Key words: Fast rusting, Backcross lines, Stem rust, Resistance, Susceptibility
Introduction
Wellington, situated in the Nilgiri hills, South India at an
altitude of 1850 m. (above msl.) is a hot spot for rusts, powdery
mildew and other foliar diseases of wheat. All the three wheat rusts,
Puccinia graminis Pers. f. sp. tritici, P. recondita
Rob. ex Desm. f. sp. tritici and P. striiformis f.
sp. tritici with a wide spectrum of pathotypes occur
throughout the year in Wellington. Rusts begin to appear on highly
susceptible genotypes on seedling after 4th week of planting under
natural conditions and thus are highly destructive.
Alien specific genes, namely, Lr9 (Aegilops
umbellulata), Lr19, Lr24 (both from Agropyron
elongatum), Lr28 (Ae. speltoides), Lr32
(Ae. squarrosa) and Lr37 (Ae. ventricosa)
confer high degree of adult plant resistance to leaf rust at
Wellington (Tomar and Menon 1998). A backcross program was initiated
during 1988 to introgress these genes into popular Indian bread wheat
cultivars which were susceptible to leaf rust. While transferring
these genes by repeated backcrossing authors observed the improved
cultivars (backcross lines) to be highly susceptible to stem rust as
compared to their recurrent parents. Investigations were, therefore,
carried out to screen the backcross lines carrying Lr28 and
Lr32 against stem rust at different plant growth stages to
know whether high susceptibility to stem rust is closely associated
with leaf rust resistance as compared with their respective recurrent
parents.
Materials and methods
The material used in the present study comprised of (a) two donor
parents viz., CS 2A/2M 4/2 carrying Lr28 and C86-8/Kalyansona
F4 having Lr32, (b) recurrent parents, namely,
C306, HD2329, J24, Kalyansona, N15439, Sonalika, WH147, WH542 and
HS240 and (c) backcross lines carrying Lr28 and Lr32.
Since Lr28 and Lr32 are dominant genes,five to nine
backcrosses were given in quick succession and backcross lines were
constituted after three generations of selfing at F3
generation. Backcross lines were similar to their recurrent parents
in all respect like height, heading, maturity and seed
characteristics except that they carried genes Lr28 and
Lr32 conferring resistance to leaf rust. The backcross lines
along with the recurrent parents were planted in paired rows. Stem
rust inoculum collected from local and farmer's field was sprayed
after 40 days of planting. Observations on susceptibility/resistance
to stem rust and its progress was recorded at regular intervals at
different stages of plant growth. The rust reactions were scored
according to the modified Cobb's scale. The experiment was conducted
for two seasons. A trial to assess yield potential of two backcross
lines carrying Lr28 was also conducted during 1997-98 season
in RBD with four replications under optimal growing conditions.