ELSEVIER Scientia Horticulturae 68 (1997) 49-58 SCIENTIA HORlICULTuRR Screening Portuguese cole landraces for resistance to seven indigenous downy mildew isolates Margarida E. Sousa *, Jo50 S. Dias, Anthio A. Monteiro zyxwvutsrqponm Institute Superior de Agronomiu, Technicul Uniuersity of Lisbon, Tupuda du Ajudcr. I300 Lisbou. Portugul Accepted 28 September 1996 Abstract Twenty-nine Brussica olerucea accessions, including Portuguese cole landraces, were screened for resistance to crucifer downy mildew using seven Portuguese isolates of Peronosporu parasitica. Seven-day-old seedlings were inoculated on the cotyledons with two 10 pl droplets of a water suspension containing 5 X lo5 conidiospores ml-‘. Host-pathogen interaction phenotype (IP) was evaluated 7 days after inoculation using an increasing susceptibility (l-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 IP 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 Cotqlo 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 (Fare) showed slightly higher aggressiveness than isolates Pp505 (Loures), Pp502 (Vila Real), Pp506 (Odemira), and Pp501 (P6voa). 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. 0 1997 Published by Elsevier Science B.V. Keywords: Bmssicu olerucea; Disease resistance; Galega kale; Perorwsporu purusiticu; Tronchuda cabbage 1. Introduction Crucifer downy mildew, caused by the oomycete Peronosporu parasitica (Pers. ex. Fr.) Fr., is a destructive pathogen of seedling brassicas which may also cause important * Corresponding author. 0304-4238/97/$17.00 6 1997 Published by Elsevier Science B.V. All rights reserved. PII SO304-4238(96)00979-X