Hop stunt viroid: A polyphagous pathogenic RNA that has shed light on viroid–host interactions

TAXONOMY: Hop stunt viroid (HSVd) is the type species of the genus Hostuviroid (family Pospiviroidae). The other species of this genus is Dahlia latent viroid, which presents an identical central conserved region (CCR) but lacks other structural hallmarks present in Hop stunt viroid. HSVd replication occurs in the nucleus through an asymmetric rolling‐circle model as in the other members of the family Pospiviroidae, which also includes the genera Pospiviroid, Cocadviroid, Apscaviroid, and Coleoviroid. PHYSICAL PROPERTIES: Hop stunt viroid consists of a single‐stranded, circular RNA of 295–303 nucleotides depending on isolates and sequence variants. The most stable secondary structure is a rod‐like or quasi‐rod‐like conformation with two characteristic domains: a CCR and a terminal conserved hairpin similar to that of cocadviroids. HSVd lacks a terminal conserved region. HOSTS AND SYMPTOMS: HSVd infects a very broad range of natural hosts and has been reported to be the causal agent of five different diseases (citrus cachexia, cucumber pale fruit, peach and plum apple apricot distortion, and hop stunt). It is distributed worldwide. TRANSMISSION: HSVd is transmitted mechanically and by seed.