Citrus Cachexia Viroid, a New Viroid of Citrus: Relationship to Viroids of the Exocortis Disease Complex

Recovery of highly purified citrus cachexia viroid (CCaV) was accomplished by serial elution following CF-11 cellulose chromatography of a 2 m-LiCl-soluble nucleic acid preparation. The alternative herbaceous host, cucumber (Cucumis sativus cv. Suyo), yielded greater quantities of the viroid than the highest yielding citrus host, citron (Citrus medica cv. Etrog). A randomly primed cDNA probe to CCaV purified from cucumber reacted positively to extracts from citron and cucumber inoculated with the same isolate of CCaV. When tested against a broad range of other citrus viroids, the CCaV cDNA hybridized to only one, CV-IIa, which has been identified as the causal agent of a mild form of the citrus exocortis disease. Because of the apparent homology between the nucleotide sequences of CV-IIa and CCaV, and a size difference of only five to ten nucleotides, these RNAs can be considered as members of a common subgroup of citrus viroids. These two viroids have been classified by bioassay reactions as the causal agents of two distinct types of citrus disease, an ‘exocortis-like’ syndrome and cachexia. The properties of and relationships between these two members of the citrus viroid II group and the definition of the exocortis and cachexia (xyloporosis) diseases are presented.