Agriculture Reference
In-Depth Information
1998). The virus has also been detected in samples of severely diseased cassava
collected even further to the west near Kinshasa, and also in Congo Republic
(Neuenschwander
et al.,
2002), and most recently in Gabon (Legg
et al.,
2004).
Currently these are the western-most areas where EACMV-UG has been detected,
but their significance is uncertain. It is unclear whether the recombinant virus is the
one causing the pandemic in eastern Africa and whether it occurs continuously from
Kinshasa and the Congo Republic in the west (longitude 15°E) to western Kenya in
the east (35°E). It is also not known whether spread has been from east to west or
west to east. These uncertainties will be difficult to resolve because of continuing
insecurity and poor communications in the region and the dearth of trained crop
protectionists and facilities for virus identification. Nevertheless, there is an obvious
threat to the important cassava-growing areas of Cameroon and westwards into West
Africa. Both parents of the EACMV-UG recombinant are known to occur in these
countries and a virulent form could arise at any time, even if not already present. It
is also likely that there will be spread westwards in Gabon and from Congo Republic
and DRC. This emphasises the importance of regular surveys to monitor the
changing situation, as in the OFDA-funded operations in eastern Africa and the
Great Lakes region. There are obvious problems of doing surveys in Central Africa
because of the difficult terrain, the vast areas at risk and chronic insecurity. It will
also be difficult to enforce quarantine controls on the movement of cassava material,
to avoid disseminating novel strains of virus or vectors into new areas. Nevertheless,
restrictions should be enforced and it is also important to introduce and propagate
CMD-resistant varieties so that they are available for distribution to farmers when,
or preferably before, the need arises.
20.4.10 Causes of the recent epidemic
The underlying reasons for the recent epidemic in Uganda are undoubtedly complex
and explanations have been sought so as to improve the effectiveness of control
measures and to facilitate predictions of the course of the subsequent pandemic in
the region and the areas most likely to be affected.
Any explanation must account for the main features of the epidemic, and
particularly:
•
the sudden increase in rates of spread of CMD that have occurred in areas
where the disease had previously spread slowly;
•
the consistent and predictable progress of the epidemic front southwards across
much of Uganda and on into western Kenya, north-west Tanzania, Rwanda and
Burundi;
•
the marked increase in disease severity and in whitefly infestations that are
associated with the onset of the epidemic;
•
the ecological characteristics of the epidemic that resemble those of a biological
invasion of new territory by a novel virus species or pathogenic variant.
