Agriculture Reference
In-Depth Information
The situation changed in the early 1990s when it became apparent that the Kumi
epidemic was an extension of the earlier one in Soroti, that much of central Uganda
was affected and that the epidemic was continuing to spread southwards. In
subsequent years the epidemic and associated whitefly infestations continued to
spread southwards into Mpigi, southern Mukono and Iganga and eventually reached
Lake Victoria near Jinja township in 1996. Other lake shore areas to the east and
west were then affected and the progress of the epidemic was monitored in detail by
sequential observations along a north-south transect in southern Mukono district
(Colvin
et al.,
2004). Kalangala Island in Lake Victoria was first affected in 1997
and the epidemic continued to spread into Masaka and Rakai and eventually reached
the south-west districts bordering Tanzania and Rwanda. Moreover, the areas that
were affected first entered a post-epidemic recovery phase as the problem abated,
cassava plantings increased and production was restored.
The extent and rapidity of the changes that have occurred in the incidence and
severity of CMD and in the varieties of cassava grown in Uganda since the epidemic
was first reported in 1988 are evident from comprehensive surveys of large areas of
the country in 1990-92 (Otim-Nape
et al.,
1998), 1994 (Otim-Nape
et al.,
2001) and
2003 (Bua
et al.,
2005). There have also been surveys of a smaller number of
representative districts in 1997 and annually between 1998 and 2001. Nevertheless,
the information available is incomplete because official access to some northern and
western districts has been restricted or impossible because of continuing insecurity.
Despite these difficulties, it is apparent that the epidemic spread across all parts of
central and southern Uganda during the 1990s and that by the end of the decade
virtually all areas were in a post-epidemic phase.
The current situation in post-epidemic areas is variable and related to the overall
status of cassava, and the extent to which CMD-resistant varieties have been
adopted. The greatest impact of the epidemic has been in Kumi, Soroti and other
districts where cassava was particularly important and where there had been an
almost total reliance on Ebwanateraka, or one of the other farmer-selected varieties
that proved to be extremely vulnerable to CMD. In these districts there was an initial
drastic decline in production as the original cassava was largely eliminated. The
subsequent recovery was associated with a big increase in varietal diversity as
farmers introduced varieties from other parts of Uganda and CMD-resistant varieties
were made available through the many different cassava rehabilitation projects. This
is apparent from the surveys that also revealed a generally high incidence of
infection in the farmer-selected varieties and much use of infected cuttings as
planting material. There was a relatively low incidence of infection in the CMD-
resistant varieties, especially Nase 3, Nase 4 and the recently released TME 14 (Bua
et al.,
2005).
Another feature of the most recent survey results is that the symptoms in the
local varieties are generally less severe than those encountered earlier at the height
of the epidemic. This is apparent even in Ebwanateraka and other vulnerable
varieties that have persisted in many areas, or reappeared after almost disappearing
for several years. Many of these varieties now grow and yield satisfactorily,
although almost totally infected. These developments can be explained by the
emergence of somewhat tolerant landraces and by the occurrence of avirulent virus
