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Materials and methods

The experimental material in the present study comprised a white seeded donor parent TR 380-14*7/ 3Ag#14 carrying the genes Sr24 and Lr24 and 19 elite Indian bread wheat cultivars. They were Sonalika, Kalyansona, NI 5439, C 306, WH 147, Lok-1, HD 2329 HD 2285, J 24, HD 2009, VL 421, UP 262, WL 711, HUW 234, PBW 226, HD 2402, HI 1077, HS 240, and WH 542. Cultivars HS 240 and WH 542 carry Petkus rye-derived resistance genes Sr31, Lr26, Yr9 and Pm8. The recurrent parents were highly susceptible to leaf rust and most of them showed high susceptibility to stem rust except HS 240 and WH 542. Since Sr24 and Lr24 are tightly linked dominant genes, five to seven backcrosses were given in quick succession within a short span of two to three years, raising three experimental crops in a year, under high natural incidence of rusts, and the genotypes phenotypically similar to their recurrent parents in all respects and carrying resistance to stem rust and leaf rust were constituted after three generations of selfing at F3 generation.

All the improved lines were screened for stem rust and leaf rust under artificial epiphytotic conditions at adult plant stage. Rust reactions were scored following standard practices. Only 10 backcross lines were subjected to seedling test with 40A and 40-1 pathotypes of stem rust under glass house conditions at temperature ranging from 27 to 30oC. The data on Barley Yellow Dwarf Virus (BYDV) incidence under natural condition was also recorded in percent infection over three years.


Results and discussion

Improved backcross lines exhibited total resistance to leaf rust at adult plant stage (Table 1). This indicates that the gene Lr24 had been introgressed into susceptible recurrent parents. However, thirteen backcross lines exhibited susceptibility to stem rust and six lines showed moderately resistant to moderately susceptible reaction to stem rust. Of the seven, five recurrent parents, namely, HD 2285, HUW 234, PBW 226, HI 1077 and HD 2402 appear to carry unknown factor(s) for low reaction (10S to 30MS) to stem rust. Cultivars HS 240 and WH 542 and their improved version showed a high degree of stem rust resistance by the presence of Sr31 derived from rye.

Sawhney and Goel (1981), Patil and Deokar (1996) reported that Sr24 conditioned seedling resistance to 19 prevailing and virulent Indian stem rust pathotypes. Sr24 displayed a high degree of resistance to stem rust for over 25 years at Wellington, a hotspot for stem rust. However, virulence for Sr24 has been reported from South Africa (Le Roux and Rijkenberg 1987). Its resistance was also overcome by a new Indian pathotype 40-1 (Bharadwaj et al. 1990) of stem rust. TR 380-14*7/3Ag#14 is resistant to stem rust by different factor(s) from Sr24. Only 10 backcross lines and their recurrent parents were tested in seedling stage with two pathotypes 40A and 40-1 of stem rust (Table 2). All the tested lines except WH 542*6/TR 380-14*7/3Ag#14 and WH 542, showed 3+ or 4 infection type to 40-1 in seedling stage. This indicates that the gene Sr24 is indeed overcome by this virulence. The seedling reaction to 40A pathotype of stem rust confirms the presence of Sr24. The line WH 542*6/TR 380-14*7/3Ag#14 and its recurrent parent WH 542 exhibited ;1 and ;1,2 reactions in seedling stage to 40A and 40-1 pathotypes by the presence of Sr31 (Goel et al. 1994) respectively. Thus, based on the pattern and type of reactions to stem and leaf rusts tested in seedling it is presumed that the segment carrying Sr24-Lr24 has been introgressed into the backcross lines.

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