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evaluation of complex agronomic traits such as
yield then begins (Figure 15.1).
There is a high level of molecular diversity
in the CMGs and evidence that certain CMGs,
when present in mixtures, employ pseudo-
recombination or reassortment strategies and
recombination at certain hot spots, such as the
origin of replication (Stanley 1995; Deng et al.
1997; Zhou et al. 1997; Fondong et al. 2000; Pita
et al. 2001), resulting in the emergence of 'new'
viruses with altered virulence. Such examples are
EACMV-UG2, an ACMV-EACMV recombinant
component A, as well as EACMV-UG3, which is
a pseudo-recombinant component B, (Pita et al.
2001). These recombinant viruses combine parts
of the genomes of both ACMV and EACMV
and were implicated in the pandemic of severe
CMD that devastated cassava in much of East
and Central Africa (Legg and Fauquet 2004).
The severity of CMD is impacted by human
and environmental factors, as it is transmitted
through cuttings by the farmers and through
the natural whitefly vectors. If the transmission
through cuttings has been the prevalent mode of
transmission for a long time (Fauquet and Far-
gette 1990), recently it has been observed that
transmission through whiteflies played a major
role in the recent pandemic in East Africa (Legg
and Fauquet 2004; Patil and Fauquet 2009).
Given that CMGs are transmitted by whiteflies,
the spread of the virus is going to depend largely
on the vector. Temperature is the most impor-
tant environmental factor controlling the size
of the vector population (Fauquet and Fargette
1990). Vector preferred temperature estimates
vary from 20 to 30 C (Fargette et al. 1994) to 27
to 32 C (Thurston 1998), but generally high
temperatures are associated with high fecun-
dity, rapid development, and greater longevity
in whitefly (Fargette et al. 1994). Increased light
intensity has also been shown to increase activ-
ity of the whitefly vector (Thurston 1998). High
rainfall - more than 1200 mm -favors more
spread than in locations that are drier and have
a shorter growing season and where less cas-
sava is grown (Bock 1983; Fargette et al. 1994).
CMD is generally more virulent in hotter and
more humid regions, while the incidence of
Cassava Mosaic Disease
CMD is the most important disease threatening
cassava production in Africa. CMGs have been
reported from many cassava-growing countries
in Africa and India and the CMD induced by
them constitutes a formidable threat to cassava
production (Legg and Fauquet 2004; Patil and
Fauquet 2009). The CMD begomoviruses are
unknown in the Americas, the center of diver-
sity for cassava.
Geminiviruses are a large family of plant
viruses with circular, single-stranded DNA
(ssDNA) genomes packaged within geminate
particles. Members of the genus Begomovirus
have caused significant yield losses in many
crops worldwide (Varma and Malathi 2003).
The genome of CMGs consists of two DNA
molecules, DNA-A and DNA-B, each of about
2.8 kbp (Stanley et al. 2004), which are cod-
ing for different proteins responsible for different
functions in the infection process.
Nine species of CMGs have been identified
between Africa and South Asia based on their
genomic sequence and phylogenetic analysis.
They include representatives of seven African
and two South Asian species, namely African
cassava mosaic virus (ACMV), East African
cassava mosaic virus (EACMV), East African
cassava mosaic Cameroon virus (EACMCV),
East African cassava mosaic Kenya virus
(EACMKV), East African cassava mosaic
Malawi virus (EACMMV), East African cas-
sava mosaic Zanzibar virus (EACMZV), and
South African cassava mosaic virus (SACMV),
all from Africa, as well as the Indian cassava
mosaic virus (ICMV) and Sri Lankan cassava
mosaic virus (SLCMV) in Asia (Fauquet et al.
2008; Patil and Fauquet 2009). This number will
probably grow, resulting from a high rate of nat-
ural recombination between geminiviruses and a
high transmission rate of whitefly vectors (Patil
and Fauquet 2009).
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