The virus, to which this chapter is devoted, was described first in 1931 by Weimer in the U.S. as a disease agent which caused mosaic symptoms in the fodder plant lucerne (Medicago sativa L.). 1 Lucerne is known to American farmers as alfalfa, a word of Spanish-Arab origin, now introduced into general American-English usage. Alfalfa mosaic virus is now the generally accepted name for the virus in Anglo-Saxon literature 2,3 but many languages have etymological equivalents of lucerne used in the 157virus name: French, virus de la mosaique de la luzerne; German, Luzerne-Mosaikvirus; Dutch, luzernemozaiekvirus; Czech, virusu mozaiky lucerny.