Viroids are the smallest autonomously replicating subviral pathogens presently known.
So far, they have been found only in higher plants where they may cause economically important diseases especially in potato, tomato, citrus, horticulturists chrysanthemum, coconut palms, avocado and hops (Tab. 1). Viroid-specific disease symptoms are growth retardation, malformation of leaves, flowers and fruits and even death of the plant.
Recent studies indicate that latent viroids which cause no obvious disease are widespread in such crop plants and ornamentals like citrus, grapevine, hops, banana, croton and hibiscus (Table 2) which have been vegetatively propagated throughout history and the nuclear stock of which had been selected for apparent healthiness mainly by visual inspection.
A typical example for this situation is the recently detected occurrance of hop latent viroid (Puchta et al., 1988) in practically all hop cultivars grown around the globe (Table 3).
Detailled biochemical and biophysical studies have shown that viroids are single-stranded, covalently closed circular RNA molecules which, due to intramolecular base-pairing, exist in their mature native state as largely double-stranded rod-like structures (Figure 1). At present about 30 viroids are known and the molecular structure of 16 of them (and of about 25 sequence variants thereof) has been elucidated (Table 1). On the basis of their sequence similarity (Table 4) these viroids can be grouped into six groups, the prototypes of which are potato spindle tuber viroid (PSTV), coconut cadang cadang viroid (CCV), hop latent viroid (HLV), hop stunt viroid (HSV), apple scar skin viroid (ASSV) and avocado sunblotch viroid (ASBV), respectively (Table 1). Their chain length ranges between 240–380 nucleotides and represents a characteristic feature of the various viroid "species".
Various lines of evidence suggest that viroid replication proceeds in a