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.
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.