Mycorrhiza, or root fungi, are commonly found in undisturbed soils. There’s no substitute for this specially adapted group of organisms that have evolved alongside plants for 450 million years. Farmers can’t produce their own mycorrhizas, nor can they import soil containing them into their fields, because this could spread unwanted or potentially pathogenic organisms such as phytophthora and fusarium. Over-fertilisation, soil disturbances and indiscriminate use of pesticides, particularly fungicides, have compromised natural mycorrhiza populations.
However, this can be remedied by applying specific inoculants. Since root fungi don’t grow like other fungi, they also can’t be produced artificially in batch culture or a bioreactor. This is because they can’t use external supplies of carbon for their energy requirements, and rely solely on the host plant for carbon. This is the major difference between mycorrhizas and other fungi used in microbial products. Trichoderma, for example, a fungus used as a biocontrol agent, isn’t mycorrhizal. South Africa is still behind other countries in the use of root-fungal inoculants.
Companies in Europe, the UK and America already produce these products. California has even passed legislation stipulating all rehabilitation projects must use inoculated plants. While South Africa has imported several of these products, their performance generally hasn’t met expectations because the isolates they use aren’t adapted to our environment. Fortunately, the new National Biodiversity Act recognises our indigenous soil microorganisms.
In the nursery and field
The term mycorrhiza means “fungusroot”, perfectly describing the relationship between these fungi and their host plants, which include most plant species. A few plants, such as cabbages, canola, proteas and macadamias are non-mycorrhizal, but this group is comparatively small. Any crop produced in a nursery should be inoculated, as growth media generally don’t contain mycorrhizas and may be pasteurised. Dryland crops also benefit from inoculation because it helps them access more of South Africa’s limited water, but it’s just as essential under irrigated conditions, ensuring better water usage and efficiency. The fungi should be applied as soon as roots develop and can be mixed into the growing media. However, as most nursery production practices rely heavily on pesticides and fertilisers which hamper the establishment of the fungi, plants may instead be inoculated when transplanted into the field.
A plant’s best friend
Mycorrhiza associate with plants in several ways, but their function remains the same. Primarily, the fungus extends the rooting system of the plant and exploits the soil’s macro- and micronutrients more effectively. Mycorrhiza transport these nutrients and transfer them to the roots through specific mycorrhizal structures, enhancing nutrient uptake and improving plant growth, even in times of stress from disease or drought. In return, the fungus receives carbon in the form of energy-rich sugars from the plant, making the association mutually beneficial.
Two main fungi types
Mycorrhizal relationships are broadly classified into endomycorrhizas and ectomycorrhizas, encompassing seven different types. The type most relevant to agriculture are the arbuscular mycorrhizal (AM) fungi, which are endomycorrhizal, meaning that it’s difficult to distinguish them from roots by sight. Photos don’t usually show the most crucial part of the fungi-root relationship, namely the very fine fungal threads that grow out from the roots and extend into the soil. These allow the plant to exploit the soil environment beyond its rooting zone, and increase the surface area for nutrient uptake. Under undisturbed conditions there could be 100m of fungal hyphae in the soil for every 1mm of root. This network can even improve the soil directly surrounding the plant. The arbuscules are finely branched, treelike structures that form within the cortical cells of the roots.
The major difference between mycorrhiza and some pathogenic fungi is that they do this without damaging the root cells. AM fungi bond with the roots of most of our crop plants, grasses, indigenous trees and shrubs. They release a special protein, a kind of superglue, into the soil, which aids soil aggregation, aeration and moisture penetration. Ectomycorrhizal (ECM) fungi are central in forestry. ECM associations form between various groups of fungi and exotic forest tree species, such as pines and oaks. ECM fungi are more host-specific than AM fungi, although some species can bond with a wide range of exotic trees.
The black truffle, for example, is an ECM fungi. Forestry nurseries in South Africa, although aware of the importance of ECM fungi to the growth of their trees, rely on natural inoculation and don’t apply a mycorrhizal inoculum. However, there’s little indication of an indigenous ECM fungal population, and the majority of these fungi were imported along with the trees. Across these categories, some oddities do occur. Eucalyptus, for example, forms a relationship with both AM and ECM fungi.
A local formulation
Dr Joanna Dames was part of a collaboration of mycorrhizal experts from several South African universities who developed Mycoroot, an effective mycorrhizal inoculant for local conditions (see box: Products available in SA). “We’ve seen results in a variety of crops, such as olives, vines, maize, chicory, shallots, vegetables, flowers and indigenous trees,” she says. “AM fungi are suitable for use on a wide range of crop and horticultural plants from apples to zucchini.” Dr Dames explains this versatility is possible because the AM fungi used are non-hostspecific.
“The products have been designed to combine several isolates of glomus species, which are suitable for use with a wide selection of crops. However, if there are particular soil problems such as high salt concentrations, heavy metal accumulation and very acidic soils, the Mycoroot team can design and adapt their product to meet these requirements. This is a long process requiring at least 18 to 24 months and would only be conducted under contract. It’s mainly suitable for large rehabilitation-type projects.”
However, Mycoroot can’t be developed in a liquid formulation that could be applied through an irrigation system. This, according to Dr Dames, is because such a product would rely solely on mycorrhizal spores, which are fairly large for fungi, have a dormancy period and are filled with lipids storing the carbon the fungus needs for its initial growth. “This prevents the product from being homogenous when in a suspension, and ensuring adequate coverage would take an enormous number of spores,” she says. “Most of the product wouldn’t reach the roots of the plants, where it’s needed, even if you try to wash it into the soil.”
How to apply Mycoroot
The plant’s roots must come into contact with the inoculant, as the fungus will only grow if it detects the presence of a host plant. One application is enough for the life of the perennial crop or trees. Several annual applications of AM fungi to fields with annual crops will ensure the fungi’s long-term survival. The population’s sustainability will depend on the initial mycorrhizal status of the soil. It also depends on reducing inorganic phosphorus applications (more complex or slowly released fertilisers are recommended) and avoiding fungicides detrimental to the mycorrhiza. – Roelof Bezuidenhout For information on mycorrhiza, compatibility with fungicides, mycorrhizal analysis of your field and other soil microbial analysis services, contact Dr Joanna Dames on 083 636 0706, Beatrix Bouwman on 083 276 7712 or Prof Colin Straker on 083 305 6176.
Products available in SA
Locally produced Mycoroot is a granular product containing indigenous isolates of AM fungi formulated in an inert carrier. The inoculants have been developed through the collaboration of several mycorrhizologists at local universities. “Mycoroot products are produced in a soil-free culture and are pathogen free. The granular formulation contains fungal spores, hyphal fragments and colonised root pieces,” says Mycoroot director Dr Joanna Dames.
“It can be applied during soil preparation, at planting, at transplant, or by mixing it into the growing media in nursery production.” The cost of the product ranges from R1 450 to R2 300 per hectare for application directly in the field. However, application depends on crop plant, planting method and plant density. For applications in the nursery the cost per plant ranges from 22c to R2,50 on transplanting, depending on plant type and size.
