Fungicide

Fungicides are chemicals that inhibit the growth of fungi. A fungus is a tiny plantlike organism that obtains its nourishment from dead or living organic matter. Some examples of fungi include mushrooms, toadstools, smuts, molds, rusts and mildew.

Fungi have long been recognized as a serious threat to plants and crops. They attack food while it is growing and after it has been harvested and is placed in storage. One of the great agricultural disasters of the second half of the twentieth century was caused by a fungus. In 1970, the fungus that causes southwest corn-leaf blight swept through the southern and Midwestern United States and destroyed about 15% of the nation’s corn crop. Potato blight, wheat rust, wheat smut, and grape mildew are other important diseases caused by fungi. Fungi can attack agricultural crops, garden plants, wood and wood products (dry rot in particular is a major problem), and many other items of use to humans.

Fungicides usually kill the fungus that is causing the damage. Sulfur, sulfur-containing compounds, organic salts of iron, and heavy metals are all used as fungicides. Other fungicide types include carba-mates or thiocarbamates such as benomyl and ziram, thiozoles such as etridiazole, triazines such as anilazine, and substituted organics such as chlorothalonil. Many non-drug fungicides have low mammalian tolerance for toxicity, and have been shown to cause cancer or reproductive toxicity in experimental animal studies.

Fungicides operate in different ways depending upon the species that they are designed to combat. Many are poisons and their application must be undertaken carefully or over-application may kill other plants in the area. Some fungicides disrupt some of the metabolic pathways of fungi by inhibiting energy production or biosynthesis, and others disrupt the fungal cell wall, which is made of chitin, as opposed to the cellulose of plant cell walls. Chitin is a structural polysaccharide and is composed of chains of N-acetyl-D-glucosamine units. Fungal pathogens come from two main groups of fungi, the ascomycetes (rusts and smuts) and the basidiomycetes (the higher fungi—mushrooms, toadstools, and bracket fungi).

Human fungal infections, such as athlete’s foot, can be treated by fungicides normally referred to as antifungal agents or antimycotics. Compounds such as fluconazole, clotrimazole, and nystatin are used to treat human fungal infections.

The first known fungicide was a naturally occurring substance, sulfur. One of the most effective of all fungicides, Bordeaux mixture, was invented in 1885. Bordeaux mixture is a combination of two inorganic compounds, copper sulfate and lime.

With the growth of the chemical industry during the twentieth century, a number of synthetic fungicides have been developed: these include ferbam, ziram, naban, dithiocarbonate, quinone, and 8-hydroxyquinoline. For a period of time, compounds of mercury and cadmium were very popular as fungicides. Until quite recently, for example, the compound methylmercury was widely used by farmers in the United States to protect growing plants and treat stored grains. During the 1970s, however, evidence began to accumulated about a number of adverse effects of mercury- and cadmium-based fungicides.

As a result of the problems with mercury and cadmium compounds, scientists have tried to develop less toxic substitutes for the more dangerous fungicides. Dinocap, binapacryl, and benomyl are three examples of such compounds.

Another approach has been to use integrated pest management and to develop plants that are resistant to fungi. The latter approach was used with great success during the corn blight disaster in 1970. Researchers worked quickly to develop strains of corn that were resistant to the corn-leaf blight fungus and by 1971 had provided farmers with seeds of the new strain.