Further reading: Avsunviroidae

Hadidi, A., Flores, R., Randles, J. W. & Palukaitis, P. (2017).  Viroids and Satellites. Oxford, UK: Academic Press. 

Hadidi, A., Flores, R., Randles, J. W. & Semancik, J. (2003).  Viroids. Collingwood: CSIRO Publishing.