Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 248

durum breeding programs but can also complement the efforts with bread wheat im-
provement akin to what pentaploid breeding (2n = 5x = 35, AABBD) efforts provide
due to bread/durum crossings. This diversification comes from numerous acces-
sions of the diploid progenitors and is extended to cover the tetraploids not being
extensively used in the current breeding programs. The allelic richness that becomes
available to exploit does have a down side and needs voicing. In case where genetic
diversity is distributed in the two ploidy level crops by combining and shuffling
their genes, a fear exists that the spectrum of divergence may be reduced or elimi-
nated thus narrowing the base making germplasm derivatives of breeding programs
prone to greater susceptibility occurrences.
The Practical Output Contribution of Novel Genomic
Diversity
Apart from the conventional mode of wheat improvement is the newer trend where
close relatives of the primary gene pool are exploited for improvement programs
and categorized as interspecific breeding. The focus here is on the D genome that
could easily be extended to the A genome diploids and novel AB tetraploids. Though
direct crossing has greater precision this group's emphasis has been on adding di-
versity to wheat across its three genomes for global handling of complex stresses.
Hence bridge crossing has been favored and has contributed on the applied scale
as evidenced from global varietal releases, registration of stocks and generation of
pre-bred materials. Information has been captured from research conducted up to
2004 in CIMMYT Mexico and reported in Annual Wheat Newsletter of 2004 with
additional outputs from similar resource materials but environmentally removed to
Islamabad, Pakistan from 2005 until the present having the group leader as the com-
mon lead person throughout.
The most significant practical usage of the D genome synthetics for biotic stress-
es to date have been for Fusarium head blight, Septoria leaf blotch, Cochliobolus
spot blotch, karnal bunt, yellow, leaf and stem rust and powdery mildew. The strat-
egy has been to identify resistance first in the primary synthetic hexaploids and then
exploit those in crosses onto elite wheat cultivars that need such improvement. Each
of these biotic stresses are briefly discussed:
FusariumHeadScab
The internationally recognized Sumai 3 has over the past few decades stood as the
resistant standard for scab resistance and if the emphasis is on Type2 resistance
then acceptance of up to 15 % infection is considered as resistant since the central
inoculated florets get damaged, thus immunity is never present. The extensive test-
ing done in Mexico by the CIMMYT Wheat Wide Crossing program researchers