1IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK; 2GBPUA and T, Pantnagar-263145, India

Background and objectives
Five groups of Brassica juncea accessions with differential resistance at the cotyledon stage to four isolates of Peronospora parasitica have been previously reported [1]. This study reports: (i) characterization of 16 differential responses in B. juncea at the cotyledon stage to 14 isolates of P. parasitica; (ii) aspects of inheritance of resistance at the cotyledon stage in two accessions from different response groups which, between them, possess resistance to all isolates; and (iii) examines the response of accessions from ten differential responses groups at the juvenile and flowering growth stages to three isolates.

Materials and methods
Twenty five accessions of B. juncea were screened under controlled conditions to 14 isolates of P. parasitica, nine from B. juncea in India (IP00A, IP02, IP03, IP04, IP04A, IP05, IP05B, IP33 and IP33A), three from B. rapa in India (IP09, IP14 and IP13A) and two from B. napus in the UK (R1 and P003). The response of cotyledons and leaves to infection was assessed on a 0-7 scale (disease reaction). Where disease reaction scored 0-3, 4-5 and 6-7, the host was considered resistant, partially resistant and susceptible, respectively [2]. To study aspects of inheritance of resistance, crosses were made involving one susceptible accession and two accessions (RESBJ-200 and RESBJ-190) with differential resistance responses to P. parasitica isolates. Experiments involving plants at the cotyledon and juvenile/flowering growth stages were scored six and ten days after inoculation, respectively.

Results and conclusions
Sixteen differential responses groups of B. juncea accessions at the cotyledon stage to P. parasitica isolates were identified (A-P). Among accessions representing these groups, fourteen were putative homozygous for resistance or partial resistance selected from seedling populations of accessions that exhibited a heterogeneous reaction to isolates from B. juncea and B. rapa. The widest resistance profiles to P. parasitica isolates were expressed by groups 'A' and 'B'. Group 'A' was susceptible to only two isolates, IP05 and IP05B, whereas group 'B' was susceptible to three isolates IP03, IP04 and IP04A. Putative homozygous lines resistant to all 14 isolates was selected from the F4 progeny from crosses involving accessions RESBJ-200 from group 'A' (selection from cv. Kranti) and RESBJ-190 from group 'B' (selection from cv. Krishna). Both selections were selfed and tested for uniformity of reactions to all isolates for three generations. The resistance of RES-200 to isolates IP00A, IP04A and IP33A seems to be conditioned by single dominant genes which were either closely linked, allelic or identical. The resistance of RES-190 to isolates IP00A, IP05B and IP33A seems also to be conditioned by single dominant genes. The gene for resistance to IP00A and IP33A in RESBJ-200 are independent from the genes for resistance to the same isolates in RESBJ-190.

Cotyledons of all accessions were markedly more susceptible than true leaves. This was mainly related to the rapidity of the necrotic reaction in restricting fungal development on leaves. However, the cotyledons gave a good indication of the resistance of true leaves. Generally, the relative differences in resistance or susceptibility between accessions remained largely unchanged, regardless of which tissue was inoculated. The cotyledonary and true leaves of each plant responded independently when inoculated simultaneously each with a different isolate of the pathogen. In some cases, resistance at the flowering growth stage was stimulated or enhanced when plants were pre-inoculated with the same isolate at the seedling and juvenile stages. It was also observed that the floral stigma of some resistant accessions were heavily infected with the pathogen which prevented pod formation. This may indicate that the genes for resistance in those accessions were either not expressed or their effect was suppressed in these parts of the plants.

The new genes for differential resistance to P. parasitica will be of interest in future studies of the genetics of the host-pathogen interaction and for breeding for disease resistance.

1. Nashaat NI, Awasthi RP 1995. Journal of Phytopathology 143, 157-9.
2. Nashaat NI, Heran A, Mitchell SE, Awasthi RP, 1997. Plant Pathology 46, 964-968.