ROOTS
CASSAVA MOSAIC AND CASSAVA BROWN STREAK VIRUS DISEASES IN AFRICA: A comparative guide to symptoms and aetiologies*
R.J. Hillocks and J.M. Thresh Natural Resources Institute, University of Greenwich Chatham Maritime, Kent ME4 4TB, UK
Introduction
Vegetatively propagated crops are prone to virus infection and cassava (Manihot esculenta Crantz) is no exception to this generalisation. At least seventeen different viruses of cassava have been described, of which eight are known to occur in Africa (Thresh et al., 1994). The main attention in Africa has been on the viruses causing cassava mosaic and cassava brown streak diseases which are the subject of this publication. Relatively little attention has been given to the other viruses of cassava or to the diseases they may cause. There is limited information on their distribution and none on their effects on growth or yield. These are serious deficiencies and emphasise the inadequate attention given to the viruses of what is arguably, the most important African food crop.
Cassava mosaic disease (CMD)
Distribution
CMD was first described in 1894 in what is now Tanzania. The disease was later reported in many other countries of East, West and Central Africa. It is now known to occur in all the cassava-growing countries of Africa and the adjacent islands and also, in India and Sri Lanka. A report of the disease in Indonesia in 1931, has not been confirmed and the mosaic disease of cassava in South America is caused by a different virus.
There are great differences between regions in the overall prevalence of CMD and in the severity of the losses caused. The available information from surveys and yield loss assessments is summarised by Thresh et al. (1997), who on plausible assumptions, estimate the losses in Africa to be 15–24%. This is equivalent to 15–28 million tonnes, compared with the FAO production estimates for 1997 of 84 million tonnes.
* This article was published in Roots, Volume 7 number 1 Special Issue, December 2000, the newsletter of the Southern Africa (SARRNET) and the East Africa Root Crops Research Network (EARRNET). Some of the photos in the published article are no longer available and a selection is presented in this text. If you wish to obtain a copy of the special issue of Roots please contact The Programme Manager, Crop Protection Programme. This publication is an output from the Crop Protection Programme of the United Kingdom Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of DFID Roots 7 (1) Special Issue December 2000
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Causal Agent(s)
When CMD was first described, the causal agent was assumed to be a virus, in the absence of any visible pathogen. This view was consistent with the results of early studies showing that the disease was transmitted by a whitefly vector, now known to be Bemisia tabaci. However, proof of the viral aetiology was not obtained until the 1970s and 1980s, when sap inoculations to herbaceous hosts were successful and virus isolates obtained in this way were purified and characterised. After initial uncertainty, the isolates were shown to cause CMD, Koch’s postulates were fulfilled and the various isolates from Africa and India were regarded as strains of a single virus of the geminivirus group and designated African cassava mosaic virus. Subsequent studies have led to the recognition of several distinct but similar viruses:
o African cassava mosaic virus (ACMV)
o East African cassava mosaic virus (EACMV)
o Indian cassava mosaic virus (ICMV)
o South African cassava mosaic virus (SACMV)
What appears to be a hybrid recombinant between ACMV and EACMV has been reported in Uganda, Kenya, Tanzania, Sudan and Democratic Republic of Congo and designated UgV. The different viruses have very similar properties and they are all members of the newly created family: Geminiviridae; Genus: Begomovirus (type member, Bean golden mosaic virus). Each of the cassava mosaic geminiviruses (CMGs) can cause CMD and there is evidence that virus combinations are more damaging than single infections.
These results have been obtained only relatively recently at the Scottish Crop Research Institute, Dundee and elsewhere (Harrison et al., 1995; Thresh et al., 1998a; Rey and Thomson, 1998). The full implications are as yet unclear and additional information is required on the distribution of the different viruses and on the interactions between them. Meanwhile, it is appropriate to refer to CMD in Africa or Asia caused by CMGs. There is no justification for referring to separate Indian, East African and South African diseases which would create needless confusion, given the limited facilities available in many parts of Africa for detecting and characterising the virus or viruses present in mosaic-affected plants.
Symptoms
The symptoms of CMD occur as characteristic leaf mosaic patterns that affect discrete areas and they are determined at an early stage of le