Cassava Mosaic Virus Disease (CMD)
|Taxonomy and distribution
Cassava mosaic virus disease (CMD) is caused by cassava mosaic geminiviruses (CMGs), in the family Geminiviridae and genus Begomovirus. Three species are recognised: African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV) and South African cassava mosaic virus (SACMV). A number of variants or strains have been described, the most widely reported of which is the Uganda variant form of EACMV (EACMV-Ug), which is a recombinant virus combining parts of the genomes of both ACMV and EACMV. CMGs are sub-microscopic pathogens viewable only with the help of an electron microscope. They comprise wo join‘sister’ particles which together are approximately 20 by 30 nm in size. The genome is made up of a pair of single-stranded closed circular DNA molecules referred to as the ‘A’ and ‘B’ particles. These are enclosed by a protein coat.
Figure 1. Distribution of cassava mosaic geminiviruses in Africa.
occurs virtually throughout the cassava growing areas of Africa, with
the apparent exception of coastal East Africa,
where EACMV occurs largely alone (Figure 1). EACMV occurs together with
ACMV in the Great Lakes zone of East and Central Africa. In parts of
Central Africa and West Africa, EACMV occurs at low frequency, and is usually
found only in mixed infections together with ACMV. EACMV-Ug occurs in Uganda,
western Kenya, north-western Tanzania, north-eastern Rwanda and southern
Sudan. There are further provisional reports from eastern Democratic Republic
of Congo (DRC) and the Republic of Congo. EACMV-Ug is expanding its range
at rates varying from 20-100km per year.
and economic importance
CMD symptoms are variable according to season and variety, but always include a chlorotic mosaic on infected leaves (Figure 2). The colour of chlorosis varies from pale green to whitish yellow and the extent of chlorosis varies from almost 100% of the leaf surface to less than 5%.
|In resistant varieties, plants may ‘recover’, a phenomenon in which newly emerging leaves of an infected plant sprout without symptoms.|
Figure 3. Whitefly-borne
dual (ACMV+EACMV-Ug) infection in a susceptible cultivar.
moderate to severe infections, leaves also exhibit crumpling, the laminae
distorted, and the size is reduced. These effects lead to the stunting
of the growth of the plant. There are no symptoms on the stem or roots.
Whilst symptoms produced by different CMGs or CMG strains may differ
in severity, the general characteristics cannot be readily distinguished.
Mixed CMG infections occur commonly and typically give rise to the most
severe symptoms (Figure 3). Reduced photosynthetic activity resulting
from chlorosis caused by CMD leads to reduced tuberization and smaller
or no yield at all. Africa-wide losses to CMD have been estimated at
between 12 and 23 million tonnes, representing 15-24% of total production.
Transmission and epidemiology
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