viroids, particles smaller than viruses which are thought capable of producing virus-like disease effects in the grapevine. Viroids can be found in nominally virus-free vines following thermotherapy, and are known to cause significant diseases for other crops. They are transmitted by vegetative propagation as for viruses, but viroid-free grapevines can be produced by tissue culture. No viroids have so far been identified with commercially important grapevine diseases. The viroid Yellow Speckle is widespread in Australia but is not known to harm the vines.
