Breakdown of Pl2, a major gene of resistance to apple powdery mildew, in a French experimental orchard

Pl2, a major resistance gene to powdery mildew originating from Malus zumi, is mainly used in apple-breeding programmes. In an experimental orchard in France, an increase of susceptibility to powdery mildew was observed on apple genotypes carrying Pl2. This increase of susceptibility could not be explained by an effect of the age of the trees, or by an effect of the amount of inoculum on expression of the resistance Pl2. It was demonstrated by tests of pathogenicity in controlled conditions that isolates of Podosphaera leucotricha sampled in this orchard were virulent to Pl2. The appearance of virulent isolates within 6 years after planting apple genotypes selected for the presence of Pl2 resulted in a breakdown of the resistance. Ten years after planting, the percentage of genotypes that were still resistant to powdery mildew varied between 2 and 56%, depending on the progenies. This suggested that additional quantitative resistance genes may influence the durability of the resistance Pl2.