Agriculture Reference
In-Depth Information
in a detailed economic assessment completed before the occurrence of the current
regional pandemic (Geddes, 1990).
Until recently there were few quantitative data to support these assertions, or on
which to base definitive estimates of the losses caused by CMD. Nevertheless,
various estimates have been made, most recently by Thresh
et al.
(1997), who
suggested overall losses in Africa of 15 to 24% of total production. This is equivalent
to 13-23 M.t., compared with estimated production at the time of 73 M.t. On these
assumptions, annual losses total US$1300 to 2300 million at a conservative value of
US$100
t-1
. In deriving these figures a broad distinction was made between three
distinct epidemiological situations, designated 'benign', 'endemic' and 'epidemic'.
The
benign
situation is currently represented by many parts of Tanzania and the
mid-altitude areas of Cameroon and Malawi, where the incidence of CMD is
generally low and mainly due to the use of partially infected stocks of planting
material. In these areas, the symptoms of CMD are not usually very conspicuous,
there is little spread by the whitefly vector (
Bemisia tabaci
) and disease control
measures are not considered essential. Some loss of crop is incurred, but it is
accepted by farmers and researchers as unimportant and there are no compelling
reasons to adopt CMD-resistant varieties or phytosanitation. Thus, the situation is
stable and there is a dynamic equilibrium between counteracting tendencies. One is
for the incidence of infection to increase, albeit slowly, due to a limited amount of
spread by vectors. The other is for the incidence to decrease because infected plants
tend to be underrepresented on further vegetative propagation. This is because the
debilitating effect of CMD on growth decreases the number of stems that are
suitable for use as cuttings and farmers tend to select these from the most vigorous
unaffected plants available. An additional factor is that infected cuttings grow less
vigorously than healthy ones, sustain greater mortality during the early stages of
growth and are more likely to be damaged during the first weeding. Moreover, CMD
is not fully systemic in infected plants, especially those infected at a late stage of
growth, and so they provide at least some uninfected cuttings. This so-called
'reversion' phenomenon has important epidemiological implications in restricting
the degeneration that would otherwise occur on successive cycles of vegetative
propagation (Fargette
et al.,
1994). The interactions between the various factors are
complex and have been modelled. These studies indicate that dynamic equilibria can
develop under different conditions and at incidences of disease determined by the
values of the assumptions made on rates of spread, the amount of reversion and the
extent to which infected cuttings are under-represented because of vigour/selection
effects (Fargette
et al.,
1994; Fargette and Vié, 1995; Holt
et al.,
1997).
The
endemic
situation is also stable, but completely different from the benign in
that there is such a high incidence of CMD that farmers have only limited access to
uninfected cuttings for new plantings. This is the situation in many important
cassava-growing areas of West and Central Africa, where farmers and even
researchers have become so accustomed to CMD that it is regarded as largely
inevitable and of no great significance. Yields are substandard, but they are accepted
as normal and specific control measures are seldom recommended or adopted.
However, farmers are discriminating in their choice of variety and adopt those that
grow and yield satisfactorily, despite the occurrence of CMD (Thresh
et al.,
1994b).
