Most studies involving determination of host range, symptom expression and mode of transmission are carried out using crdue virus obtained from an infected plant. Virus purification is undertaken with the objectives of obtaining virus preparations of both high quality and good yield. In identifying an unknown virus, the usual process involves the stepwise determination of as many characters as may be required to match the information about an unknown virus with that of a known, previously-described virus. Plant virologists recognize that there is no universally applicable set of characters for defining virus species. Some carlaviruses occur in relatively high concentrations in their natural or experimental host and have been readily purified. Accurate diagnosis often depends on a comparison of many distinct properties of the agent. Some of these properties can be determined using crude sap, but in general such properties are capable of providing only a tentative diagnosis, or of assigning the causal virus to a virus Family or Group.