Fungi are living plants which lack the chlorophyll or green colouring matter which helps to build up the starches etc. used by the plant. Theyon organic matter either in a living plant or on decomposing organic material and are then termed parasites; as such, fungi are responsible for numerous diseases infecting , fruits, vegetables etc. Mildews, cankers, rusts and so on are typical examples. The gardener can control many of these diseases provided he realises that in most instances treatment with a fungicide (whether spraying or dusting) is preventive rather than curative. The aim is usually to prevent infection of healthy foliage.
It should be emphasised that by no means all fungi are harmful, some being of great value to human beings. Mushrooms and truffles are examples of edible fungi and the Penicillium fungi furnish the well-known antibiotic penicillin.