Fungicides are a class of pesticides that are marketed specifically for the purpose of killing or inhibiting the growth of fungus. Fungus are defined under the Federal Insecticide, Fungicide, and Rodenticide Act as "any non-chlorophyllbearing thallophyte (that is, any non-chlorophyllbearing plant of a lower order than mosses andliverworts), as, for example, rust, smut, mildew, mold, yeast, and bacteria, except those on or in living man or other animals and those on or in processed food, beverages, or pharmaceuticals." Although the United States statutory definition excludes fungi that would grow on food, beverages, and pharmaceuticals, biologically these are fungi. Thus, in the United States, products designed to kill fungi are regulated by the U.S. Environmental Protection Agency as pesticides and/or by the Food and Drug Administration under food and drug law (a chemical may fall under the purview of both agencies).

The benefits of fungicide use have been many. In agriculture, fungicides control pests that may rob water and nutrients from crop plants or may cause food spoilage as the products are brought to market. Fungicides may also prevent the growth of fungi that produce toxins, such as aflatoxins. Fungicides also have important industrial applications and are important in preserving the purity and safety of certain pharmaceutical agents.

In 1997 there were an estimated $0.8 billion in sales of fungicides in the United States, about 7 percent of the total pesticide market. In 1997, worldwide, 5.7 billion pounds of pesticides were used, of which 0.5 billion were fungicides. Of the1.2 billion pounds of conventional pesticides used in the United States in 1997, a total of 81 million pounds of fungicides were used; 79 percent of the use was in agriculture. Generally, the United States has experienced a downward trend in total fungicide use since 1970.

There are numerous classes of fungicides, with different modes of action as well as different potentials for adverse effect on health and the environment. Most fungicides can cause acute toxicity, and some cause chronic toxicity as well. Hexachlorobenzene, now banned or severely restricted in most parts of the world, has been associated with human poisoning from contaminated seed grain and poisoning of infants from misuse in laundry solutions. Metam sodium and other thiocarbanates are skin irritants that can cause reactive airway disease at low doses and severe toxicity and even death at high doses. The ethylene bis dithiocarbamates (EBCDs) are suspected human carcinogens and are tightly regulated in the United States.

Organic mercurials have caused severe acute and chronic toxicity. Worldwide, there have been a number of incidents of treated seed grain fed to people, with disastrous consequences in terms of acute poisoning and damage to fetuses. Phenyl mercuric acetate is no longer used as a paint preservative in the United States because it off-gases elemental mercury into the air, with the potential for causing toxicity to young children. Organotin compounds also have serious human toxicity and are very toxic to the environment; their use is banned or severely restricted in most of the world. Likewise, due to human toxicity concerns, cadmium is no longer used as a fungicide in the United States.

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