A complete set of disomic addition lines of Secale cereale cv. Imperial rye in Triticum aestivum cv. Chinese Spring used in this study, was received from Dr. S.M.Reader, PBI, Cambridge, UK. A high yielding but Karnal bunt susceptible commercial wheat variety WL711 developed by the Punjab Agricultural University, Ludhiana was used as the recurrent parent for the transfer of rye chromosomes.
The addition lines, recipient variety WL711, F1's and subsequent backcrossed progenies were grown in the field during wheat crop seasons 1985-94 following standard agronomic practices. 'Me presence of rye chromosomes in successive backcross generations was monitored cytologically by fixing spikes in Carnoy's solution and preparing squashes from PMCs in 2 per cent acetocarmine. Two to three spikes of the cytologically confirmed plants were inoculated by Neovossia indica isolates prevalent in north India at the boot stage for screening against Karnal bunt following the technique described by Aujla et al. (1982). The data on percent incidence of Karnal bunt was recorded by counting the total number of infected grains among the inoculated heads at maturity.
Results
The incidence of Karnal bunt under artificial inoculations of the isolate PBW 154 to seven disomic addition lines (2n = 44,21 W" + 1R") of rye and their F1's with WL711 is given in
Table1. Disomic addition lines with rye chromosomes 1R, 2R, 3R, 5R and 7R in Chinese Spring background and their monosomic addition lines in F1's with WL711 were susceptible to Karnal bunt. The percentage of infection ranged from 1.4 to 21.1 as compared to 12.3 percent in Chinese Spring.
Only two out of the 12 F1 monosomic addition plants of 2R were free while the remaining plants had an average 14.8 percent incidence. The 4R and 6R disomic addition lines and their F1's monosomic addition with WL711 were completely free from Karnal bunt infection indicating that the chromosomes 4R and 6R of rye carry gene(s) for resistance to Karnal bunt. Karnal bunt incidence in 4R and 6R monosomic addition plants (2n = 43) and the euploid derivatives (2n = 42) in various backcross generations is given in Table 2. The monosomic addition plants carrying 4R and 6R chromosomes in all the five backcross generations were free from Karnal bunt whereas the euploid segregants were susceptible with a range of incidence from 9.2-35.4 percent. The recurrent cultivar WL711 had 22.8 per cent mean infection with a range of 9.1 to 35.5 percent over five years of testing with a particular isolate (PBW 154) of Neovossia indica.
However, in BC5F2 both the 4R and 6R monosomic addition plants and their euploid segregants were found to be susceptible on inoculation with the mixture of the previous and a new isolate (C-21) found virulent on a Karnal bunt resistant triticale variety TL 1210.
In 1993-94, the BC5F3 plants of both the monosomic addition chromosomes were inoculated with both the fungal isolates separately. The 4R and 6R monosomic addition plants remained free from the disease incidence on inoculation with the old isolate used upto BC5 generation (PBW 154) but were susceptible to the new isolate (C-21) showing varying degrees of infection (Table 3).
Disomic addition lines of both 4R and 6R chromosomes have been isolated from the selfed progenies of BC5 which are completely fertile and morphologically similar to the recipient parent WL711. These lines were free from the new isolate.