The relationship between cotyledon and adult plant resistance to downey mildew (Peronospora parasitica) in Brassica oleracea

410 accessions of a "core collection" of Brassica oleracea were screened at the cotyledon stage for resistance to downy mildew with isolates P501 (Portugal), P005 and P006 (both UK). Seedlings were assigned to one of six previously described interaction phenotypes. Seed of 20 accessions that showed a high proportion of seedlings in non-sporulating classes and of four accessions with an intermediate and a high proportion of seedlings in sporulating classes was used to produce plants which were assessed for response to downy mildew as adult plants in a field trial in Central Portugal. Field evaluation employed a five class scale of increasing susceptibility based on the number of infected leaves per plant and on the size of sporulating lesions. The incidence of plants exhibiting field resistance was higher than cotyledon resistance with some accessions showing more than 95␘f plants with no sporulating lesions. The accessions with more than 20␘f the seedlings in non-sporulating classes following inoculation with any of the isolates all expressed moderate to high resistance in the field. There was no accession with higher resistance at the cotyledon than at adult plant stage. In contrast, accession HRI 9846 (ornamental kale) was very susceptible at the cotyledon stage and yet was resistant in the field.