Characterization and inheritance of partial resistance to downy mildew, Peronospora parasitica, in breeding material of broccoli, Brassica oleracea convar. botrytis var. italica

Twenty doubled haploid broccoli lines from breeding material were evaluated for resistance to Peronospora parasitica at the seedling stage. All lines supported sporulation of the pathogen, but to varying extents and intensities. Partial resistance of the more resistant lines, 'br8' and 'br9', reduced conidia production on cotyledons by 50-70% compared with the most susceptible lines. Inoculation of the two most resistant lines with 13 isolates of different geographical origins revealed that 'br9' showed a rather uniform level of resistance to all isolates while 'br8' showed some isolate specificity. Partial resistance was evaluated in six of the broccoli lines in a half diallel set of crosses. Disease assessment of seedlings showed that additive genetic effects explained 45.8% and 31.8% of the total variation of sporulation score and conidia production, respectively. This suggests that recurrent selection for partial resistance to P. parasitica in early generation inbreds or in populations of broccoli will be efficient to obtain cotyledon resistance.