Agriculture Reference
In-Depth Information
tend to aggregate on the dark green areas of the laminae that leads to crowding and
can be expected to promote emigration.
These findings provide an explanation for the increased whitefly activity and
virus spread that have been such a marked feature of the epidemic in Uganda and
elsewhere. Spread is also likely to be facilitated by the increased concentration of
virus that occurs in plants dually infected with ACMV and EACMV-UG (Harrison
et al.,
1997b). Nevertheless, additional studies are required on the complexities of
the interaction between virus, host and vector. It is particularly important to explain
how the limited amount of dark green tissue produced by severely diseased plants
can generate such large populations of whiteflies. There is also a need to account for
the large population of whiteflies and direct feeding damage now being reported on
Nase 4 (SS4) and several other recently introduced varieties, even though these are
resistant to CMD and seldom become infected and express symptoms.
20.4.11 The rôle of varietal diversity
The recent epidemic in Uganda was undoubtedly exacerbated by the widespread
cultivation of Ebwanateraka and several other farmer-selected varieties that proved
to be extremely vulnerable to the severe form of CMD. Ebwanateraka is thought to
have originated in Soroti district in the 1970s and was first included in official
variety trials there in 1983. CMD was not a problem in the area at the time and in
these circumstances of low infection pressure, the inherent vulnerability of
Ebwanateraka to CMD was not apparent and the variety was soon grown extensively
in Soroti and several other districts, where it largely displaced the many other local
varieties being grown previously. The history of Bao and other vulnerable varieties
is likely to be similar to that of Ebwanateraka, but it is less well documented.
Many other varieties of local origin are grown in Uganda and farmers often grow
mixtures of several different varieties, especially in areas to the west and south
where cassava is grown mainly for local consumption. There is similar genetic
diversity in many other parts of Africa, and a rapid turnover in the varieties grown
(Nweke
et al.,
1994). This occurs as new ones become available from introductions
or by selection from self-sown seedlings and as existing varieties fail because of
their unacceptable vulnerability to one or more of the prevailing pests or diseases, or
for some other defect. Varieties that are severely affected by CMD grow so badly
that they are unlikely to survive in areas where a virulent form of the disease is
prevalent and, inevitably, there will be a conscious or unconscious trend towards the
cultivation of varieties that escape or tolerate infection. Conversely, vulnerable
varieties can emerge and may even become dominant in areas of low inoculum
pressure where other selection criteria are paramount.
These considerations are consistent with the trends observed in Uganda where
the impact of the 1990s epidemic was greatest in Soroti and other areas where
cassava was being widely cultivated; here there was limited diversity and the main
varieties grown were extremely vulnerable to infection. In contrast, in areas of
diversity there were marked shifts in the relative importance of the different varieties
being grown. Some vulnerable ones that were widely grown almost completely
