Agriculture Reference
In-Depth Information
disappeared, whereas others appeared or increased in importance because they
continued to provide acceptable yields, even when infected. These developments
have been evident in many districts of Uganda and indicate how farmers can adjust
to the occurrence of severe CMD epidemics if there is a sufficient degree of
diversity amongst the local varieties being grown, or if they have ready access to
such material from elsewhere.
An obvious difficulty with this approach is that the adjustment takes years to
achieve, during which time substantial losses occur, farmers have considerable
difficulty in maintaining production and food security is jeopardized. Furthermore,
the outcome is not entirely satisfactory in that many of the local varieties being
grown became totally infected, albeit with a generally mild form of CMD. This
emphasises the need to improve the health status of local varieties and also the scope
for deploying resistant genotypes of the type available from IITA and elsewhere.
These were the main objectives of the various official interventions made in Uganda
in response to the epidemic, as discussed in the following section.
20.4.12 Control measures
In 1988, there was only limited research in progress on CMD in Uganda and the
development and release of virus-resistant varieties was not a high priority of the
National Cassava Programme. Moreover, there was little information from previous
research done in Uganda or elsewhere in Africa on which to base effective control
measures. This necessitated a greatly increased commitment to research on CMD
and its control. Much progress has been made, as is evident from the many
publications that have appeared since the first version of this chapter was completed
in 1997. Moreover, the experience gained in Uganda has been invaluable in
mounting similar control programmes in Kenya, Tanzania and elsewhere.
Because of the massive impact of the CMD epidemic, it was necessary to
introduce control measures on a large scale in Uganda before definitive results were
obtained and suitably resistant varieties were selected and made available.
Accordingly, and since 1989, various organizations funded relief and rehabilitation
schemes, but only some were arranged in close collaboration with research staff
(Otim-Nape
et
al.,
1994, 2000). This explains why different varieties and approaches
were adopted in mounting the various projects and why the results were very
variable and not always successful (Otim-Nape
et al.,
2000). Much of the variation
in outcome can be explained by differences in the resistance of the varieties released
to farmers and in the circumstances and way in which they were deployed. It soon
became evident that plantings made soon after the onset of the epidemic were likely
to be unsuccessful unless very resistant varieties were used. In epidemic areas
whiteflies were generally numerous, sources of inoculum were abundant, there was
seldom much separation between plantings and the inoculum pressure was so high
that susceptible varieties soon succumbed. Moreover, because of the mobility of the
whitefly vector this occurred even if somewhat isolated sites were selected, roguing
was practised and all infected plants were removed from the immediate locality
before planting commenced.
