Biology Reference
In-Depth Information
than CMG-free plants, yields of plants infected
by ACMV were reduced by 42%, those infected
by severe strains of EACMV-UG by 68%, and
those with mixed ACMV
(Fauquet and Fargette 1990). Legg and Thresh
(2003) estimated an annual economic loss of
US$1.9-2.7 billion, making CMD, in economic
terms, one of the most globally important plant
virus diseases.
Productivity and stability of cassava are key
major objectives of breeding. Stability of pro-
duction is highly associated with resistance and
tolerance to major biotic stresses. CMD is one
of the major biotic stresses that severely lim-
its productivity and stability in cassava, and
accordingly CMD resistance is a primary objec-
tive and of paramount importance to successful
cassava breeding programs in Africa and India.
The aim of breeding for resistance is to pro-
duce cultivars with improved resistance to sev-
eral CMGs and high yields that persist under a
wide range of environmental conditions. Fortu-
nately genetic resistance to important diseases of
cassava has been identified (Hillocks and Wydra
2002; Ceballos et al. 2004).
The emergence of new viral diseases and the
increasing evolutionary capacity of the viruses
and their vectors to adapt and remain highly vir-
ulent is a huge challenge to breeding programs.
Durability of resistance is therefore a priority as
a breeding objective in Africa and India. Identi-
fying and pyramiding different virus-resistance
genes will therefore provide stable resistance
against a broad spectrum of CMGs. This is
very important and much more pertinent given
the recent discovery that CMGs can synergize
and cause unusually severe symptoms leading to
almost total yield loss in infected plants. Gene
pyramiding in cassava will require molecular
marker-assisted breeding to expedite the process.
Cassava's center of origin is Latin Amer-
ica, where huge cassava genetic resources are
available for genetic improvement. Wild Mani-
hot germplasm offers a wealth of useful genes
for the cultivated M. esculenta species (Hahn
et al. 1980; Chavarriaga et al. 2004). The Inter-
national Center for Tropical Agriculture (CIAT,
the Spanish acronym) has utilized wild Mani-
hot species for genetic improvement of key
novel traits for which genetic variation is highly
EACMV-UG infec-
tions by 82% (Owor 2002), highlighting the
impact of the synergistic interaction between
these two geminiviruses.
+
CMG Situation in Africa
Despite the demonstrated presence of EACMV-
UG in West Africa (Tiendrebeogo et al. 2009;
Akinbade et al. 2010), we have not yet witnessed
the emergence of a CMD pandemic in this part
of the African continent. The dominant viruses
infecting cassava in West Africa are ACMV and
EACMCV (Fondong et al. 2000 ; Ogbe et al.
2006; Alabi et al. 2007). On the other hand, it
is clear that these viruses have the capacity to
synergize in cassava fields and cause the drastic
“candle stick symptom” (Figure 15.2; Fondong
et al. 2000; Alabi et al. 2007). It is probable that
the explanation for why they have not yet done
this resides in the fact that the superabundant
whitefly populations from East Africa have not
been able to move and become established in
West Africa (Legg and Fauquet 2004). This does
not mean that this could not happen in the com-
ing years, and the co-prevalence of ACMV and
EACMCV, capable of a synergistic interaction,
in many West African countries suggests that the
viral components for a possible pandemic are
already present in West Africa. This also sup-
ports the breeding of cassava CMD-resistant cul-
tivars to prevent such a pandemic.
Breeding for CMD Resistance
In the last 75 years cassava mosaic disease
has severely affected cassava production in
Africa. The continued threat of a CMD pan-
demic in Africa could undermine production
gains achieved in the recent past. The economic
impact of CMD, as estimated by studies inves-
tigating possible yield loss worldwide has been
calculated at something between 20% and 95%
Search WWH ::




Custom Search