Fungicide, FUHN juh syd, is a substance used to kill or retard the growth of fungi that are harmful to human beings, other animals, or plants. Diseases caused by fungi can destroy or seriously damage food crops. In the 1840's, a fungal disease destroyed most of the potato crop in Ireland, where people depended heavily on potatoes for food. About 1 million people died from starvation during this disaster, called the Great Irish Famine. A fungus also caused the chestnut blight that killed most of the American chestnut trees in the United States during the early 1900's.