Citrus Tristeza Virus (Closteroviridae)

Abstract

For the last 90 years, long before the virus nature of the tristeza disease was suspected and recognized Citrus tristeza virus (CTV), genus Closterovirus, family Closteroviridae was considered as a serious threat to the citrus production. The costs of CTV infections varied geographically and temporally, especially since growers worldwide have recognized the grave consequences of CTV epidemics. A variety of control measures including replacing sensitive rootstocks, prevention of spread with propagation material and wide scale use of cross-protection for handling stem pitting related problems are now commonly used in attempts to control the range of CTV induced diseases. Despite these efforts, the virus, which apparently originated in Asia, continued its spread and is now present throughout most citrus growing areas of the world, with only a few of the Mediterranean basin countries and some Western USA areas still free of massive infections. However, even these areas are now under a serious threat due to the recent spread of the most efficient aphid vector to nearby areas. CTV is naturally transmitted in a semi-persistent mode by several aphid species. The brown citrus aphid (Toxoptera citricida) is the most efficient vector because its great abundance on citrus trees and its high efficacy of vectoring highly pathogenic isolates. The symptoms of CTV depend on the nature of the invading isolate and the sensitivity of the citrus host. The most dramatic symptom referred to as “quick decline” results from rapid death of trees grafted onto sour orange rootstocks. Other CTV related diseases such as “stem pitting” of grapefruit or sweet orange trees are not related to a specific rootstock and do not result in tree death, but are often associated with stunting and poor productivity. Symptomless CTV isolates are used to provide protection (‘cross-protection’) against stem pitting, but not against the quick decline.