Fungi are vital for soil and plant health

July 6, 2009
By: Mark Kesby, Projects Officer

Soil fungi are microscopic plant-like cells that grow in long threadlike structures called hyphae and make a mass called mycelium. The mycelium absorbs nutrients either from the soil, surface organic matter or the roots that it has colonised.

Fungi are important for decomposing woody material, increasing nutrient uptake by plant roots, improving plant resistance to disease, pests and drought and improving soil structure.

Fungi can be divided into three groups based on how they function; decomposers, mutualists and pathogens.

As their name suggests, decomposers convert dead organic matter into fungal biomass (ie their own bodies) and organic acids, giving off carbon dioxide in the process. They are essential for the decomposition of hard woody material including cellulose, proteins and lignin, some of which is highly resistant to breakdown. By consuming the nutrients in the organic matter, they play an important role in immobilising and retaining nutrients in the soil. As well, the organic acids that these fungi produce help create organic matter that is resistant to degradation.

Mutualists develop mutually beneficial relationships with plants by colonising the roots. Here they help the plant to obtain nutrients from the soil eg phosphorous and get energy from the plant in return. Their mass hides roots from pests and pathogens and provides a greater root area through which the plant can obtain nutrients.

Mycorrhizal fungi are the best know of the mutualists. They grow partly inside plant roots with hyphae extending well out into the soil. Up to 5 metres of hyphae can be extracted from a gram of soil! There are four groups of mycorrhizal fungi - arbuscular (VAM) being the most common and agriculturally important. This fungi has arbuscles which are growths formed inside the plant root that have many small projections going into the cells. Around 90% of plants have some sort of association with these fungi. Groups of plants that don’t host VAM include the Cruciferae family (mustard, canola, broccoli), Chenopodiaceae (spinach, beets, saltbush) and Proteaceae (banksia, macadamia).

The dark, round masses inside the cells of this clover root
are vesicules for the arbuscular mycorrhizal fungus.

Pathogen fungi include the well known groups Verticillium, Phytophthora, Rhizoctonia and Pythium. These organisms penetrate the plant and decompose the living tissue, creating a weakened, nutrient deficient plant or death. Soils with high biodiversity have been shown to suppress soil-borne fungal diseases. Suppression mechanisms include the suite of native organisms out-competing the pathogenic organisms, physically protecting roots and providing better nutrition to the plant.

There are several things that you can do to encourage soil fungi. A constant supply of food (organic material), host plants (where required), water and minimal disturbance are important. Tillage has a disastrous effect on fungi as it severs the hyphae and breaks up the mycelium. Long fallows combined with non-host crops (eg canola) will decimate the population of VAM. Subsequent highly VAM dependent crops (faba beans, cotton) or even moderately dependent crops (eg sunflowers and sorghum) will suffer major yield loss. This is known as long fallow disorder.

Not surprisingly, broad spectrum fungicides are toxic to fungi! Their use will result in a decline in beneficial types. Herbicides are not generally thought to affect fungi directly.

Further information can be sourced from two excellent websites:

• www.dpi.nsw.gov.au/agriculture/resources/soils/biology

• www.soilhealthknowledge.com.au/