Family: Pospiviroidae

Genus: Apscaviroid

Distinguishing features

Circular ssRNAs between 293 and 369 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) identical to that of the reference variant of the type species of the genus (apple scar skin viroid, ASSVd) and the terminal conserved region (TCR) (Figure 3. Pospiviroidae). Replication most probably occurs through an asymmetric rolling-circle model by analogy with potato spindle tuber viroid (PSTVd) (Figure 1.Viroids). 

Biology

Members of the genus Apscaviroid (apscaviroids) are reported to occur worldwide, particularly those affecting genera Citrus and Vitis. Several of them may cause severe losses to economic relevant fruit crops, while others do not induce visible symptoms. The most common pathway of apscaviroid spread is provided by the use of infected plants for grafting or propagation. Seed transmission has been reported for ASSVd (Hadidi et al., 1991) and grapevine yellow speckle viroid 1 (GYSVd-1) (Wan Chow Wah and Symons 1999), while it has been experimentally excluded for apple dimple fruit viroid (ADFVd) and pear blister canker viroid (PBCVd) (Di Serio et al., 2017). ASSVd has been reported to be transmitted to herbaceous hosts in the greenhouse by the whitefly Trialeurodes vaporariorum (Walia et al., 2015), but the epidemiological relevance of this finding is unclear. No role for natural vectors has been shown in the natural transmission of other apscaviroids (Tessitori 2017, Habili 2017). 

Species demarcation criteria

Viroids with rod-like or quasi rod-like conformation, with the TCR, a CCR identical to that of members of the type species of the genus, less than 90% sequence identity and distinct biological properties with respect to the members of the genus, are classified in different species (Di Serio et al., 2014). 

Related, unclassified viroid-like RNAs

Virus names and virus abbreviations are not official ICTV designations. 

* Autonomous replication has not been conclusively shown