Crop diversity fruit trees and banana plants) are cloned. A single pest or disease could threaten a whole crop due to this uniformity ("genetic erosion"). A well-known historic case was the Irish Potato Famine of 1845-1847, where a vital crop with low diversity was destroyed by a single fungus. Another example is when a disease caused by a fungus affected the monocultured 1970 US corn crop, causing a loss of over one billion dollars in production. A danger to agriculture is wheat rust, a pathogenic fungus causing reddish patches, coloured by its spores. A virulent form of the wheat disease, stem rust,
Brown blotch disease blotch disease causes large economic losses due to a perceived decrease in quality, even though it does not cause any health problems in humans. Brown blotch disease is observed in mushroom farms all over the world. In the UK, the Netherlands and France, 15% of the crop weight is affected by brown blotch disease, while in Italy it is even worse; in the worst years, 40% of the total weight of mushroom crops is affected by brown blotch disease in Italy. Because brown blotch disease affects all mushroom species, exotic mushrooms grown in eastern countries such as Japan, China, and