Association of East African cassava mosaic virus-Uganda (EACMV-UG) with cassava mosaic disease in Sudan

Cassava mosaic disease (CMD) caused by whitefly-transmitted begomoviruses has a dramatic impact on cassava production in Africa. A severe CMD epidemic has been caused by a recombinant strain of East African cassava mosaic virus (EACMV-UG), which has been spreading steadily from north-central Uganda into East and Central Africa. This virus presents a significant threat to cassava cultivation on the continent. EACMV-UG, in mixed infection with African cassava mosaic virus (ACMV), leads to severe disease symptoms (Gibson et al., 1996), the decline of the crop and often results in complete loss of harvestable roots. Various distinct begomoviruses implicated in CMD have been reported from significant cassava growing regions in Africa (Fodong et al., 2000; Were et al., 2004; Ariyo et al., 2005) revealing a complex virus situation. Some of these begomoviruses cause severe disease symptoms in cassava. However, only in mixed infections of the recombinant strain EACMV-UG and ACMV has an epidemiological significance been demonstrated.

Accordingly, a virus survey was conducted in 2002 in the major cassava growing regions in southern Sudan. Leaf samples were taken and stem cuttings established in the field at the Agricultural Research and Technology Corporation, ARTC, Wad Medani, Sudan. Total DNA extracts from leaves were subjected to differential PCR using specific primers for all ACMV and EACMV species and strains defined to date (Ariyo et al., 2005). Of 14 samples with conspicuous CMD symptoms, 13 contained the severe form of the recombinant EACMV-UG strain. Using several infected cassava samples, this finding was confirmed by sequence analysis of PCR amplicons obtained from DNA A genome fragments (rep gene, intergenic region and coat protein gene). ACMV was found in one sample as a single virus infection and in two other samples in a mixed infection with EACMV-UG.

The detection of EACMV-UG in almost all of the samples tested provides evidence for the potential epidemiological significance of this virus in the occurrence of CMD in southern Sudan.