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 ÔAÕ and ÔBÕ expressed the widest resistance profiles to these isolates. Group ÔAÕ was susceptible to isolates IP05 and IP05B, moderately resistant to isolate IP33 and resistant to all other isolates. Group ÔBÕ 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 F 4 progeny of crosses involving lines RESBJ-200 from group ÔAÕ (selection from cv. Kranti) and RESBJ-190 from group ÔBÕ (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.
The response of cotyledons of oilseed rape (Brassica napus ssp. oleifera) accessions to infection by isolates of Peronospora parasitica under controlled conditions was assessed on a 0–7 scale (disease reaction). In interactions scored 0–3, 4–5 and 6–7, the host was considered resistant, partially resistant and susceptible, respectively. Accession RES‐26, selected from the spring oilseed rape cultivar Janetzkis, was partially resistant to isolate R1 and resistant to isolate P003 of P. parasitica, which distinguishes it from three previously described differential response groups (‘A’, ‘B’ and ‘C’) of accessions in B. napus. The resistance of RES‐26 to isolate P003 seemed to be conditioned by a single, partially dominant gene and the resistance of RES‐02, which belongs to group ‘A’ (resistant to R1 and P003), by two independent partially dominant genes. The gene for resistance to P003 in RES‐26 is either closely linked, allelic or identical to one of the two genes for resistance in RES‐02. Resistance of RES‐02 to R1 is conditioned by a single, incompletely dominant gene. The genes for resistance to isolates R1 and P003 in RES‐02 are either closely linked, allelic or identical. The cotyledonary leaves of each seedling responded independently when inoculated simultaneously each with a different isolate of the pathogen.
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