avsunviroid

Avsunviroid is a viroid within the family Avsunviroidae. Its best-known member is Avocado sunblotch viroid (ASBVd), the causal agent of avocado sunblotch disease. Viroids are the smallest infectious pathogens known, consisting solely of short, circular RNA without a protein coat. Avsunviroids replicate in plant chloroplasts and are characterized by hammerhead ribozymes that catalyze self-cleavage during replication.

ASBVd has a small circular single-stranded RNA genome of about 246 nucleotides. It forms branched structures and is capable of intracellular replication via a rolling circle mechanism in chloroplasts. The hammerhead ribozymes on both polarity strands enable self-cleavage and ligation during replication, a feature that distinguishes avsunviroids from other viroids that replicate in the nucleus.

ASBVd infects avocado trees (Persea americana). Infection causes avocado sunblotch disease, historically including sunken, pale blotches on fruit and bark streaking; leaves may show mottling or yellowing and growth may be retarded. Symptoms vary with cultivar and environmental conditions, and asymptomatic infections are common, complicating diagnosis.

Natural spread occurs primarily through movement of infected plant material, such as budwood and grafts, and can be facilitated by pruning wounds. Transmission through seeds is unreliable and not a major route. Diagnosis relies on molecular techniques including RT-PCR, nucleic acid hybridization, and sequencing; symptoms alone are not definitive. Management centers on using certified virus-free stock, roguing and removing infected trees, sanitizing equipment, and restricting movement of plant material from infected areas, as there is no cure for viroid infection.