Biology Reference
In-Depth Information
Crossing Block(s)
Source population for
target ecology
Selection
scheme
Screen for:
resistance to diseases and
pests, root conformation (pre-
and post-harvest handling)
and plant characteristics
Seedling nursery 50,000-100,000
seedlings (no replication)
Year 1
Further screen for:
resistance to diseases and pests,
root conformation (pre-and post-harvest
handling) and plant characteristics
Evaluate for yield and food quality
characteristics
Further screen for:
resistance to diseases and pests, root
conformation (pre- and post-harvest
handling) and plant characteristics
Further evaluate for
yield and food quality
characteristics
Clonal evaluation 500-3,000
clones (1 replication)
Year 2
Preliminary yield trial 100-200
clones (2 replications)
Year 3
Advanced yield trial about
50 clones (4 replications)
Year 4
NARS
Evaluate for yield, food
quality and adaptation
in multilocations
Uniform yield trial 15-25
clones (4 replications)
Year 5
NARS
Feedback
On-farm trials (2-5 elite clones),
multiplication and release
Fig. 15.1.
Cassava breeding scheme at the International Institute of Tropical Agriculture, IITA used mainly in African
national breeding programs.
shifts to low heritability traits such as root yield.
The clones that show outstanding performance
in the regional trials are released as new culti-
vars, and eventually incorporated as parents in
the crossing nurseries. The long selection cycle
that results in the release of few cultivars implies
that the adaptation of molecular tools for cas-
sava genetic improvement is required to make
the process more efficient in order to reduce the
long process involved in classical breeding.
Breeding objectives depend on the ultimate
use of the crop. Genetic variability is limited in
M. esculenta
and, therefore, interspecific crosses
with other
Manihot
species to introgress useful
alleles have often been attempted. Stability of
production is associated with resistance or toler-
ance to major biotic stresses, and the emphases
vary with the target environments. Cassava is
normally propagated vegetatively, with stem cut-
tings. This process easily disposes the plant to
pests and disease, as planting materials are re-
used in the subsequent growing season, leading
to higher inoculum pressure, especially in sus-
ceptible cultivars, which have the tendency to
accentuate the rapid spread of diseases. In Africa
and India the key biotic constraint is CMD. From
the early years of research into CMD, it was
apparent to workers that cultivars varied in their
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