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Cassava
and sweetpotato are very important starchy
root crops in many parts of the tropics where
they are vital staples for over half a billion
people. These two root crops play significant
roles in the farming and food systems in
Eastern Africa
and
both are often grown on the same farm. They
store well in the soil as a famine reserve
crops, have high productivity per unit area
and perform relatively well in marginal soils,
which makes them ideal crops for food
security. These crops appeal to low income
earners because they offer the cheapest source
of food calories and can be used as cash
crops. They can also be processed to produce
industrial starch and livestock feeds.
Because
of their significance, cassava and sweetpotato
are high priority commodities in the research
and development agenda of national
agricultural research programmes in the
eastern Africa region. Thus
sustainable cassava and sweetpotato production
can contribute significantly towards the
millennium development goals.
However,
productivity of these two important crops is
limited by both biotic and abiotic constraints
leading to poor yields at farm level. Among
the leading biotic stress are cassava mosaic
virus disease (CMVD), cassava brown streak
virus disease (CBSVD), cassava bacterial
blight, cassava mealybugs and green spider
mites. Sweetpotato production is mainly
constrained by sweetpotato virus disease
complex (SPVD) and weevils.
In
recent years, increased prevalence of cassava
and sweetpotato diseases particularly those of
viral origin has been reported. The
devastating CMVD, CBSVD and SPVD epidemics
have been associated with great yield losses,
and in many cases the once elite cultivars
have become extinct.
Viral
diseases account for up to 50% yield losses.
Cassava and sweetpotato are difficult crops to
breed using conventional methods due to low
seed set and a phenology that is highly
influenced by environment e.g. time to
flowering. In the case of sweetpotatoes,
conventional breeding is relatively
intractable due to challenges of sexual
hybridization, resulting from hexaploid nature
of the crop. However, several cultivars have
been reported in the region with possible
varying levels of resistance/tolerance to
diseases, and nutritional and agronomic
attributes. In many respects the high level of
genetic diversity observed in farming
communities also represents the farmers’
selection, mainly from spontaneous
recombination. Hence the landraces may possess
high frequencies of genes required for
adaptation to biotic stresses and for food
quality characteristics. This raises
possibilities for improvement using available
germplasm.
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