Progress on strain differentiation of Citrus tristeza virus and its application to the epidemiology of citrus tristeza disease

Citrus tristeza virus (CTV) occurs in most citrus producing regions of the world, and it is the most serious viral pathogen of citrus. With the recent establishment of the brown citrus aphid, Toxoptera citricida, its most eff icient vector, on Madeira Island (Portugal) and in Florida (USA) and the co untries of the Caribbean Basin, the impact of CTV is likely to increase in these regions. Since there are many strains of CTV and CTV infections frequ ently occur as mixtures of several strains, it is necessary to be able to d istinguish the strains for regulatory purposes, disease management and epid emiology. We describe the evolution of techniques developed to detect CTV a nd to differentiate the individual strains, and present the results of test s using these latest methods on CTV isolates from mainland Portugal, Madeir a Island and Florida. Mild and decline-inducing strains of CTV were detecte d in mainland Portugal and mild, decline-inducing and severe stem pitting s trains on Madeira Island. In Florida we demonstrated the presence of infect ions that reacted with probes made against stem pitting strains not previou sly detected there. It is concluded that CTV presents a significant threat to citrus production in mainland Portugal, on Madeira Island and in the nei ghbouring countries of the Mediterranean Basin, as well as in Florida, else where in the USA and throughout the Caribbean Basin, especially following t he widespread establishment of T. citricida throughout the region. (C) 2000 Elsevier Science B.V. All rights reserved.