Development of apple powdery mildew on sources of resistance to Podosphaera leucotricha, exposed to an inoculum virulent against the major resistance gene Pl‐2

The major resistance gene Pl‐2 in apple has recently been shown to be overcome by a virulent population of powdery mildew, Podosphaera leucotricha, in genotypes of the fourth generation after introgression of Pl‐2 from crab apple, Malus zumi. Tests of pathogenicity in controlled conditions and observations in an orchard showed that the initial progenitor of Pl‐2, MAL 68/1, was overcome by a virulent inoculum. As a consequence, all selections derived from MAL 68/1 may lose their resistance to powdery mildew, if Pl‐2 is used alone. Sources of resistance other than Pl‐2 remained efficient to the virulent population: M. hupehensis, M. mandshurica, M. robusta, M. sargentii, M. sieboldii, D12, Mildew Immune Selection and ‘White Angel’. The combination of Pl‐2 with quantitative resistance genes resulted in a high level of resistance.