Agriculture Reference
In-Depth Information
There is also increasing use of improved varieties, selected at IITA or elsewhere
partly for their ability to resist or tolerate infection with CMD. Stocks of these
varieties that are known to be free of CMD are seldom available, the removal
(roguing) of diseased plants is seldom practised and the benefits to be gained from
adopting such phytosanitation measures are unclear.
In ecological terms, the
epidemic
situation is one of acute instability and
represents an extreme perturbation of the dynamic equilibrium to be expected
between an obligate parasite and its host(s). As in the endemic situation, there is a
high incidence of CMD, but symptoms are so severe that plants are stunted and
produce little or no yield. Food security and economic livelihoods are threatened and
the whole future of the crop in the area is jeopardized. Clearly, this is an
unsustainable situation and farmers have little alternative but to decrease the area of
cassava grown and switch to other crops, at least until such time as the epidemic
abates or effective control is achieved using resistant varieties or by other means.
For these reasons, the epidemic situation is unlikely to be widespread or
sustained for long and has seldom been reported. However, this may be because of
the limited research on cassava in many parts of Africa. At least some outbreaks
may be overlooked or unreported, or their full significance is not appreciated until
farmers have worked out their own solutions by changing variety or decreasing the
area of cassava grown. There are only brief accounts of the early epidemics
associated with the expansion of cassava production in East and West Africa in the
l920s and 1930s and of the more recent epidemics in Cape Verde (Anon., 1992) and
Akwa Ibom State, Nigeria (Anon., 1993). The 1940s epidemic in Madagascar is
relatively well documented (Cours
et al.,
1997). The recent epidemic in Uganda has
been followed in unprecedented detail and is considered here.
20.3 CASSAVA AND CASSAVA MOSAIC DISEASE IN UGANDA
Cassava was first reported in Uganda during the latter part of the nineteenth century
and it was grown on only a limited scale until the 1920s, when production expanded
rapidly. As in many other parts of Africa at the time, this expansion was soon
followed by reports of CMD, which was first recorded in Uganda in 1928. By 1934,
CMD was of considerable importance in many areas, including what are now Soroti
and Kumi administrative districts. (Fig. 20.1 locates the different districts of Uganda
mentioned in this chapter). There was almost total infection of the main varieties in
the worst affected areas. New varieties and breeding lines introduced from Tanzania
were evaluated for resistance and concerted breeding efforts led to the selection of
CMD-resistant varieties. These were multiplied rapidly in the 1950s and distributed
in quantity to farmers (Jameson, 1964). The use of such varieties, and the measures
of selection and roguing, were enforced by local government statute and led to the
control of the disease for several decades. This satisfactory situation seems to have
continued in the 1970s and into the 1980s. However, little detailed information is
available on the incidence of CMD during this period and the disease received only
limited research attention until new problems were reported in the late 1980s and the
latest investigations began.
