strain protects trees against infection with a severe strain of CTV. Bud-stock trees should be inoculated with a mild CTV strain at least four to six months prior to propagation. They should then be kept in a greenhouse, under aphid-free conditions. It is also recommended that nurseries of young plants grown for propagation should be kept vector-free. Top-grafting with pre-inoculated buds onto interstock trees infested with severe CTV is not effective, because the tree has little protection against the disease. It is necessary to spray nursery plants and young trees with insecticide occasionally, to control aphids. This should retard any re-infection with the virus. A biological approach has been to bring in a parasitoid wasp from Asia that naturally controls A. spiraecola. This approach was stopped when T. citricida arrived due to it not being a parasite of the more important aphid. There is a natural parasite of aphids in Florida in the gall midge family Cecidomyiidae that attempts to keep aphid levels down, however due to the influx in aphid population it has not been able to keep populations down. Importance CTV is the most economically important and damaging virus of citrus trees. It can be spread quickly and do damage not only by killing trees with sour orange rootstock, but also by stem pitting normal citrus trees. It has killed more than 80 million trees worldwide, mainly in South Africa since 1910, Argentina (10 million) and Brazil (6 million) since 1970, and the U.S. (3 million) since 1950. With the spreading of T. citricida the severity and impact has increased dramatically in Central America and the U.S. In Spain there has been a progressive decline in production from over 40 million sweet orange and mandarin trees. References External links ICTVdB - The Universal Virus Database: Citrus tristeza virus Family Groups - The Baltimore Method Closteroviridae Viral citrus diseases