Agriculture Reference
In-Depth Information
suggesting that simple selection based on the presence of
these compounds was not so straightforward (Marti
et al
.
1993). More recently, Stevenson
et al
., 2009 identified
chemical compounds in one Ugandan sweet potato variety
New Kawogo that are directly toxic to weevils and increase
mortality although development was not hindered for those
larvae that survived. These findings suggest that potential
for breeding improved African varieties with resistance to
C. puncticollis
and
C. brunneus
.
the infection can spread to progeny roots. Thus dipping
seed roots in fungicide before planting is a recognised
practice. Some cultivar resistance does exist.
Rhizopus
rot can be caused by several species of
Rhizopus
but mainly
R. stolonifer
. This is characterised by
soft watery lesions, and copious development of coarse
white mould with globular sporangia, turning from white
to black. In store the infection spreads fairly rapidly to
adjacent roots creating 'nests' of rotting. This pathogen is
both soil and airbourne.
Other important rots include Sclerotium rot caused by
Corticium rolfsii,
Soil Rot/Pox caused by
Streptomyces
ipomoeae
and Scurf caused by
Monilochaetes infuscans
(Snowdon 1991).
Post-harvest rots of sweet potato
Many of the rotting pathogens are present in the soil, and
infect through wounds made during harvesting. Thus,
harvesting practices to minimise damage, curing, and fun-
gicide treatment immediately after harvest are all effective
ways to reduce losses.
Low temperature storage reduces rot incidence. The
main post-harvest rots of sweet potato are described by
Snowdon (1991), and these are summarised below.
Black rot of sweet potatoes is caused by
Ceratocystis
fimbriata
and this is an important rot of sweet potatoes in
most regions of the world. The main symptoms are slightly
sunken brown/black circular lesions, and a bitterness
throughout the flesh due to phytoalexin production. Crop
rotation is a good preventative measure as the pathogen
does not attack many other crops.
Fusarium
rot is caused for example by
F. oxysporum
or
F. solani
. The type of decay is variable, either a dry decay
from the ends or a surface rot of pale brown circular
lesions. Some cultivars exhibit resistance, and therefore
there is potential in breeding for resistant varieties.
Java black rot caused by
Botryodiplodia theobromae
was first observed in roots imported to the USA from
Java. This disease is common throughout the tropics.
Infected tissue is yellowy-brown and firm, later darken-
ingto black. It is important to make sure that any roots
used for planting material are free from infection, as
STORAGE, HANDLING AND PROCESSING
OF YAMS
The yam tuber is prone to mechanical damage during
harvest from the soil when simple traditional tools such as
cutlass and hoe are used as these readily cut or injure the
large-sized tubers. Any rupture of the tuber caused by
damage or injury leads to deterioration and dehydration.
Despite its obvious advantages curing does not appear to
be widely practised in West Africa. Research undertaken in
Ghana, Cote d'lvoire and Nigeria has estimated that
10-50% of yams produced and harvested are lost in storage
(Amusa
et al
. 2003, Rees & Bancroft 2003).
Ideal storage conditions for most species are 15-16°C and
70-80% relative humidity (Anon 1982). Under these
conditions storage for 6-7 months is possible. In West Africa
where most yams are grown temperature control is not an
option, and ambient storage leads to shorter dormancy and
more rapid sprouting. Storage structures used range from pits
or mounds of tubers placed directly on the ground to
constructions where shade is provided, the tubers are off the
ground and have a degree of ventilation. A simple classifica-
tion of types of storage is given in Table 18.9. The maturity
Table 18.9
Simple Classification of Yam Storage Structures and Their Characteristics.
Storage structure
Characteristics
Mounds/clamps
Prone to insect, nematode damage - maintain tubers at high humidity.
Pits
Prone to insect, nematode damage - maintain tubers at high humidity.
Mounds/clamps/pits
+
Shade (tree or
Prone to insect, nematode damage - maintain tubers at high humidity,
lower temperature.
built shelter
Structures that raise tubers above the
ground (wooden or bamboo shelf).
Protect tubers from pests - maintain tubers at lower humidity.
Structures that increase ventilation (yams
tied to poles or hung, yam barns).
Protect tubers from pests, and maintain tubers at lower humidity. Labour
intensive. Allow easy inspection of tubers for rots and sprouting.
