They generally are used for soil fumigation. Most older fungicides sprayed on leaves and fruit are of this type. Most protectant fungicides are relatively stable by themselves. Generally they are relatively insoluble in water and resist removal or chemical change by water, yet must be toxic to fungi. Often a chemical change is brought about by the fungus, the host, or the environment before toxicity occurs.
Toxicity simply means the ability to do damage to the fungus cells.
Fungicides may act to produce a toxic reaction in the fungus in several different ways. Propiconazole and triadimefon act this way on cereal rusts. The permeability of the cell wall may be affected, causing a leaking of nutrient materials from the cell. Some fungicides may combine with essential metals in a way that they become unavailable for normal cell functions, including the functioning of essential enzymes. Other fungicides may inhibit respiration, or nuclear division, or may break dormancy of spores.