Seedling and adult plant resistance to downy mildew (Peronospora parasitica) in cauliflower (Brassica oleracea convar. botrytis var. botrytis)

Seedlings of six cauliflower cultivars (Brassica oleracea convar. botrytis var. botrytis) were assessed for resistance to a Danish isolate of Peronosp ora parasitica, under controlled conditions. Resistance, characterized by r estricted sporulation and necrotic dark flecks at the inoculation site on t he cotyledons, was expressed in the hybrids 9306 F1, 9311 F1, and the open pollinated cultivar Perfection. Testing of the parent lines and F2 generati ons of the two resistant hybrids suggested that resistance was a dominantly inherited trait controlled by a single gene. Inoculation of the cultivars with seven isolates, from different geographical origins, showed that the r esistance was isolate specific. The two hybrid cultivars expressing cotyled on resistance and two hybrids expressing susceptibility were assessed for a dult plant resistance under field conditions. The AUDPC (Area Under the Dis ease Progress Curve), based on disease incidence and severity, revealed sig nificant differences between the cultivars. At harvest, the cultivars exhib ited significantly different levels of defoliation and curd attack. The cul tivars 9306 F1 and 9311 F1 showed high levels of resistance in all assessme nts, whereas the two cultivars exhibiting susceptibility at the seedling st age, 9304 F1 and 9305 F1, also exhibited susceptibility through the adult p lant stage. Thus, the resistance exhibited under field conditions resembled that identified at the seedling stage under controlled conditions. The res ults suggest that cotyledon resistance similar to that described could prov ide resistance throughout the adult plant stage, including curds.