Agriculture Reference
In-Depth Information
In contrast, such an approach achieved at least partial success when adopted
during the later post-epidemic stage when there had been a big reduction in the
amount of cassava grown, sources of infection were relatively few, planting was
done in large blocks and stringent roguing was promoted. Even greater success was
achieved where suitably resistant varieties were available, but the problem was to
produce sufficient quantities of such material to meet the enormous demand,
especially in districts where only vulnerable varieties had been grown previously.
This explains the importance of the early Gatsby, Vision Terudo and other NGO-
funded projects, in which the emphasis was on CMD-resistant varieties that offered
prospects of a lasting solution to the CMD problem.
Cassava production has been restored in Uganda and economic assessments have
been made of the great benefits obtained from the substantial investment in
agricultural research and development necessitated by the epidemic. However, there
is a need to avoid an undue dependence on the few resistant varieties that have been
released to date. It is also important to improve the generally unsatisfactory health
status of the many local varieties that are still being grown and which are almost
totally infected. Only limited attention has been given to this aspect of control and
the most appropriate measures to adopt and the benefits to be gained have not been
determined. It is likely that much could be achieved by selecting healthy planting
material and by adopting other phytosanitation measures. There is also scope for
evaluating mild strain protection and appropriate crop disposition and for exploiting
biodiversity as a means of control by using intercrops or varietal mixtures of
resistant and susceptible varieties (Thresh and Otim-Nape, 1994). Another priority is
to obtain the funding required to utilize the experience gained in Uganda and mount
effective rehabilitation projects on a suitably large-scale in Kenya, Tanzania and
other countries affected more recently. Ideally, stocks of resistant varieties should
also be introduced to areas ahead of the epidemic for use when required. This has
seldom been possible, because of the limited funds allocated and the understandable
emphasis on areas already affected and in greatest need.
20.5 GENERAL EPIDEMIOLOGICAL FEATURES OF CASSAVA MOSAIC
DISEASE
CMD is likely to be an unfamiliar disease to many readers of this volume.
Nevertheless, it is important in providing an example of general epidemiological
features of wide applicability:
•
As a 'new encounter' disease
(sensu
Buddenhagen, 1977) arising from the
juxtaposition of an exotic crop and an indigenous pathogen. Several other
important African virus diseases are of this type, including groundnut rosette,
maize streak and cacao swollen shoot (Thresh, 1985).
•
As one of the many tropical diseases to have caused serious problems following
an intensification of cropping practices (Thresh, 1985; Bos, 1992). Rice tungro
in South-East Asia and rice yellow mottle and maize streak in Africa are other
well-known virus diseases of this type.
