Induction of systemic resistance in broccoli (Brassica oleracea var. botrytis) against downy mildew (Peronospora parasitica) by avirulent isolates

The aim of this study was to investigate whether pre-inoculation of susceptible brassica plants with avirulent isolates of Peronospora parasitica (downy mildew) could induce resistance against virulent isolates. The accessions Milady (broccoli) and Billabong (cauliflower) susceptible to the P. parasitica isolates FP15 and FP16 were used. Isolates FP06, FP07, FP11, and FP17 are avirulent on Milady but virulent on Billabong. Both accessions are nonhosts of Fusarium oxysporum, P. parasitica from Arabidopsis thaliana, and Bremia lactucae. Seven-day-old seedlings were first inoculated with the inducing organisms and then challenged 0–15 days later with the virulent P. parasitica isolates. Neither F. oxysporum, B. lactucae, the virulent brassica P. parasitica isolates, nor Noco2 induced resistance in Milady and Billabong against the virulent isolates. However, pre-inoculation of the seedlings of Milady with the avirulent isolates induced resistance against the virulent isolates (FP15 and FP16), and disease reduction ranged between 49% and 70%. The induced resistance (IR) was systemic and lasted at least 15 days. Seedlings expressing IR accumulated the pathogenesis-related proteins PR-2 and PR-5 but not PR-1, PR-3, and PR-9.