Abstract

Evaluation of 64 synthetic hexaploid wheat accessions selected at CIMMYT Mexico was carried out against Puccinia recondita f.sp. tritici pathotype 77-5 and 77-6, at seedling and adult plant stages in controlled and field conditions, to identify new untapped sources of leaf rust resistance. Out of these, 31 were highly resistant at seedling and adult plant stages, while 7 were highly resistant at adult plant but highly susceptible at seedling stage. Five accessions were observed to be susceptible at both stages. The study indicated presence of a number of genes responsible for imparting seedling and field resistance in various synthetic hexaploid wheat lines which could be incorporated as new sources for the improvement of leaf rust resistance in bread wheat.

Introduction

Leaf rust caused by Puccinia recondita f.sp. tritici is the most important and destructive diseases on wheat in India (Joshi et al., 1986; Evermeyer and Browder, 1974; Sawhney et al., 1977). Pathotypes 77-5 and 77-6 were the most virulent and have among other genes combined virulence for both Lr23 and Lr26, the most frequent resistant genes in Indian wheat cultivars (Anonymous, 1992). Several successful attempts were made to develop resistant wheat lines to leaf rust but after some time varieties became susceptible due to occurrence of new virulence. Exploitation of specific rust resistance genes to combat these changes is therefore of paramount importance.

Synthetic hexaploid wheat was produced by crossing durum wheat (2n=4x=28, AB genomes) with A. tauschii (2n=2x=14, D genome) with the objective of exploiting new genetic variability for resistance or tolerance to abiotic and biotic stresses in D-genome of T. tauschii. Synthetic hexaploids have been reported as having resistance to diseases such as karnal bunt, caused by Tilletia indica Mitra (Multani et al., 1988), leaf rust, Puccinia recondita Eriks - (Kerber and Dyck, 1969; Singh et al., 1998) and stripe rust, Puccinia striformis Westend. (Ma et al., 1995).

The objective of this study was to evaluate the synthetic lines and identify new resistant lines for most virulent leaf rust resistance pathotypes which could be effectively used in broadening the genetic base for resistance in bread wheat.

Materials and Methods

Leaf rust pathotypes 77-5 (121 R 63-1), 77-6 (121 R 55-1) and a mixture of pathotypes (12-2, 77-2, 77-5, 104-2) were obtained from DWR Research Station, Shimla. Sixty-four synthetic hexaploid wheats (T. turgidum x T. tauschii) selected at CIMMYT, Mexico and being maintained at Agharkar Research Institute were screened for resistance to races 77-5 and 77-6 at the seedling stage. Seedlings were inoculated as per standard procedure (Browder, 1971). A set of isogenic lines for specific Lr genes and standard differentials was also tested to determine the virulence pattern of the leaf rust isolates used. The study was conducted in a temperature controlled room maintained at 20 + 1°C and 80% relative humidity with a photoperiod of 18 hrs day length (10,000 lux) using flourescent lamps.

Adult plants were tested with a mixture of pathotypes (12-2, 77-2, 77-5, 104-2) in the field during the 2000-01 and 2001-02 crop seasons along with Lr lines and standard differentials. Each accession was grown in a single row of one meter length and spaced 50 cm apart. After every 20 rows a highly susceptible cultivars was sown for better spread of inoculum. Inoculation was done by making a spore suspension in water with a few drops of Tween 80. Standard inoculation methods were practised and measures to create optimum conditions for maximum disease spread were taken. Rust severity was recorded according to the scale suggested by Peterson et al. (1948).

Results and Discussion

The pathotypes 77-5 and 77-6 used in this study showed the following avirulence/virulence characteristics in seedling tests:

77-5: (121 R 63-1) PLr9, 18, 19, 21, 24, 25, 28, 29/p Lr1, 2, 3, 10, 11, 12, 13, 14, 15, 16, 17, 20, 22, 23, 26, 27 + 31, 30, 33, 34

77-6: (121 R 55-1) PLr9, 16, 18, 19, 20, 21, 24, 25, 28, 29, 32, 41/p Lr1, 2a, 3, 10, 11, 12, 13, 14, 15, 17, 15, 23, 26, 27, 30, 33, 34

Isogenic lines Lr9, Lr19, Lr21, Lr24, Lr25, Lr28 and Lr29 showed response 0-0 at seedling stage and adult stage to these pathotypes, while Lr27, Lr32, Lr35, Lr23 responses were immune to moderately resistant at adult stage confirming their effectiveness at all growth stages.

