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Discussion
The resistance obtained to stem and stripe rusts in the
constituted line in the crosses involving HW 2005 may be due to the
resistance already present in the recurrent parents. However, the
cross involving CLRP 6 x HW 2005 resulted in the low susceptibility
to stripe rust with the rust severity of TS. The res-gene Lr24
is known to be very effective against all the leaf rust virulences
prevalent in India (Sawhney 1985a; Kochumadhavan et al. 1988). The
constituted lines may also carry the stem rust res-gene Sr24,
since Sr24 and Lr24 are tightly linked (McIntosh et al.
1977). The res-gene Sr24 also has been reported to be
effective against all the virulence of stem rusts prevalent in India
(Sawhney 1985a) except against the virulence 40-1 (Bhardwaj et al.
1990). However, Sr24 might provide only additional resistance
to other stem rust virulence than the virulence 40-1. Several workers
have developed lines carrying Sr24+Lr24 (Sears 1973;
McIntosh 1976; Kochumadhavan et al. 1988; Brahma et al. 1996) and
this gene continued to offer good resistance to leaf rust.
Recently several near isogenic lines of popular Indian cultivars
carrying A. elongatum derived resistance gene
Sr24+Lr24 viz. Kalyansona, Sonalika, WH 147, WH 542, C
306, Lok-1, HD 2329, HUW 234, PBW 226, NI 5439, HD 2285, HD 2402, HI
1077, HD 2009, UP 262, VL 421, WL 711 and HS 240 has been developed
(Kochumadhavan pers commun) at IARI-Regional Station, Wellington and
they are resistant against all the leaf rust races prevalent at
Wellington. Though some of the alien genes possess undesirable traits
including Lr24 (Gupta 1985), the lines which carries this
specific gene gave superior yields when compared to non carriers
(Brahma et al. 1996).
The yield tests of a number of backcross derivatives revealed that
with suitable selection procedure it was possible to achieve superior
or equal yield in comparison to the recurrent parent combined with
alien derived resistances to rusts without any undesirable effects
(Sawhney 1994).
The cross obtained with Veery 's' x PH 127 was resistant to all the
three rusts and it was supposed to carry two alien source derived
genes (Secale cereale derived linked gene Sr31, Lr26
and Yr9 and Ae. umbellulata derived Lr9).
Further study on the yield parameter and interactive nature of two
alien segments needed. Presence of these gene combination is expected
to give durable rust resistance. Durable resistance due to a single
gene such as Sr31 in the IB/IR translocation, for resistance
to stem rust are known but many cultivars that gave prolonged
resistance to stem rust were found to have a combination of genes
(McIntosh and Watson 1982). Van der Plank (1963) stated that
resistance derived through certain gene interaction is likely to be
more stable as compared to resistance determined by single gene
pair.
The effectiveness of Lr9 has already been reported by several
earlier workers. The successful transfer of Lr9 into
Kalyansona gave excellent resistance to leaf rust (Sawhney
1985b,1987). Kochumadhavan (1996, pers commun) transferred Ae.
umbellulata derived gene Lr9 and developed near isogenic
lines of several popular Indian bread wheat cultivars which are
conferring good resistance against all the prevailing leaf rust races
at Wellington.
The recurrent parents CPAN 4166, CPAN 4167 and CPAN 4168 when crossed
to HW 2037 (carrying Ae. speltoides derived gene Lr28)
the resultant progenies were resistant to stem, leaf and stripe
rusts. The donor parent is susceptible to both stem and stripe rusts
and hence the resistance to these rusts in the constituted line must
be due to the resistance already present in the recurrent parents to
stem and stripe rusts. The alien gene Lr28 was found to offer
high degree of field resistance (Kochumadhavan pers commun) and it is
reported to confer seedling resistance against ten important leaf
rust races (Sawhney and Goel 1983; Sawhney 1985a).
Though the alien genes Lr9, Lr24 and Lr28 offered
excellent resistance, many of these genes are highly influenced by
environmental factors as well as age, genetic background of the host
plants (Browder 1981; McGregor and Manners 1985). The alien genes
Lr9, Lr19, Lr24, Lr28, Lr31 and Lr32 were highly
resistant and the disease severity on the lines carrying these genes
never exceeded 10 MR (Gupta 1985). Person (1959) extended a concept
by suggesting that if a cultivar possesses two or more race specific
resistance genes, its pathogen could possess genes for
pathogenecity/virulence, each one corresponding to a particular
resistance genes. The existence of gene to gene interactions
therefore suggests that breeding for disease resistance must be a
continuous process. Hence, control of diseases through maintenance of
genetic diversity between cultivars for their resistance characters
constitutes another strategy that will reduce the possibilities of
major epidemics due to rapid spread of matching pathogenic races.
This is possible by improved breeding stocks with unexploited
resistance genes, particularly from alien sources, in agronomically
desirable backgrounds for easy recovery of resistance without
associated undesirable effects (Sawhney 1994). Hence, the developed
lines are quite useful by way of offering not only wheat lines with
genetic diversity and resistance to combat the rust but also will act
as genetic source for future breeding programs.
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