Tropical Root Crops - Crop Post-Harvest: Science and Technology - page 438

retention of retention of carotenoids in processed products
made from orange fleshed varieties because of the potential
benefits to health and nutrition. Bechoff et al . (2010) found
high losses in carotenoids during drying and storage. They
showed that total carotenoid retention during drying varies
more with variety than type of dryer (solar or sun).
Carotenoid loss was generally correlated with high initial
moisture content and high carotenoid content in fresh sweet
potato roots. Hot air cross flow drying however retained sig-
nificantly more provitamin A than other drying methods
tested. However, while some provitamin A is lost during
drying, much greater losses can occur during storage (about
70%) and this was not affected by the packaging (storage at
ambient temperature). For low cost storage of sweet potato
chips at ambient temperature, the losses of carotenoids dur-
ing storage are therefore considered to be more of a con-
straint to the utilisation of dried orange fleshed sweet potato
than losses occurring during drying (Bechoff et al . 2009).

roots cannot be stored in-ground for any significant period
of time. Surveys of root quality in the markets of Tanzania
have shown that, at certain times of the year, 15-20% of
roots that are sent to the market may be spoiled by
infestation (Kapinga et al . 1997). This is an underestimation
of the total levels of loss, since farmers usually leave
infested roots in the field.
There are a number of species of sweet potato weevil;
Cylas puncticollis and C. bruneus are the most prevalent
species in East Africa, while C. formicarius is the most
abundant in North America and the Far East. The female
sweet potato weevil lays eggs singly in cavities excavated
in either the vines or exposed/easily accessible roots. The
developing larvae tunnel while feeding inside the vine or
root and are the most destructive stage. Pupation takes
place within the larval tunnels and adults emerge after a
few days. Plants may wilt or even die as a result of extensive
stem damage, and damage to the vascular system can
reduce the size and number of storage roots. While external
damage to roots can affect their quality and value, internal
damage can lead to complete loss.
Several attempts have been made to breed for resistance
to Cylas spp. Variation in susceptibility to infestation among
sweet potato cultivars has been reported (Mullen et al .
1985, Stathers et al . 2003 and references therein). Breeding
programmes have lead to the release of cultivars in the
United States with a degree of resistance to Cylas formi-
carius . The most likely resistance mechanisms include:
escape via deep rooting (as weevils can only burrow short
distances); or early maturity (enabling farmers to harvest
roots before the onset of the dry season and the subsequent
increase in Cylas spp. populations); or non-preference
related to the chemical composition of the roots of different
cultivars. However, the rate of success in breeding for
non-preference has been slow, leading some breeders to
conclude that an adequate source of resistance may not
exist within the sweet potato germplasm (Talekar 1987).
Nevertheless, there are numerous reports of variation
among varieties in susceptibility to weevil attack. Stathers
et al . (2003) have related reduced susceptibility to deeper
rooting. There is some evidence of resistance based on
chemical composition of the root surface. The levels of two
triterpenoid components; boehmeryl acetate and boehmerol,
which are known ovipositional stimulants (Wilson et al .
1990), differed significantly in sweet potato cultivars that
differed in susceptibility to the C. formicarius elegantulus
(Summers). Selecting varieties with low levels of these
components may be a route to the selection of less preferred
and therefore less susceptible cultivars. However, consider-
able chemical variation within a cultivar was also reported

PESTS AND DISEASES OF SWEET POTATO

Sweet potato weevil
Damage to sweet potato storage roots by insect pests, even
when it occurs before harvest, can be considered a
post-harvest problem as it reduces both the nutritional and
economic value of the storage roots and can reduce shelf life.
The most important insect pest of sweet potato storage
roots worldwide is the sweet potato weevil ( Cylas spp.,
Coleoptera: Apionidae). In certain areas of East Africa, the
so-called rough weevil ( Blosyrus spp.), which damages the
surface of the root, is also starting to gain economic importance.
Sweet potato weevils constitute a major constraint to
sweet potato production and utilization worldwide
(Villareal 1982, Sutherland 1986, Chalfant et al . 1990,
Lenne 1991). Yield losses as high as 60-97% have been
reported (Ho 1970, Subramanian et al . 1977, Mullen 1984,
Jansson et al . 1987, Smit 1997). Even low levels of infesta-
tion can reduce root quality and marketable yield because
the plants produce unpalatable terpenoids in response to
weevil feeding (Akazawa et al . 1960, Uritani et al . 1975)
and consumers will pay only reduced prices for roots
damaged by Cylas spp. (Ndunguru et al . 1998). Sweet
potato weevils are a particularly serious problem under dry
conditions, because the insects, which cannot dig, can
reach roots more easily through cracks that appear in the
soil as it dries out. In much of East Africa, the sweet potato
crop matures after the end of the rains, and root bulking,
which has a tendency to shift the soil, often exposes roots
providing easy access for Cylas spp. It is for this reason
that during the dry season, unlike cassava, sweet potato