The Grouping of Citrus Viroids: Additional Physical and Biological Determinants and Relationships with Diseases of Citrus

Abstract

A consensus catalogue for the grouping of citrus viroids (CV) utilizing the physical parameters of size and nucleotide sequence homology as tested with group-specific cDNA probes has been furthered developed. Analysis of cachexia disease sources by sequential polyacrylamide gel electrophoresis (sPAGE) has revealed a third member of the CV-I1 Group, designated as CV-IIc. A viroid characterized by a similar size and sequence homology is common to all "xyloporosis" disease sources analyzed. Chrysanthemum, which has been identified as a selective host for CEV and the Group I1 viroids, produces a leaf spotting reaction when inoculated with CV-IIa, the viroid inducing the most mild reaction in citron. Mixed infections of viroids from different groups has indicated a range of responses on citron including: 1) both enhanced and delayed severe dwarfing symptoms of CEV, 2) synergism resulting in severe dwarfing and epinasty mimicking CEV symptoms, and 3) a variation in leaf symptoms ranging from no effect to severe epinasty. Field testing of pure citrus viroids and mixed viroid inoculations of commercial scion on exocortis sensitive rootstock combinations has been initiated. Analysis of the viroid profiles of citrons inoculated with field sources of some diseases of suspected viroid etiology such as, "xyloporosis" of Palestine sweet lime, "gummy bark" of navel orange, and "Kassala" of grapefruit, indicated the presence of a complex pattern of several viroids.