A number of infectious plant diseases result from viruses. Major crops that are seriously affected by virus diseases include: tobacco, potato, sugar beet and cane, peach, orange, cotton and wheat. Most of the plant viruses impair or destroy chlorophyll, causing the plant to wilt or die; some viruses, though, stunt or otherwise deform plants without seriously affecting the chlorophyll.
The importance of virus diseases on crop plants has increased tremendously in the past 50 years. Since 1900, more than 200 new plant viruses have been discovered. Many of them have done widespread damage to crop plants. Curly top caused almost complete abandonment of the sugar beet industry in parts of western USA from 1926 to 1932 and still causes severe injury to tomatoes, beans and a number of other crops. Sugarcane mosaic caused extensive losses to the sugarcane industry in the USA, Argentina, Brazil, and other countries beginning about 1917. Spotted wilt has become widespread and now causes losses to tomato and other crops in many parts of the world. Since 1940, swollen shoot has caused extensive damage to the cacao industry of west Africa. Virus diseases of citrus trees have become more destructive and from 1936 to 1946 tristeza caused the loss of 7 million orange trees in the state of Sao Paulo, Brazil, alone. It has attacked or now threatens millions of trees in various tropical and subtropical areas. This increase in destructiveness of virus diseases and in the number of known viruses has come about largely as a result of the expansion of agricultural enterprises and the increased movement of plants and plant products in recent years.
Virus diseases of plants are widespread, and many of them cause economic loss. The whole organization of the potato industry is conditioned by the necessity for minimizing virus diseases. For more than a century it has been known that if a farmer in the UK continued to use his own seed potatoes for successive crops, degeneration progressed gradually until the great majority of the plants were malformed and stunted. Among other important crop diseases caused by viruses are spotted wilt of tomato, tobacco mosaic, leaf curl of cotton in the Sudan, and swollen shoot disease of cacao. The tobacco mosaic virus besides affecting many members of the nightshade family (Solanaceae), which includes tobacco, tomato and potato, occurs on some 30 species of plants in 14 other families.
Viruses originate in local areas all over the world. Through long association native plants have developed a tolerance to the local viruses that enables infected individuals to survive with little injury. When crop plants are introduced into an area they frequently become subject to infection with the native viruses, against which they have had no opportunity to develop resistance. Such a virus may cause extensive losses to a crop plant, not only in the areas of original distribution of the virus, but also in the other areas to which it may spread on the recently attacked crop plant. Virus diseases produce a wide range of symptoms and types of injury on plants. Sometimes they kill the plant in a short time, as with spotted wilt and curly top on tomato. More often they cause lesser injuries that result in reduced yields and lower quality of product. With respect to the general symptoms produced, most viruses are of two rather clearly defined groups: those that cause mottling or spotting of leaves, and those that cause a yellowing leaf, curling, dwarfing, or excessive branching, but little or no mottling or spotting.