A Definition of Citrus Viroid Groups and Their Relationship to the Exocortis Disease

Summary Nucleic acid extracts from citrons (Citrus medica cv. Etrog) displaying mild and moderate symptoms associated with the exocortis disease were analysed by sequential and denaturing PAGE which revealed the presence of several viroids. A comparison was made of electrophoretic patterns displaying one or more distinct citrus viroids from field isolates of citrus with exocortis. Citrus viroids were characterized by the physical parameters of electrophoretic mobility, chromatography on CF-11 cellulose and hybridization to cDNA probes of the well characterized citrus viroids, citrus exocortis viroid, CV-Ib from the 'citron variable viroid' isolate, and citrus cachexia viroid. These characteristic properties combined with biological distinctions in the host range and symptom expression suggested a scheme for the organization of the citrus viroids into five major groups. The association of the symptoms induced by these citrus viroids in citron cv. Etrog, their organization into individual viroid groups and their presumed relationship to the exocortis disease of citrus are discussed.