Based on their infection types at seedling stage and field response at adult plant stage the synthetic hexaploid wheats were grouped into three sets:

Group I: Thirty one (48.4%) accessions expressing high levels of resistance both at seedling and adult plant stage with infection types (ITS) ranging between 0 to 2 and field score between F to 30 MR with the mixture of races. This group conferred rust resistance at the seedling stage and a high level of resistance at the adult plant stage (Table 1).

Group II: Seven (10.9%) accessions in the susceptible catagory (ITS 3 to 4) on seedlings with both rust pathotypes and low severity on adult plant F to 40 MR in field tests, suggesting that these accessions possess adult plant resistance (APR) (Table 2).

Group III: Five accessions showing high susceptibility in seedling (ITS 3 to 4) and high disease severity on adult plants in field tests (40 S to 80 S) (Table 3).

This study showed that synthetic hexaploid wheat have resistance to highly virulent pathotypes both at seedling and adult plant stages. Singh et al. (1998) reported a high degree of genetic variability for leaf rust resistance at both seedling and adult plant stages. Similar cases were reported by Kerber and Dyck (1969). Zhaug and Knott (1993) found that all the seedling resistance genes they tested in durum wheats were effective at the adult plant stage.

For inducing genetic resistance in wheat cultivars by hybridization, selection of resistant source is of prime importance. The studies indicated that synthetic hexaploid wheat lines were available with seedling resistance to virulent pathotypes of leaf rust and also adult plant resistance. Both types may be of immence value to bread wheat resistance breeders for incorporating durable resistance into commercial varieties.

References

Anonymous, 1992. MEHTAENSIS. Compiled by DWR Regional Res. Station, Shimla. Vol. 12 (3).

Browder LE, 1971. Pathogenic specilization in cereal rust fungi, especially Puccinia recondita f.sp. tritici. Concept, methods of study and application. United States Department of Agriculture Technical Bulletin 1432, 51 pp.

Evermeyer MG, Browder LE, 1974. Effect of leaf rust and stem rust in 1973 Kansas wheat yields. Plant Disease Reporter 58, 469-471.

Joshi LM, Singh DV, Srivastava KD, 1986. Wheat and wheat disease in India. In : Joshi LM, Singh DV and Srivastava KD (ed) Problems and progress of wheat pathology in South Asia, 1-19.

Kerber ER, Dyck PL, 1969. Inheritance in hexaploid wheat of leaf rust resistance and other characters derived from Ageilops squarrosa. Canadian Journal of Genetics and Cytology 11, 639-647.

Ma H, Singh RP, MujeebA, Mujeeb Kazi A, 1995. Resistance of stripe rust in Triticum turgidum, T. tauschii and their synthetic hexaploids. Euphytica 82, 117-124.

Multani DS, Haliwal HS, Singh P, Gill KS, 1988. Synthetic amphiploids of wheat as a source of resistance to karnal bunt (Neovossia indica). Plant Breeding 101, 122-125.

Peterson RF, Campbell AB, Hanula AE, 1948. A diagrammatic scale for estimating rust intensity of leaves and stem of cereals. Canadian Journal of Research 26, 496-500.

Sawhney RN, Nayar SK, Singh SD, Chopra VL, 1977. Virulence pattern of the Indian leaf rust races on lines and varieties of wheat with known Lr genes. SABRAO Journal 9, 13-20.

Singh SS, Sharma DN, Mehta H, 1998. Resistance to Puccinia recandita tritici in
synthetic hexaploid wheats. Indian Journal of Genetics 58, 263-269.

Zhaug H, Knott DR, 1993. Inheritance of adult plant resistance to leaf rust in six durum wheat cultivars. Crop Science 33, 694-697.