Impact of major gene resistance management for sunflower on fitness of Plasmopara halstedii \(downy mildew\) populations

1 Inra-UBP, UMR 1095, 234, avenue du Brézet, 63100 Clermont-Ferrand, France 2 Inra-CNRS, UMR 441, BP 52627, 31326 Castanet-Tolosan, France 3 Inra-UBP, UMR 1095, 24, avenue des Landais, 63177 Clermont-Ferrand, France 4 CETIOM, ENSAT, 18 Chemin de Borde-Rouge, BP 32607 31326 Auzeville-Tolosane France 5 Inra, UMR santé végétale (Inra-Enita), centre de recherches de Bordeaux, la Grande Ferrade, BP 81, 33883 Villenave d’Ornon cedex, France Abstract: Changes in virulence of Plasmopara halstedii populations under different major gene (Pl) management strategies were studied over 5 years continuous cropping of one sunflower hybrid under netting cages. Strategies were monoculture of forms of the hybrid with 1 gene or with combinations of 2 genes, alternation of different genes, and mixtures of several different forms of the hybrid. Monoculture with single resistance genes led to loss of efficient resistance after 3 years, with high levels of disease and increased variability of the pathogen, whatever the Pl gene used. Combinations of genes, alternation and mixtures gave longer term control of downy mildew. In particular, combinations of resistance genes coming from both female and male parents of the hybrid (such that even impurities had a resistance gene) gave the best control and least variation in pathogen virulence. Results are discussed with the object of durable control of downy mildew by all methods available.