Characterization of a viroid associated with avocado sunblotch disease

A viroid has been purified from avocado leaves infected by sunblotch disease and designated the avocado sunblotch viroid. It is a covalently closed circular RNA molecule with a molecular weight lower than that of chrysanthemum stunt viroid and citrus exocortis viroid while hybridization analysis with 32P-labeled complementary DNA indicated that it is a single RNA species. It could be detected as a stainable RNA band on polyacrylamide tube gel electrophoresis of partially purified extracts of only two of four avocado isolates with positive symptoms of sunblotch disease. However, the viroid was detected in all four isolates by hybridization analysis with 32P-complementary DNA; this procedure has potential use for the rapid indexing of sunblotch disease since the viroid was not present in an isolate of healthy avocado. It has yet to be shown that the viroid is the causative agent of sunblotch disease.