Seedling and adult plant response of some hexaploid wheat lines to different variants of leaf rust pathotype

V.C. Sinha, Rajiv K.Sharma, A.P. Sethi and B.S. Malik

Division of Genetics, IARI, New Delhi 110012, India

Vertical resistance (VR) genes provide hypersensitive type of reaction and confer a high level of resistance throughout the life cycle of the plant. However, this type of resistance exerts a high level of selection pressure on the pathogen population which then mutates and evolves resistant strains ultimately rendering the resistant gene ineffective in the long run. Horizontal resistance (HR) genes or what are commonly known as adult plant resistance (APR) genes, on the other hand, provide race nonspecific, moderate level of resistance to the carrier genotypes. Such genes do not exert a strong selection pressure on the pathogen population and let pathogens coexist but at a much lower population density level and thereby ensuring that the pathogen does not cause serious economic losses. This also confers a long lasting durable resistance. The study under report was conducted to assess the seedling and adult plant response of some newly developed bread wheat lines against an array of leaf rust pathotypes so as to identify the potential donors of adult plant leaf rust resistance for use by breeders.

The study made use of a collection of 28 elite wheat lines developed through pedigree breeding method for yield improvement. These lines were screened in seedling stage (Stakman et al. 1962) against selected leaf rust pathotypes (77-5, 77-6, 77-7, 77-2 and 12-2) under glass house conditions. The same set of genotypes was also screened under field conditions at adult plant stage (Loegering 1959) after creating field epidemic with the mixture of test pathotypes. Based on the data recorded in seedling and adult stage the lines DW1237, DW1239, DW1240, DW1229, DW1234, DW1242, DW1243, DW1246 and DW1232 were categorized into low (L) and very low (VL) field response to leaf rust as against seedling susceptibility to most of the pathotypes (Sawhney et al. 1996). Results revealed a high degree of APR in these lines which can serve as donors for this trait in wheat breeding program (Table 1). Out of the 28 lines screened, nine have exhibited (parentage in Table 2) desirable adult plant resistance. However, these lines seem to have diverse genetic bases as is indicated by their differential reaction to the individual pathotypes as well as to a mixture of the latter. Among the known APR genes, Lr22a and Lr34 have been shown to exhibit very low (resistant) and low (moderately resistant) response under field conditions (Sawhney and Sharma 1992). It is likely that some of these lines carry Lr34 since most materials originating from CIMMYT carry this gene. The resistant lines identified carrying APR genes will serve as important source to introgress the APR genes into future breeding program.

References

Logering WQ (1959) Method of recording cereal rust data. USDA, Int Spring Wheat Nursery, USA.

Sawhney RN, Sharma JB and SharmaDN (1992) Genetic diversity for adult plant resistance to leaf rust in near isogenic lines used in Indian wheat. Pl Breed 109: 248-254.

Sawhney RN, Sharma JB, Sharma DN and Nagarajan S (1996) Adult plant resistance sources for breeding wheat with durable resistance to leaf rust. Cereal Rust and Powdery Mildew Bull 24: 29-34.