Screening Portuguese cole landraces for resistance to seven indigenous downy mildew isolates

Twenty-nine Brassica oleracea accessions, including Portuguese cole landraces, were screened for resistance to crucifer downy mildew using seven Portuguese isolates of Peronospora parasitica. Seven-day-old seedlings were inoculated on the cotyledons with two 10 mu l droplets of a water suspension containing 5 x 10(5) conidiospores ml(-1). Host-pathogen interaction phenotype (IF) was evaluated 7 days after inoculation using an increasing susceptibility (1-9) scale. The screening revealed high variable reaction for downy mildew resistance within Portuguese cole landraces. Some accessions were very susceptible with most of the plants scoring in TP classes 7 and 9, but most of the accessions showed high variability in interaction phenotype with plants scoring in all IP classes. Four accessions belonging to landraces 'Couve Algarvia','Couve Murciana' and 'Couve Coracao de Boi' expressed high resistance to all downy mildew isolates, with over 42% of seedlings in the non-sporulating IP class 1. Downy mildew isolates Pp504 (Batalha), Pp503 (Condeixa), and Pp507 (Faro) showed slightly higher aggressiveness than isolates Pp505 (Loures), Pp502 (Vila Real), Pp506 (Odemira), and Pp501 (Povoa). There was no significant interactions between accession and downy mildew isolate, which shows that the geographic origin of the host and pathogen had no influence on the interaction phenotype. This study confirms the high potential of Portuguese cole landraces as sources of resistance to downy mildew and has identified accessions with a high valuable resistance. (C) 1997 Published by Elsevier Science B.V.