Plant Sunflower resistance to French races of downy pathology mildew (Plasmopara halstedii) S Mouzeyar, J Philippon P Walser, F Vear D Tourvieille De Labrouhe INRA, Station d’Amélioration des Plantes, Domaine de Crouelle, F63039 Clermont-Ferrand cedex, France (Received 26 March 1994; accepted 31 May 1994) Summary — Three races of downy mildew (1, A, B) have been observed on sunflowers in France. Until recently, all genotypes resistant to race A were resistant to the less virulent races 1 and B. Some genotypes have now been identified as resistant to races 1 and A but susceptible to race B. Helianthus annuus = sunflower / Plasmopara halstedii = downy mildew / sélection aux races françaises de mildiou (Plasmopara halstedii). Il existe en France 3 races de Plasmopara halstedii, agent du mildiou du tournesol, Helianthus annuus L : la race 1, la race B et la race A. Tous les génotypes résistants à la race A étaient également résistants aux 2 autres races. Cette possibilité pouvait être exploitée par les sélectionneurs en utilisant uniquement la race A pour sélectionner pour la résistance simultanée aux autres races françaises. Cependant, la mise en évidence du comportement de la lignée QHP1 et de 2 populations, HAR4 et HAR5, qui sont résistantes à la race A mais sensibles à la race B, remet en question cette stratégie de sélection et imposerait l’utilisation de 2, voire des 3 races françaises. Résumé — Résistance du tournesol Helianthus annuus = tournesol/Plasmopara halstedii Downy mildew of sunflower (Helianthus annuus L) is caused by Plasmopara halstedii (Farl) Berl and de Toni. It is an obligate parasite with physiological races that are capable of attacking a variable range of sunflower genotypes. Resistance is controlled by major genes denoted PI. At present, 7 races and 16 PI genes have been reported (Gulya et al, 1991a, 1991b). In France, 3 different races have been found (Tourvieille et al, 1989): race 1, which was identified in 1966 (Louvet and Kermoal, 1966); race A, which is similar to the American race 4 and was identified in 1988; and race B which is similar to the American race 3 and was identified in 1989. * Correspondence and reprints = mildiou / sélection TableI presents the interactions of these 3 with a range of differential sunflower genotypes. The contamination of differentials and assessment of resistance were made according to the method described by Mouzeyar et al (1993, 1994), by observation of both fungal sporulation and microscopic development of resistance reactions. Until recently, race A was considered as the most virulent and all genotypes resistant to this race were also resistant to the less virulent races. This meant that breeders could only carry out selection programmes using the most virulent race in order to breed for resistance to all 3 races. races been observed that the inbred line QHP1 and the populations HAR4 and HAR5 are resistant to race A but susceptible to race B (less virulent). This means that it will be necessary to check that resistance sources are effective against all French races in breeding pro- However, it has now Sackston WE, Viranyi F, Masirevic S, Rashid KY (1991b) New races of the sunflower downy mildew pathogen (Plasmopara halstedii) in Europe and North and South America. J Phytopathol 132, 303-311 Gulya TJ, grammes. Louvet J, Kermoal JP (1966) Le mildiou menace-t-il la culture du tournesol en France? C R Acad Agric 52, 896-902 In addition, since this result is different from that obtained with American races 3 and 4 (Gulya, personal communication), more detailed studies of the identification of French races are Mouzeyar S, Tourvieille De Labrouhe D, Vear F (1993) Histopathological studies of resistance of sunflower (Helianthus annuus L) to downy mildew (Plasmopara halstedii). J Phytopathol 139, 289297 necessary. REFERENCES Gulya TJ, Miller JF, Viranyi F, Sackston Proposed internationally standardized identification of 11-20 race Plasmopara WE (1991a) methods for halstedii. Helia 14, Mouzeyar S, Tourvieille De Labrouhe D, Vear F (1994) Effect of host-race combination on resistance of sunflower (Helianthus annuus L) to downy mildew (Plasmopara halstedii). J Phytopathol (in press) Tourvieille D, Mouzeyar S, Lafon S, Regnault Y (1991) Evolution des races de mildiou (Plasmopara helianthi) sur tournesol en France. Proc 3rd Int Conf Plant Diseases ANPP, Bordeaux, 777-784