Circular ssRNA genomes of 246–301 nt depending on species and sequence variants. The most stable secondary structure is a rod-like or quasi-rod-like conformation with five domains, a central conserved region (CCR) similar to that of members of the genera Pospiviroid and Hostuviroid, and the terminal conserved hairpin (TCH) (Figure 3. Pospiviroidae). Single or double cytosine residues occur at a specific locus in the genomic RNA, whose TR domain in members of the type species (Coconut cadang-cadang viroid) is variable in length as a result of 41-, 50- or 55-nt duplications that generate larger variants containing 287–301 nt (Haseloff et al., 1982). As disease progresses from the early to the mid stages, the small genome forms are replaced by the large forms. Dimeric forms of coconut cadang-cadang viroid (CCCVd) coexist with their corresponding monomeric forms. Replication most probably occurs through an asymmetric rolling-circle model by analogy with potato spindle tuber viroid (PSTVd) (Figure 1.Viroids). At the subnuclear level, CCCVd distributes throughout the nucleoplasm but it mainly is concentrated in the nucleolus (Bonfiglioli et al., 1994).
Members of the genus Cocadviroid infect monocots (CCCVd; coconut tinangaja viroid, CTiVd) or dicots (hop latent viroid, HLVd; citrus bark cracking viroid, CBCVd). CCCVd and CTiVd only infect members of the family Arecaceae(Vadamalai et al., 2017). The geographic distribution of CCCVd includes the Philippines, Malaysia and Sri Lanka, and CTiVd appears restricted to Guam. HLVd and CBCVd have broader geographic distributions (Vadamalai et al., 2017, Lavagi et al., 2017). The natural host range of HLVd is restricted to commercial hop, Japanese hop (Humulus japonicus) and Urtica dioica (Lavagi et al., 2017). CBCVd has a broader natural host range that includes citrus species and hops (Lavagi et al., 2017). Low rates of CCCVd transmission by seed and pollen transmission have been reported (Pacumbaba et al., 1994), while HLVd is neither seed nor pollen transmitted (Matoušek and Patzak 2000, Matoušek et al., 2008). CCCVd and CTiVd infections may be lethal to coconut palm (Rodriguez et al., 2017, Wall and Randles 2003). CBCVd causes bark cracking in citrus plants, and more recently it has been found to induce a severe disease in hop (Jakse et al., 2015).
Species demarcation criteria
Viroids with molecular features (rod-like conformation, CCR, TCH) identical to that of members of the type species in the genus,less than 90% sequence identity and distinct biological properties with respect to the other members of the genus, are classified in different species (Di Serio et al., 2014).