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This volume of the series The Plant Viruses is devoted to viruses with rod-shaped particles belonging to the following four groups: the toba­ moviruses (named after tobacco mosaic virus), the tobraviruses (after to­ bacco rattle), the hordeiviruses (after the latin hordeum in honor of the type member barley stripe mosaic virus), and the not yet officially rec­ ognized furoviruses (fungus-transmitted rod-shaped viruses, Shirako and Brakke, 1984). At present these clusters of plant viruses are called groups instead of genera or families as is customary in other areas of virology. This pe­ culiarity of plant viral taxonomy (Matthews, 1982) is due to the fact that the current Plant Virus Subcommittee of the International Committee of Taxonomy of Viruses is deeply split on what to call the categories or ranks used in virus classification. Some plant virologists believe that the species concept cannot be applied to viruses because this concept, according to them, necessarily involves sexual reproduction and genetic isolation (Milne, 1984; Murant, 1985). This belief no doubt stems from the fact that these authors restrict the use of the term species to biological species. According to them, a collection of similar viral isolates and strains does constitute an individ­ ual virus, i. e., it is a taxonomy entity separate from other individual viruses.