Spread into Rwanda of the severe cassava mosaic virus disease pandemic and associated Uganda variant of East African cassava mosaic virus (EACMV-Ug)

Cassava is a key food security staple in the Great Lakes zone of East/Central Africa. During the 1990s, a devastating pandemic of unusually severe cassava mosaic disease (CMD) spread through Uganda and into the neighbouring countries of Kenya and Tanzania, caused principally by the emergence and spread of the Uganda variant of East African cassava mosaic virus (EACMV-Ug)(Geminiviridae: Begomovirus) (Otim-Nape et al., 1997). In July 2000, a survey was conducted in Rwanda to determine the presence or absence of severe CMD and EACMV-Ug. Twenty-six farmers' cassava fields were assessed in the major cassava-growing regions of the country. At each site visited, assessments were made of CMD incidence, severity (using a 2-5 scale) and infection type (whitefly- or cutting-borne), and leaf samples were collected from two symptomatic plants for virus diagnosis. A standard PCR diagnostics procedure was used, in which specific primer pairs for African cassava mosaic virus (ACMV) and EACMV-Ug were used for detection (Zhou et al., 1997).

CMD incidence was low throughout the areas surveyed (26.7%) and there were no clear differences between administrative regions. By contrast, CMD symptoms were markedly more severe in the north-eastern Umutara administrative region (score 3.6)(Figs. 1/2) than in the other areas surveyed (2.4) and in Umutara, most infection was whitefly-borne (83%) whilst in other areas infection was largely cutting-derived (81%). These contrasts in patterns of CMD severity and infection type matched virus distributions. ACMV was detected in 26 samples, collected from locations throughout the surveyed area. EACMV-Ug, however, was only detected in 6 samples, all from Umutara. One EACMV-Ug infected plant was also infected with ACMV. Mean severity scores for virus-tested samples were: ACMV (2.9), EACMV-Ug (4.0), ACMV+EACMV-Ug (5.0). These data together provide evidence for the recent spread into Rwanda of the EACMV-Ug associated pandemic of severe CMD, almost certainly through migration of viruliferous whitefly populations from the neighbouring countries of Uganda and/or Tanzania, which had been affected in previous years (Legg, 1999). As a consequence of this development, there is an urgent need for CMD control, through the deployment of host plant resistance, as has been done in Uganda, Kenya and Tanzania.