Fungicide

A fungus is a tiny plant-like 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 both while it is growing and after it has been harvested and 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.

Chestnut blight is another example of the devastation that can be caused by fungi. Until 1900, chestnut trees were common in many parts of the United States. In 1904, however, chestnut trees from Asia were imported and planted in parts of New York. The imported trees carried with them a fungus that attacked and killed the native chestnut trees. Over a period of five decades, the native trees were all but totally eliminated from the eastern part of the country.

It is hardly surprising that humans began looking for fungicides (substances that will kill or control the growth of fungi) early in history. The first of these fungicides 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. The most serious effects were observed among birds and small animals who were exposed to sprays and dusting or who ate treated grain. A few dramatic incidents of methylmercury poisoning among humans, however, were also recorded. The best known of these was the 1953 disaster at Minamata Bay, Japan. At first, scientists were mystified by an epidemic that spread through the Minamata Bay area between 1953 and 1961. Some unknown factor caused serious nervous disorders among residents of the region. Some sufferers lost the ability to walk, others developed mental disorders, and still others were permanently disabled. Eventually researchers traced the cause of these problems to methylmercury in fish eaten by residents in the area.

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.