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Differential Responses of Brassica oleracea and B. rapa Accessions to Seven Isolates of Peronospora parasitica at the Cotyledon Stage. D. Silue, Prince de Bretagne Biotechnologie, Pen Ar Prat, 29250 Saint Pol de Leon, France. N. I. Nashaat, IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK, and Y. Tirilly, Laboratoire de Microbiologie et Biochimie, ESMISAB, Universite de Bretagne Occidentale, Technopole Brest-Iroise, 29280 Plouzane, France. Plant Dis. 80:142-144. Accepted for publication 16 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0142.

Nineteen accessions of Brassica oleracea var. italica (broccoli), 31 of B. oleracea var. boirytis (cauliflower), two of B. oleracea var. capitata (cabbage), three of B. rapa subsp, rapifera (turnip), one of B. rapa subsp, pekinensis (Chinese cabbage), and three of Raphanus sativus (radish) were tested for their response to isolates of Peronospora parasitica (downy mildew) at the cotyledon stage. Of the seven isolates tested, four were from crops of cauliflower in France, two from oilseed rape (B. napus subsp, oleifera) in the UK, and one was from mustard (B. juncea) in India. Twenty-one differential responses to P. parasitica isolates from B. oleracea and two from B. rapa were identified. All Raphanus sativus accessions were resistant to all seven isolates. Accessions for which seedling populations exhibited a heterogeneous reaction to some isolates were classified in a separate category. The differential resistance to P. parasitica identified in B. oleracea and B. rapa can be used for future studies of the genetics of the host-pathogen interaction and for breeding for disease resistance