Biotic Stress and Crop Improvement: A Wheat Focus Around Novel Strategies - Crop Improvement: New Approaches and Modern Techniques - page 241

es has increased and the benefits have also resulted (Mujeeb-Kazi et al. 2008b ).
Maximum benefits on a practical scale has come from the closely related wild pro-
genitors like the D genome diploid grass Ae. tauschii that has resulted in variet-
ies in China, Spain, Afghanistan, Ecuador and an abundance of advanced varietal
candidate lines globally possessing biotic stress resistances and high yield levels
(David Bonnett, Personal Communication). Specifying the contribution of special
resources towards biotic stresses a brief consideration covers spot blotch, Septoria
tritici , Karnal bunt, Fusarium graminearum, powdery mildew, yellow rust and stem
rust, substantiated by inputs from other resources that are more divergent are out-
puts for spot blotch and stem rust.
Germplasm for Combating Biotic Stresses
The wheat family members are distributed within three gene pools; primary, sec-
ondary and tertiary (Jiang et al. 1994 ) and their utilization ranges from a relative
ease to complex based upon genetic distance and genetic affinity traversing from
perfect homology to genomic homeology. Details to elucidate the species distribu-
tion and range are provided in Dewey ( 1984 ), Kimber and Feldman ( 1987 ).
a. The conventional resource. The globally available accessions in ex situ gene
banks number approximately 800,000 of which 3 % are of wild wheats (Valk-
oun 2001 ). Those categorized as conventional wheats, land races and grouped
into winter, spring and facultative fall in this section from which special men-
tion will be made of some land races and naturally originated wheats that have
been in extensive use since mid-1970s that carry the spontaneous translocation
T1BL.1RS. National land races approach 1,000 in Pakistan of which 112 are
widely studied and have been evaluated for various parameters. They are further
unique diversity candidates to exploit for wheat improvement.
b. The unique wild/exotic gene pool resources including the diploid progenitors of
the primary and secondary gene pool plus various tetraploids with their contribu-
tion of derived usable stocks which address biotic stresses. In extensive use have
been the D genome followed by the A and in very limited use so far the B(S)
genomes. Capturing interest also are the tetraploids Triticum dicoccum , T. dicoc-
coides , T. carthlicum and the hexaploid T. spelta .
c. The tertiary gene pool species with their diversity made user friendly and specifi-
cally targeted for a biotic stress of current global significance and a potent threat
to wheat production will be addressed. The contributions from this pool have
been tabulated and reported by Mujeeb-Kazi et al. ( 1987 , 1989 , 2008a ), Sharma
and Gill ( 1983 ), Sharma ( 1995 ), Friebe et al. ( 1996 ), Mujeeb-Kazi (2003 , 2005 ,
2006a ), Trethowan and Mujeeb-Kazi ( 2008 ), Ogbonnaya et al. ( 2013 ) elucidat-
ing that almost all major stresses encountered in wheat cultivation plots can ben-
efit from alleles of value present in these resources.