Fungi

Free-living fungi

These fungi make a living by scavenging on organic matter in the
soil. Sometimes when you break open an old piece ofwood, you can see a mass of white. This is fungal hyphae, slowly degrading the wood. You can also sometimes find the thin white strands of fungal hyphae under the litter of the forest floor.Along with bacteria, fungi are responsible for most of the fertility of soils, especially soils in native vegetation. They are able to decompose just about anything and release all the nutrients contained within the materials, to be used by plants for growth. This electron micrograph of fungal hyphae was taken from wheat straw collected from a field. The hyphae play a major role in breaking down the straw, and returning the nutrients locked up in the straw back to the soil.


Parasitic and pathogenic fungi

These bare patches are where soil-living fungal diseases killed the plants 		Some of the most serious crop diseases in the history of human civilization have been soil-borne, parasitic or pathogenic fungi. In the 1700's, the potato crop in Ireland was badly affected by a fungal disease called potato blight (Phytophthora). More than 3 million people died as a result of the subsequent lack of food, and many more left the country and migrated to America.

Mushroom forming fungi
Mushrooms come in all shapes and sizes, some of them very beautiful, some of the downright ugly. Most of these are ectomycorrhizal fungi. Some fungi are edible and quite delicious (see left).

You should be very wary of eating any fungi you find in the wild. Anything brightly coloured is certain to be highly toxic. Anything with white gills is also very toxic. The field mushroom has a white cap and brown gills. This is the safest mushroom to pick and eat. Otherwise, you are better off buying your mushrooms from the shop and not eating those you find in the soil.   

Bacteria, yeasts and viruses

Decomposers
Bacteria and yeasts, along with fungi are responsible for most of the fertility of soils. They are able to decompose just about anything. In fact, bacteria are now being used to help clean up toxic soils, because they are the only organism that can break down the substances. This electron micrograph shows bacteria found in soil that help break down organic materials

A source of novel chemicals
Isolating unknown species of yeasts on petri dishes to look for new pharmaceuticals
The antibiotic, penicillin, was first found when a fungal contaminant was noticed to inhibit bacterial colonies growing on an agar plate. There is now a whole industry looking for new chemicals that are produced by microorganisms for medicines, pesticides, and industrial uses.  For example, the protein produced by the bacteria Bacillus thurengiensis has been used to kill certain insect pests (see genetic engineering - Environmental Impact of Pest Control). The toxin is so useful, the gene that produced the toxin in the bacteria was isolated and transferred to cotton plants. These genetically transformed cotton plants now are able to produce the SAME protein, and protect itself against damage by moths. 

Diseases
Bacteria, yeasts and viruses are responsible for a wide range of diseases on crops and native plants. Many of these diseases spend at least a part of their life cycle in the soil. For example, a bacterial disease of tomatoes is found in the soil. When rain, or irrigation water hits the soil, it splashes some soil onto the tomato leaf. The bacteria are then able to attack the leaf. In Australia, bacterial blight of tomatoes (see below) can reduce yields by up tp 50%.

This is as healthy tomato leaf
.

This tomato leaf has become infected by a bacteria called Pseudomonas. The symptoms of this disease are curled up leaves and black spots.

Photographs on this page courtesy of
Prof. Sally Smith, Dr. Susan Barker and Dr. Ian Singleton, University of Adelaide
Dr. Vadakattu Gupta, CRC for Soil and Land Management
Robin Coles, Primary Industries South Australia