very soluble at the low pH of peaty soils.

Ectotrophic Mycorrhizas
These type of mycorrhizas are very widespread and many species of tree are associated with ectomycorrhizas. Like the ericoid mycorrhizas, they form a sheath outside the root tip of the tree without penetrating it. There is again an exchange of nutrients between plant and fungus. The tree receives mainly nitrogen and phosphorus from the fungus and the fungus receives carbohydrate exudates from the tree. These ectomycorrhizas can live quite successfully without an association with a living tree. They can live on decaying organic matter and most of the toadstools seen on a forest floor are the fruiting bodies of ectomycorrhiza. However, it is more beneficial for the fungus to associate itself with tree roots as it involves less energy to take carbohydrates from tree roots rather than to secrete enzymes in order to break down the litter on the woodland floor.

Endotrophic (Arbuscular) Mycorrhizas
These types of mycorrhizal fungi are associated mainly with soft tissue plants but they also associate with some types of woody subjects. These fungi do not form a sheath around plant roots. Instead the hyphae (strands) of the fungus penetrate the root of the host plant and grow within the cell walls. They form structures within the root that are called arbuscules. These are the sites of the nutrient exchange. As with the other types, mineral nutrients are passed to the plant from the fungus and carbohydrates are passed to the fungus. Unlike ectomycorrhizas, endomycorrhizas cannot survive without the host plant. They cannot obtain their carbon by decomposition. They do not produce toadstools. Their spores are invisible to the naked eye and are formed in the soil. It is estimated that about 70% of the herbaceous plants of the world are associated with endotrophic mycorrhizas.

These endomycorrhizas give more benefits to the host plants than the other types of mycorrhizas. They give, as others do, an improved uptake of phosphorus and both ammonium and nitrate to the plant. They also give an improved uptake of trace minerals to the plant, eg zinc and copper. However, they also give other benefits to the plant. These include an increase in the resistance to drought, to diseases and to insect pests.

In turf, where endomycorrhizas are being applied externally, the uptake of phosphorus and nitrogen is not very relevant as there is usually enough of both nutrients available to the grass. The main benefits of using these arbuscular mycorrhizas are associated with their ability to improve drought resistance, disease resistance and to banish pests. In golf greens, annual meadowgrass (poa annua) is not generally wanted: the finer bent and fescue grasses are those that greenkeepers wish to develop. Poa annua is one of approximately 30% of plant species that do not form mycorrhizas. If endomycorrhizas are introduced into a green containing poa annua, it is reduced whereas the fine bent and fescue grasses that are mycorrhizal, increase.

Discussion

As mentioned above there are some groups of plants that do not associate with mycorrhizas and others which are so weakly mycorrhizal that they might as well not be! Most members of the brassica family - broccoli, brussels sprouts, cabbage and cauliflower - fall into this category. We also understand that azaleas and rhododendrons similarly do not associate with mycorrhizas.

 
Our Approach to Mycorrhizal Fungi

We have a mycorrhizal Viresco™ product that contains a mixture of 11 different mycorrhizal fungi spores along with other micro-organisms that are in our Viresco™ Foliar. Four of these mycorrhizas species are ectotrophic and the other seven are endotrophic.

We suggest that this product, called Viresco™ Mycorrhiza, is used routinely with all manner of plants except those such as heathers and orchids for which no benefit would arise.

Many of our customers are growers of exhibition leeks. Geoffrey Swaddle in his "Leek Book - Growing and Showing of Exhibition Leeks", published in 1988, has a chapter entitled "Other Ideas and What Next". In this chapter he refers to mycorrhizal fungi. He writes:

"There are fungi which can grow on the roots of leeks. These fungi live in mutual benefit (symbiosis) with the leek. The fungi are able to spread very well in the soil exploring a greater volume of soil in a better manner than the leek roots themselves. They enable the leek to obtain more phosphate (P₂O₅) and micro-nutrients than the leek roots could without the fungi. In return the fungi receive sugars from the leek. These fungi are called Mycorrhiza (fungus-root).

Leeks (and onions) as long as they remain disease-free, grow better on the same site than if they are rotated with other vegetables. One reason for the improvement is the increase in fertility, particularly due to the build-up of organic matter. The writer has, for several years, believed that the presence of mycorrhiza has been another reason for leeks (and onions) growing better in one position than when they are moved to a new site. Recent research seems to confirm this belief.

It may be possible in the future to obtain cultures of mycorrhiza for inoculating leeks to increase their size"

Geoffrey Swaddle's ideas have come to life as these cultures are now available in our Viresco™ Mycorrhiza.

Leeks and onions are strongly mycorrhizal. At one UK university's department of biology, when experiments are being done with mycorrhizas, leek plants are more often than not used as the plant subject.

Viresco (UK) Ltd
Tel: 44 (0)1845 525585
Fax: 44 (0)1845 523133
Email:
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