The twenty one bread wheat lines were selected for the present study (Table 1) and these lines were subjected to multilocational evaluation for yield, disease resistance and quality traits before they were promoted as final year entries (prospective cultivars) in advanced varietal trials under All India Coordinated Wheat Improvement Project (AICWIP, ICAR) during 1995 (Initial Varietal Trial), 1996 (Advance Varietal Trial 1st Year) and 1997 (Advance Varietal Trial, Final Year) (Jag Shoran et al. 1997). Field response of genotypes to yellow rust was evaluated at Directorate of Wheat Research, Karnal, Haryana during normal crop seasons of 1996 and 1997. Seeds of each genotype were sown in a 2 meter row with a space of 20 cm between the rows. Three spreader rows (row to row gap of 15 cm) of mixture of five susceptible cultivars (Agra Local, Kathia Local, NP4, Lal Bahadur and C306) were planted on four sides of the plot on 3 different dates with interval of 15 days. The crop was fertilized at the rate of 120 Kg N/ha (split equally at seedling and stem elongation), 60 Kg P/ha and 40 Kg K/ha. Irrigation was provided at crown root initiation, stem elongation, flowering and grain formation stages of crop growth.

The yellow rust epidemic was initiated by inoculating 4 week old plants of spreader rows with the uredosporic dust having equal proportions of three pathotypes, N(46S102), P(46S103) and K(47S102). Inoculations were done following the syringe and spray techniques described by Joshi et al. (1988). Using the modified scale (Peterson et al. 1948), average disease severity on upper three leaves was first recorded at 40 days after inoculation when the susceptible control variety, Agra Local had reached 40% disease severity. The infection types (TR, R, TMS, MS, TS and S) were recorded by following McNeal et al. (1971) and defined as TR: necrotic/chlorotic flecks in traces without sporulation, R: necrotic/chlorotic stripes without sporulation, TMS: light sporulation, necrotic/chlorotic stripes in traces, MS: intermediate sporulation, necrotic/chlorotic stripes, TS: abundant sporulation, necrotic/chlorotic stripes in traces and S: abundant sporulation without chlorosis/necrosis. In all five readings of rust intensity were taken at weekly intervals. Area under disease progress curve (AUDPC) was calculated using a computer package developed at CIMMYT, Mexico (CIMMYT 1988). The relative AUDPC (percent) for each cultivar was calculated by dividing its actual AUDPC by susceptible cultivar Agra Local's AUDPC. The values of AUDPC were interpreted as below:

Group 1. Less than 1% AUDPC of susceptible check
Variety remained rust free and represented the strong vertical resistance, which may not impart a durable protection and likely to be lost through pathogenic adaptations. In some cases, disease initiated as necrotic/chlorotic flecks without sporulation (TR-infection type) on leaf surface and attained a final disease rating not more than 10R. This type of reaction was also considered to represent the strong vertical resistance again.

Group 2. 1 to 20% AUDPC of susceptible check
Rust initiated as abundantly sporulating but chlorotic stripes (MS-infection type). Subsequently, the progress of rust development remained slower and was restricted to 1- 20% of the susceptible check. These genotypes were interpreted to express a high degree of slow rusting.

Group 3. 21-50% AUDPC of susceptible cheek
The incipient rust intensity was represented by abundantly sporulating non-chlorotic/necrotic stripes on the entire surface of leaf. But, further development was restricted to 21-50% of the susceptible check. These genotypes were supposed to represent a lower degree of slow rusting character.

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