The results obtained in the study are presented in Table 1, Table 2 and Table 3. About 32 pathotypes of stem rust have been reported from the Nilgiri hills over the last four decades. However, prevalent pathotypes are about 20 with major and minor fluctuations periodically. The line CS 2A/2M 4/2 carries the Ae. speltoides derived gene Lr28 and Ae. comosa derived genes Sr34 and Yr8 (both tightly linked). Both Lr28 and Yr8 are highly effective at Wellington whereas Sr34 is totally ineffective. Selections were done only for leaf rust resistance in all the backcross generations, thus eliminating the linked genes Sr34 and Yr8. Sr34 is reported to be ineffective to 19 Indian stem rust pathotypes in the seedling stage (Patil and Deokar 1996). Backcross lines carrying Lr28 and Lr32 when tested with a mixture of stem rust pathotypes including 40-1, a new virulence on Sr24, were found susceptible in adult plant stage (Table 1 and Table Table 2). The high level of resistance to stem rust in backcross lines WH542*6//CS 2A/2M 4/2, WH542*6//C86-8/Kalyansona F₄, HS240*6//CS 2A/2M 4/2 and HS240*6//C86-8/Kalyansona F₄ is due to the presence of Petkus rye derived dominant gene Sr31 (Nayar 1996).

In crosses with respective parents as well as with a stock carrying specific gene Sr26 conferring resistance to stem rust, the factor for fast rusting to stem rust in the backcross lines carrying Lr28 and Lr32 was observed as recessive in nature. Therefore it is suggested that an undesignated recessive factor (sr ) for fast rusting is tightly linked with Lr28 and Lr32. The gene Sr26 is located on chromosome 6AL while Lr28 and Lr32 are present on 4AL and 3DS chromosomes respectively. Therefore, the gene(s) for fast rusting (undesignated) is non-allelic to Sr26, hence the use of term dominance is inappropriate. The gene Sr26, in particular, is present in hemizygous condition in F₁ hybrid, its allelic form is absent because of its alien origin. However, the fast rusting has been observed in the hybrids involving backcross lines with respective recurrent parents, the nature of the gene(s) can be called as recessive.

Fast rusting in the above cases is characterized by very short latent period, increased frequency of penetration, very large pustule size and increased pustule expression, increased number of uredinia per unit area of host surface and sporulation, appearance of stem rust pathogen in early vegetative phase and rapid progress and higher intensity even before maturity. Short latent period and very large pustule size and early wide coalescence were prominent. Rapid progress led to broken stems at an early stage and resulted in highly shrivelled grains. Caution should therefore, be exercised in deploying cultivars carrying Lr28 or Lr32 in areas highly prone to stem rust and this is particularly true in the case of southern hills, peninsular and central regions of India. However, highly effective major dominant genes like Sr26, Sr31, Sr32 and Sr36 along with Lr28 or Lr32 protect the cultivars from stem rust and leaf rust pathogens.

Out of 18, two backeross lines carrying Lr28 were evaluated for yield (Table 3) at New Delhi during 1997-98 season. Normally, stem rust does not appear in Delhi conditions because of low temperature during the crop growth period. No infection was noticed on the backeross lines as well as on the recurrent parents. However, leaf rust infection was observed on the recurrent parents with moderate intensity. The yield differences were not high as both the backeross lines yielded as good as the recurrent parents. Since HD2329 and WH147 showed susceptibility to leaf rust the real effect of Lr28 on grain yield could not be assessed, however, it can be concluded that resistance incorporated in HD2329 and WH147 protected the genotypes in yield loss from leaf rust infection which is a distinct advantage. Cultivar Sunland was registered in New South Wales, Australia as prime hard cultivar in 1992. Although no detrimental effect appear associated with the presence of Lr28, the durability of resistance is likely to be low (McIntosh et al. 1995). In view of the effectiveness of Lr28 and Lr32 against a mixture of races of leaf rust, the use of both the gens in wheat improvement is advocated.


Acknowledgment

The authors express their sincere gratitude to Dr. R. A. McIntosh, University of Sydney, Australia for supplying the seed of donor parents. We are thankful to Shri S. Bojan for technical help.


References

McIntosh RA, Wellings CR and Park RF (1995) An atlas of resistance genes. In: Alexa Cloud-Guest (ed) Wheat Rusts. University of Sydney Australia. 1-200.

Nayar SK, Prashar M and Bhardwaj SC (1996) Resistance breeding for new virulent pathotypes of rust. 35th All India Wheat Workers Workshop IARI New Delhi.

Patil JV and Deokar AB (1996) Host parasite interactions between lines and varieties of wheat with known Sr genes and races of stem rust. Cereal Rusts and Powdery Mildews Bulletin 24: 91-97.

Tomar SMS and Menon MK (1998) Adult plant response of near-isogenic lines and stocks of wheat carrying specific Lr genes against leaf rust. Indian Phytopathol 51: 61-67.

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