Variabilité de l’agressivité de Plasmopara halstedii, étude de sa stabilité en test de laboratoire

Abstract

Evolution of pathogenicity in Plasmopara halstedii was analysed by successive artificial infections of two strains with different levels of aggressiveness on two sunflower genotypes not carrying major gene resistance but which show different levels of partial resistance. Symptoms, in particular of plant size and sporulation density, confirmed the differences between the two lines observed in the field. The parasite populations studied showed considerable adaptation to infection conditions: the percentage of plants showing severe symptoms increased by 5% at each infection cycle, for both pathogen strains and both sunflower genotypes. These first laboratory results need to be confirmed in the field to judge the possible durability of quantitative resistances which could be introduced in sunflower breeding programmes.

Keywords

• mildew
• sunflower
• pathogenicity
• adaptation
• selection pressure