New genes for resistance to downy mildew (Peronospora parasitica) in oilseed rape (Brassica napus ssp. oleifera)

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, 3-5 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.