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 natural plants and human crops. They attack food both while it is growing and also after it has been harvested and placed into 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 southern 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 disasters 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...