The Worldwide Threat from Destructive Isolates of Citrus Tristeza Virus-A Review

This paper reviews the effects of extremely destructive forms of citrus tristeza virus (CTV) which poses serious threats to citrus industries worldwide. These include Capao Bonito CTV in Brazil, navel orange stem pitting CTV in Peru, stem pitting 12B CTV found in the university orchards in Southern California, severe grapefruit stem pitting CTV found in South Africa, recent forms of CTV responsible for decline of sweet orange on sour orange rootstock in Florida and Israel and other severe CTV isolatesreported from Spainand elsewhere. Many ofthesedestructive CTVisolates are transmitted by Toxoptera citricidus but most can be transmitted by Aphis gossypii at relatively high levels of efficiency. The impact of recent changes in aphid transmissibility and population dynamics, and the threat of movement of T. citricidus into new regions of the world are reviewed. The appearance and impact of new strains or mutants of CTV differing in pathogenic capacities or in aphid transmissibility are discussed. Methods for the identification of new or destructive isolates of CTV are also reviewed. Concepts for prevention which include quarantine, eradication and education are presented. The immediate need is to test for presence of CTV in those countries where sour orange is the predominant rootstock. Also, to test for and eliminate very destructive forms of CTV, to strengthen quarantine laws and regulations, and to educate scientists, nurseryman and growers to the dangers involved in budwood importation and virus or vector spread.