Virus indexing in the New South Wales citrus improvement scheme

Abstract

A virus indexing program was initiated in the 1950s to test local citrus clones with commercial potential, for the presence of citrus exocortis viroid and psorosis. As indexing techniques improved, clones have also been tested for the presence of citrus cachexia viroid and other viroids, citrus infectious variegation virus, citrus tatter leaf virus, and strain severity of citrus tristeza virus. Of 123 candidate clones tested, only 2 were infected with psorosis virus. Crinkly leaf (infectious variegation virus) was widespread in lemons but has been eliminated by nucellar embryony. The widespread incidence of citrus exocortis and other lower molecular weight viroids has been reduced by screening mother trees on Poncirus trifoliata stocks, biological indexing, and, more recently, by sequential polyacrylamide gel electrophoresis of RNA extracts. Cachexia is rarely found in Australian citrus clones.