Citrus Tristeza Virus - Management

Management

When CTV was first discovered quarantine was the best management strategy, now quarantining only works for areas where a small amount of trees are infected. The other approach that was adopted after T. citricida came to the U.S. was destroying any tree in which the budwood was not free of CTV. This is a more drastic measure but must be done due to how fast T. citricida can spread the virus. If there is any CTV in the area, avoid grafting trees on sour orange rootstock and instead graft on tristeza-tolerant rootstock. Since the virus has been in Asia a long time they have used rootstock from trifoliate orange, Sunki, and Shiikuwasha (C. depressa Hayata) for many years. Some hybrids, such as Troyer citrange or Swingle citromelo, show promise as resistant root-stock. Also, using scion varieties tolerant to stem pitting is recommended. The production of virus-free trees by shoot-tip grafting or heat treatment is very important. If it is possible to keep the field permanently free of CTV, the planting of virus-free trees is practical. In areas where it is difficult to find a virus-free field, preinoculation with a mild CTV 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 parasitic 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 Cecidomiidae that attempts to keep aphid levels down, however due to the influx in aphid population it has not been able to keep populations down.