For example, if fungal pathogens have adverse effects on agriculture, why not use pathogens to attack plants that we don't want to cultivate?

In agriculture, fungal pathogens may be useful for the biological control of weeds. In the United States, 13 of the top 15 weeds are introduced species, and in Canada, 78 of the 107 most troublesome weeds were introduced to the country. The best way to control such alien plants is to introduce diseases from their home territories. This policy has a 67% overall success rate.

For example, Fusarium oxysporum against cocaine plantations ...

... or Pleospora papaveracea against heroin poppies.

These activities raise important questions of ethics (because they amount to biological warfare), not to mention delicate international politics (drug producing countries are unlikely to invite ‘attacks’ on their own territory).

And it’s a very small step from targeting a narcotics crop to targeting a food crop.

Humans have been using fungi for many thousands of years. This is the corpse of Ötzi, the Ice Man.

One was a mass of fibrous material of a fungus with a long history of use as a tinder, so clearly it was part of the Iceman’s fire-making kit.

The other two were pieces of a bracket fungus threaded onto leather thongs. The fungus (actually called Piptoporus betulinus) is known to accumulate antiseptics and chemicals (known as secondary metabolites) able to reduce fatigue and sooth the mind. Perhaps they were chewed on when the going got tough on his trek over the Alps.

Today's fungal products

Today, alcohol and citric acid are the world’s most important fungal metabolites in terms of production volume, although, penicillin is still an important antibiotic. Most antibiotics that we use today actually originate from bacteria, but fungi produce some other useful pharmaceuticals.

The fungal product called cyclosporin is capable of suppressing the immune response in transplant patients so as to avoid organ rejection.

Another natural compound obtained from fungi that has great medical value is called mevinolin. This is produced by the fungus Aspergillus terreus and is the basis of the ‘statins’ which are used to reduce cholesterol levels (high cholesterol levels being considered to be a risk factor in heart disease).

Three compounds derived from mevinolin, Pravastatin, Simvastatin and Lovastatin, had worldwide sales that put them individually in the top ten selling pharmaceuticals in 1995, with combined sales valued at 5 billion U.S. dollars.

Some other fungal products

Most of the steroids in clinical use today are modified during manufacture by using fungi to make specific chemical transformations that may avoid anything up to twenty or thirty steps of pure chemistry. Using fungi in this way allows specific compounds to be made which would be otherwise very difficult, impossible, or just too expensive to produce by direct chemical synthesis.

Ergot toxins in low, and controlled, concentrations are valuable drugs causing dilation of veins and a decrease in blood pressure as well as contraction of smooth muscles. The ergot alkaloids are now produced by fermentation.

Mushroom cultivation is the most obvious way in which we exploit fungi.

World production of mushrooms in 1996 was estimated to be 5.8 million metric tonnes; with overall annual point-of-sale value is in the region of 30 billion US-dollars.

In the mid 1970s Agaricus accounted for over 70% of total global mushroom production; today, it accounts for 30% even though total production tonnage has more than doubled in the intervening years.

‘Exotic’ mushrooms (like Lentinula and Pleurotus) are now routinely found alongside Agaricus in supermarkets around the world.