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Results and
discussion
Crosses between Unnath Kalyan Sona and the three Indian wheat
cultivars were highly successful. Screening for rust resistance were
made under natural conditions. The lines developed after five
successive backcrosses showed completely resistant to yellow rust
(Table1).
The lines alsoexhibited complete resistance to stem rust, although
the original Unnath Kalyan Sona was susceptible to the same rust
(Table1).
It is not known what caused the breakdown of Sr24 gene in
Unnath Kalyan Sona, despite the fact that the linked leaf rust
resistant gene Lr24 is still effective. Crossing over between
the two genes was ruled out since there is close linkage between them
(McIntosh 1988). Susceptability of a resistant line may be due to
spread of new races with matching pathogenicity. However, under the
same experimental conditions at Wellington, the only stem rust race
40-1 (62G29-1) that knocks down Sr24 did not show any symptoms
on the original Australian hexaploid wheat donor from which the
linked gene complex Sr24 + Lr24 was derived. It has
also been shown that under Wellington conditions, the leaf rust
resistant gene Lr24 is epistatic over Sr24. Therefore,
only resistance showed by Lr24 will be seen (Kochumadhavan et
al 1988). However, the lines developed after the backcrosses showed
completely resistance to stem rust and leaf rust along with yellow
rust resistance.
Restoration of stem rust resistance in Unnath Kalyan Sona after
crossing with three Indian Wheat cultivars (resistant to yellow and
stem rust at Wellington) was probably due to transfer of an
additional unidentified and undesignated stem rust resistant gene
from donor parents to the recepient parent, and this gene is
combination with Sr24 provided the durable stem rust
resistance in the newly developed lines. It was already reported that
rust resistance provided by single genes is not completely effective
and that durable rust resistance is provided only by combination of a
few resistant genes (Roelfs 1988, McIntosh 1988, Roelfs et al 1992).
Singh and McIntosh (1986) reported that durable stem rust resistance
in wheat cultivar Kenya Plume is due to eight genes. Since the
constituted lines developed resistance simultaneously to yellow rust
and stem rust, the undesignated yellow rust resistant gene and
undesignated stem rust resistant genes might be linked to each
other.
References
Kochumadhavan K, Tomar SMS and Nambisan PNN (1988) Transfer of
rust resistance genes into commercial cultivars of wheat. Annual
Wheat News Letter 34: 54-55.
McIntosh RA (1988) The role of specific genes in breeding for durable
stem rust resistance in wheat and triticale. In: Breeding Strategies
for resistance to the rusts of wheat (eds NW Simmonds and S Rajaram),
CIMMYT Report, pp 1-9.
Roelfs AP (1988) Resistance to leaf and stem rusts in wheat. In:
Breeding strategies for resistance to the rusts of wheat (eds NW
Simmonds and S Rajaram), CIMMYT Report, pp 10-22.
Roelfs AP, Singh RP and Saari EE (1992) Rust diseases of wheat.
Concepts and methods of disease management. CIMMYT Report, p 81.
Singh RP and McIntosh RA (1986) Genetics of resistance to Puccinia
graminis tritici and Puccinia recondita tritici in Kenya
Plume wheat. Euphytica 35: 245-256.
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