fungicide, any toxic substance used to kill or inhibit the growth of fungi. Fungicides are generally used to control parasitic fungi that either cause economic damage to crop or ornamental plants or endanger the health of domestic animals or humans. Most agricultural and horticultural fungicides are applied as sprays or dusts. Seed fungicides are applied as a protective covering before germination. Systemic fungicides, or chemotherapeutants, are applied to plants, where they become distributed throughout the tissue and act to eradicate existing disease or to protect against possible disease. In human and veterinary medicine, pharmaceutical fungicides are commonly applied as topical antifungal creams or are given as oral medications.
Bordeaux mixture, a liquid composed of hydrated lime, copper sulfate, and water, was one of the earliest fungicides. Bordeaux mixture and Burgundy mixture, a similar composition, are still widely used to treat orchard trees. Copper compounds and sulfur have been used on plants separately and as combinations, and some are considered suitable for organic farming. Other organic fungicides include neem oil, horticultural oil, and bicarbonates. Synthetic organic compounds are more commonly used because they give protection and control over many types of fungi and are specialized in application.
Cadmium chloride and cadmium succinate are used to control turfgrass diseases. Mercury(II) chloride, or corrosive sublimate, is sometimes used as a dip to treat bulbs and tubers; it is highly toxic to humans. Strobilurin compounds are used in industrial agriculture to kill various types of mildews, molds, and rusts. Other substances occasionally used to kill fungi include chloropicrin, methyl bromide, and formaldehyde, though the use of these fungicides is regulated or banned in many countries. Many antifungal substances occur naturally in plant tissues. Creosote, obtained from wood tar or coal tar, is used to prevent dry rot in wood.
Fungicides kill pathogenic or parasitic fungi by disrupting their critical cellular processes. For example, many fungicides bind with specific enzymes to interrupt the metabolic pathways involved with cellular respiration. However, as with herbicides, insecticides, and antibiotics, the overuse of fungicides has led to the evolution of resistance in certain fungal species. Fungicide resistance, in which a fungal population displays decreased sensitivity to a given fungicide, can occur rapidly, as a single fungus may produce millions of spores.