Differential response and genes for resistance to Peronospora parasitica (downy mildew) in Brassica juncea (mustard)

The response of a wide range of Brassica juncea accessions to 14 isolates of Peronospora parasitica, 12 from India (IP00A, IP02, IP03, IP04, IP04A, IP05, IP05B, IP33 and IP33A were derived from B. juncea; IP09, IP14 and IP13A from B. rapa) and two from B. napus in the UK (R1 and P003), was screened. Sixteen differential host response groups to these isolates (classified as groups A‐P) were identified. Groups‘A’and‘B’expressed the widest resistance profiles to these isolates. Group‘A’was susceptible to isolates IP05 and IP05B, moderately resistant to isolate IP33 and resistant to all other isolates. Group‘B’was susceptible to isolates IP03, IP04 and IP04A, and resistant to the other isolates. Putative homozygous lines resistant to all 14 isolates were selected from the F₄ progeny of crosses involving lines 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 RESBJ‐200 to isolates IP00A, IP04A and IP33A seems to be conditioned by single dominant genes. The resistance of RESBJ‐190 to isolates IP00A, IP05B and IP33A was also conditioned by single dominant genes. The gene for resistance to IP00A and IP33A in RESBJ‐200 seems to be independent of the genes for resistance to the same isolates in RESBJ‐190. The new genes for differential resistance to P. parasitica will be of value in future studies of the genetics of the host‐pathogen interaction and for breeding for disease resistance.