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Soil Fungi

What are fungi?

Fungi are primarily organisms that cannot synthesise their own food and are dependent on complex organic substances for carbon. Specialized fungi can be pathogenic on the tissues of plants, while others form mutually beneficial relationships with plants and assist in direct nutrient supply to the plants (e.g. mycorrhizal associations).

Many fungi play a very important role in the recycling of important chemical elements that would otherwise remain locked up in dead plants and animals. In the decomposition of plant debris, certain fungi are particularly important because of their ability to derive their carbon and energy requirements from the break down of dead and decaying plant cell walls, cellulose and lignin. They are much less dependent on water than other microorganisms, but interactions with other microbes, temperature and nutrient availability will have an effect on their activity. Fungal activity is greatest in decomposing leaves and wood, and tends to diminish in the later stages of decomposition when bacteria become more dominant.

What are mycorrhizal fungi?

Mycorrhizas are associations between fungi and plant roots that can be beneficial to both the plant and the fungi. The fungi link the plant with soil by acting as agents of nutrient exchange. The fungi receive carbohydrates as energy from the host plant root whilst nutrients such as phosphorus and zinc are passed back into the plant roots from the soil. Mycorrhizal associations may also reduce attack from root pathogens and increase the tolerance of the plant to adverse conditions such as heavy metals, drought, and salinity. In general, mycorrhizas play an important role in sustainable plant productivity and maintenance of soil structure.

Mycorrhizal associations occur on almost all terrestrial plants and are not as plant-specific as other plant-microbe associations that formed between some plants (e.g. legumes) and bacteria (e.g. rhizobia).

“Some plant species, such as crucifers (i.e. broccoli) are unusual in that mycorrhizal symbioses are absent. Species with fine root hairs and many root hairs are not as dependent on mycorrhizae as species with well-defined tap roots” (Coyne, 1999).

Mycorrhiza

Are there different types of mycorrhizal associations and what plants are involved with them?

There are four main kinds of mycorrhizal fungi: arbuscular, ectomycorrhizal, ericoid and orchid mycorrhiza. Most agricultural plants, vegetables and orchard plants form arbuscular mycorrhizal associations. Ectomycorrhizal associations are less common in disturbed ecosystems and are more common on perennial plants than annuals. Ornamental plants form associations from each of the four groups and orchid mycorrhizal associations are formed only by orchids.

What are Vesicular Arbuscular Micorrhizae?

The most common type of mycorrhizas are the arbuscular mycorrhizas. They are found in natural ecosystems as well as in agricultural areas, are common on both perennials and annuals, and form associations with most agricultural plants. Two exceptions among agricultural crops are canola (oilseed rape) and lupin.

Arbuscules are believed to be the major site where the carbon and nutrient exchange between plant and fungus occurs. Named because of their "tree-like" structure, arbuscules are created by repeated branching of hyphae once they enter a cell within a plant root.

Vesicles are structures formed inside a cell within the plant’s roots. They can be regular or irregular in shape and many times wider than the hyphae on which they form. Therefore, vesicles are usually very distinctive and in some species can resemble spores. These structures are known as a place for storing nutrients. Only three of the five genera of arbuscular mycorrhizas (Glomus, Acaulospora and Entrophosphora) form vesicles.

The fungi are grouped according to the size, shape and wall structure of their spores. The spores are approximately spherical or ovoid in shape and usually have thick walls which allow them to survive harsh environmental conditions. When soil conditions are favourable, the spores germinate and hyphae grow from the spore, entering roots and establishing mycorrhizal associations.

Spores are between 30 and 500 micrometres in size - this is between 30 thousandths of a millimetre and half a millimetre. Identifying and classifying the spores is therefore something that must be done with the aid of a microscope.

Vesicles inside a root

What are the benefits of mycorrhizal associations?

Mycorrhizal fungi are characterised by very thin hyphae, which are between 1 and 10 thousandths of a millimetre in width. These hyphae explore the soil for nutrients, transport them back to the host-plant, and help bind soil particles into aggregates. The hyphae form networks between neighbouring soil particles, between roots and soil particles, between roots on the same plant, and between roots of different plants (even different types of plants). They also form networks inside the roots they colonize. These networks of hyphae are also referred to as mycelium.

Mycorrhizas extend the volume of soil explored by the plant, a characteristic that is especially important for phosphorous which does not move in the soil solution as nitrogen does. There is some evidence that the fungi may help the plants tolerate drought. When phosphorus is scarce in soil, plants that have developed mycorrhizas on their root systems have greater access to and take up more phosphorus others. Trace elements, copper and zinc behave in a similar way to phosphorus in soil and plant roots must explore the soil to intercept them.

Although arbuscular mycorrhizas can enhance plant growth in phosphorous deficient soils, the extent to which this occurs in the field in agricultural and natural environments is difficult to measure.

Refer to ‘Soils are Alive’ newsletters Volume 1 Number 2 “Arbuscular mycorrhizal fungi” by Professor Lyn Abbott and Volume 3 Number 2 “Mycorrhizas and Olive Trees: Letting nature takes it course” by Tom Ganz, Professor Stan Kailis and Professor Lyn Abbott.

Where do plant pathogenic fungi come from?

Disease-causing microorganisms have always been inherent members of any living community. In natural ecosystems, characterised by uncontrolled and changeable conditions, their population growth is impeded by the scattered distribution of host plants and, in the case of fungal pathogens, by their dependence on rainfall at the time of spore germination. In managed systems, however, such as agriculture and horticulture, monocultures of crop plant species provide an unconstrained food supply for a pathogen. Irrigated systems also provide a constant supply of water which can enable spores to germinate and cause disease in accessible host plants.

What are the most important fungal soil pathogens in Australia?

Climatic patterns can affect the types of fungal pathogens that are dominant in a region. For example, low fertility soils favour necrotrophic pathogens over biotrophic pathogens. Necrotrophic pathogens are distinguished from biotrophs because they kill host tissue prior to colonisation. Biotrophic pathogens include powdery mildew, downy mildew, rust, nematodes and viruses. Biotrophs live on living tissue and die when the host plant dies.

What soil conditions favour the growth of fungal pathogens?

The soil conditions that exist at the opening of the cropping season (warm-moist soils and low microbial activity) can favour the growth of a pathogen.

How do cultural practices affect the incidence or spread of fungal pathogens?

Reduced tillage practices help maintain infested residues at the surface of soil, increasing the damage to young seedlings. Conventional cultivations bury this inoculum source which gets broken-down more rapidly by soil microorganisms than when on the soil surface. Rotations with susceptible hosts can increase the inoculum potential of the pathogen in soil. Certain herbicides also increase the disease severity (e.g: the disease caused by the Take-all fungus and root rot caused by Rhizoctonia).

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