Cassava mosaic virus disease pandemic

Cassava mosaic virus disease (CMD) is so prevalent in some countries that it is regarded as a normal feature of cassava plants; little attempt is made to control the disease and impaired, sub-standard yields are accepted as normal. There are no definitive estimates of the losses sustained but they are likely to exceed 12 million tonnes in Africa alone. In Uganda, however, the situation is completely different in that a severe epidemic now affects much of the country. The symptoms associated with the epidemic are exceptionally severe and lead to almost complete crop failure in some of the most widely grown varieties. Crops become virtually worthless and are frequently abandoned, causing severe food shortages. Deaths have occurred in some of the worst affected areas in which the population is almost entirely dependent on cassava. In Kenya an epidemic was first reported in 1995 in areas close to the border with Uganda and, by 1997, a large tract of western Kenya was severely affected to the north and west of Kisumu, which is only 120 km from the border with Tanzania. This means that Tanzania is now at risk of spread from Kenya as well as from Uganda. The epidemic has spread so quickly and extensively as to justify the use of the term 'pandemic'. It is associated with unusually large population densities of the whitefly vector (Bemisia tabaci) and with the occurrence of a novel cassava mosaic virus. This has been characterised at the Scottish Crop Research Institute in the UK and shown to have the properties of a hybrid between the two geminiviruses previously shown to cause CMD in Africa. Extensive research is in progress in Uganda by scientists of the National Root Crops Programme in association with staff of the UK's Natural Resources Institute (NRI), the International Institute of Tropical Agriculture (IITA) and the Scottish Crop Research Institute. Virus-resistant varieties have been introduced or developed locally that withstand both the original virus and the novel one reported recently. These varieties are being multiplied and distributed to farmers so that they can replace the mainly vulnerable ones that were being grown. Considerable progress has been made in restoring production in areas that were first affected. Many organizations are involved in disseminating the resistant varieties but much remains to be done before cassava production and food security are restored, especially in the southern districts of Uganda. The pandemic has important implications for quarantine and plant protection services. This is particularly so because of the current threat of drought in many areas, and the long delay before adequate stocks of virus-resistant varieties can be built up to replace those being grown in affected areas and in the adjacent areas now at risk. Professor Thresh and Dr Otim-Nape would be interested to hear from anyone with information on the history of CMD in Uganda or elsewhere, and on the various approaches to control. They would be particularly interested to learn of possible precedents for the current pandemic in Uganda. There is a particular dearth of information on the disease situation in southern Sudan and eastern Zaire, where the Ugandan pandemic may have originated. Professor J M Thresh Natural Resources Institute University of Greenwich Chatham Maritime Kent ME4 4TB UK Dr G W Otim-Nape Namulonge Agricultural Research Institute PO Box 7084 Kampala UGANDACassava mosaic virus disease (CMD) is so prevalent in some countries that it is regarded as a normal feature of cassava plants; little attempt is made to control the disease and impaired, sub-standard yields are accepted as normal. There are no..