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Materials and methods
Seeds of nine Thatcher (Tc) near-isogenic lines carrying the
genes Lr21, Lr22a, Lr22b and Lr38 through
Lr43 were obtained from Dr. P.L. Dyck (195- Dafoe Road,
Manitoba, Canada) and used for the present work. Two Indian cultivars
Agra Local and WL711 (Lr13) were used as leaf rust susceptible
cultivars. The wheat lines possessing Lr genes transferred
from T. tauschii and the two susceptible cultivars were sown
in plastic trays in the glass-house. First leaf of seven day-old
seedlings was inoculated with uredospore-talc mixture of each of the
races, 12, 77, 77-1, 77-2, 77-3, 77-4, 77-5, 77-5, 77-6 and 104-2,
separately. The seedlings were inoculated at 100% relative humidity
for 24 hr and kept in separate glass-houses maintained at 20 plus or
minus 1C. The infection types (ITs) were recorded 14 days later,
following McIntosh et al. (1995).
For adult plant tests 15-20 seeds of the line were sown in 2 m long
paired rows, spaced 30 cm apart. These lines were surrounded by two
spreader rows of each of the susceptible cultivars Agra Local and
WL711. The spreader rows as well as the lines carrying genes from
T. tauschii were sprayed with water suspension of uredospores
of the race 77-5 on alternate days. Race 77-5 is the most frequent
and virulent amongst the Indian races and attacks all Indian
cultivars carrying Lr genes originating from T.
aestivum (Saini et al. 1998, Sawhney et al. 1998). The
inoculations were continued till the leaf rust started appearing on
susceptible cultivars. The field was irrigated adequately to ensure
high humidity needed for the leaf rust development. Disease reaction
was recorded as per cent rust severity on modified Cobb scale
(Peterson et al. 1948) as well as pustule type (Roelfs et al. 1992)
during 1996-1999.
Results and discussion
Nine Lr genes transferred to T. aestivum from T.
tauschii, as already mentioned, were evaluated for resistance
against nine prevalent pathotypes of P. recondita tritici at
the seedling stage in the glass-house and against race 77- 5 at the
adult stage in the field during 1996-1999 (Table
1). Lines carrying the genes Lr22a and Lr22b and
cultivars Agra Local and WL711 showed high (susceptible) infection
types (3 or 3+) against all the leaf rust races used. On
the other hand, the line carrying the gene Lr41 showed low
(resistant) reaction against all the pathotypes. The line carrying
Lr42 mainly differed from Lr41 in showing
susceptibility against pathotype 104-2. The lines carrying the genes
Lr39 and Lr40 showed high infection types against race
12, 77 and 104-2 only. The reaction pattern of these two lines was
different from that of the other T. tauschii derived Lr
genes.
At the adult stage, the leaf rust reaction during the years 1996-1999
was 60S on the line carrying the gene Lr22b which is
comparable to the susceptible cultivars Agra Local and WL711. The
disease reaction ranged from free to traces (Tr.) on Lr41 to
20 MS-40 MS on line carrying Lr42 during this period. The
field reaction on lines with the genes Lr21, Lr22a and
Lr43 against race 77-5 was low even though these lines showed
high seedling reaction. These three genes appear to confer adult
plant resistance against race 77-5. Since 77-5 attacks all the
Lr genes originating from T. aestivum, Lr genes
effective against this pathotype can provide useful and diverse
resistance for strategic use in breeding programs. According to
Mclntosh et al.(1995), Lr21 has potential for use in breeding
but it remained largely unexploited. Sawhney (1997) reported the
successful use of Lr21 in wheat for leaf rust management. The
genes Lr22a, Lr40 and Lr41 derived from T.
tauschii conferred high level of adult plant resistance to race
77-5 and thus have potential for use in wheat improvement. Except for
104-2, the line carrying the gene Lr42has shown seedling
resistance against all the races including 77- 5 but this line has
shown relatively low adult plant response of 20MS-40MS. Such response
is unlikely if the gene Lr42 is stable for expression at high
temperature (30C and above) which is prevalent when leaf rust
reaction is recorded in this part of the country.
Access to diverse genetic stocks possessing different Lr genes
is an essential prerequisite for a dynamic crop improvement program.
Therefore, the genes Lr21, Lr22a, Lr40 and Lr41 derived
from T. tauschii can be generally utilized against the P.
recondita tritici races prevalent in India.
Acknowledgments
The authors thank Dr. P.L. Dyck of Agriculture Canada for the
supply of seeds of the lines used for this work.
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