Chapter 28 - Avocado Sunblotch Viroid

Abstract

Avocado sunblotch viroid (ASBVd) is a 247 nucleotide, single-stranded, circular RNA that is the type species of both the genus Avsunviroid and the family Avsunviroidae. As such, it replicates in the chloroplast, utilizes a symmetrical rolling circle mechanism of replication and has distinctive hammerhead ribozyme structures that autocatalyze RNA cleavage. ASBVd has a very narrow host range, restricted in nature to avocado. The most recognizable symptoms of infection are the sunken, yellow to purplish longitudinal scars or broad spots that appear on the surface of the fruit. Infected trees are often asymptomatic and do not display overt foliar or fruit symptoms, although fewer and marginally smaller fruit may be produced. Sequence variation is common, occurring mostly in terminal loops outside the highly complementary regions. Some sequence variants have been associated with symptoms such as leaf bleaching, leaf variegation, and symptomless carrier. ASBVd is effectively controlled through the implementation of an accredited nursery scheme that ensures nuclear and multiplication trees used to source seed and budwood are regularly tested as free from ASBVd.