Viroids are nonprotein-coding, small, circular RNAs infecting plants in which they may induce specific symptoms. Five different viroids have been identified in the grapevine in the period elapsed from 1985 to 1990. Since then, no new viroid has been reported from grapevines until the application of next-generation sequencing allowed the discovery of an additional viroid and a new viroid-like RNA. Possibly, new small, circular RNAs will be identified in the future by metagenomic approaches, but bioassays, which are time intensive and require phytopathological expertise, will always be needed for establishing conclusively their true identity as viroids. Although viroids generally do not elicit severe symptoms in grapevines, some of them are the agent of diseases in certain environmental conditions or in combination with certain viruses. Some of grapevine-infecting viroids may cause severe diseases in other crops. This chapter reviews the molecular, biological, and epidemiological features of viroids and viroid-like RNAs infecting grapevines and the methods for their detection and control and discusses the future perspectives of research.
