Agriculture Reference
In-Depth Information
Little cassava is exported, but some roots are used locally to feed livestock or
poultry, or to prepare starch or alcoholic products. Production is almost exclusively
by small-scale farmers, many of whom are in the poorest sectors of society. The
very diverse methods of production and utilization were assessed in ten countries
during 1990-94 in the Rockefeller-funded Collaborative Study of Cassava in Africa
(Nweke, 1994). This showed big differences between areas in the importance and
overall status of cassava. However, in all countries there is great varietal diversity
and the crop is usually intercropped with one or more other species.
Cassava is routinely propagated vegetatively using hardwood stem cuttings.
Vegetatively propagated crops are particularly prone to virus infection and cassava is
no exception. At least 18 different viruses or putative viruses have been described, of
which eight are known to occur in Africa (Thottappilly
et al
., 2003). The most
important of these are the whitefly-borne geminiviruses (Family:
Geminiviridae
;
Genus:
Begomovirus
), which cause CMD. This was first reported in 1894 in what is
now Tanzania. The disease was soon shown to be transmissible by grafts and by
whiteflies and, in the absence of a visible pathogen, was attributed to virus infection.
However, no virus particles were detected until the 1970s, when sap inoculations to
cassava and also to
Nicotiana clevelandii
and several other herbaceous host species
were successful. The geminivirus so isolated was later characterized and caused the
typical symptoms of CMD when returned to cassava, so fulfilling Koch's postulates
(Bock and Woods, 1983). Subsequently, three similar but distinct cassava mosaic
geminiviruses (CMGs) were distinguished, of which two occur in Africa and the third
in the Indian subcontinent (Swanson and Harrison, 1994). The two African CMGs
were at first considered to have distinct and largely non-overlapping distributions, but
subsequent studies on a wider range of isolates have shown the situation to be more
complex than had been assumed. Moreover, five additional CMGs have been
distinguished recently (Fauquet and Stanley, 2003) and a novel recombinant virus or
strain has been associated with the current CMD pandemic in East and Central Africa
(Deng
et al.,
1997; Harrison
et al.,
1997a; Zhou
et al.,
1997).
There is little information on the early history of CMD in Tanzania or elsewhere
in Africa. The disease seems to have become increasingly important in the 1920s
and 1930s when cassava production expanded rapidly, and serious problems were
reported in many countries and research began in several. During this period, the
first attempts were made to select CMD-resistant varieties from those being grown
at the time in Africa, or introduced from elsewhere and resistance breeding
programmes were initiated (Jennings, 1994). These continue in several countries
and, since 1971, the most influential has been at the International Institute of
Tropical Agriculture (IITA), Ibadan, Nigeria. The overall research effort on CMD
has been considerable, as evident from the sequence of reviews and proceedings
(Fauquet and Fargette, 1988, 1990; Jennings, 1994; Thresh and Otim-Nape, 1994;
Thresh
et al.,
1994a; Otim-Nape
et al.,
1996; Thresh
et al.,
1998a,b; Legg, 1999;
Legg and Thresh, 2000, 2004; Calvert and Thresh, 2002; Thottappilly
et al.,
2003;
Thresh and Cooter, 2005). Indeed, the disease has received more attention than any
other virus disease of an African food crop, yet it remains prevalent in many areas
and is still regarded as the most important disease of cassava in Africa. Moreover,
CMD was rated as the most important vector-borne disease of any African food crop
