EARLY SELECTION FOR PARTIAL RESISTANCE TO POWDERY MILDEW, PODOSPHAERA-LEUCOTRICHA (ELL ET EV) SALM IN APPLE PROGENIES

Selection of apple for partial resistance to powdery mildew in the nur sery before planting trees in the orchard can only be relied upon if g enotypic differences in expression do not depend on plant age. Five pr ogenies between susceptible cv. Melrose and five partially resistant g enotypes (four CPRO-DLO selections from the cross 'Elstar x Prima' and cv. Prima itself) were assessed for mildew resistance following natur al infection in the nursery during two successive seasons. Per progeny 60 randomly taken trees were transplanted into the orchard and their mildew resistance levels were assessed for three more years. The mean mildew resistance of the progenies in the nursery was related to that of their resistant parent, but differences in mean resistance levels i n the orchard were not consistent with those in the nursery. The indiv idual correlations of resistance between nursery and orchard also vari ed per progeny. In general the second year's assessment in the nursery gave the best prediction of the orchard performance over three years. In conclusion, nursery selection for partial mildew resistance can be effective, but the efficiency varies between populations.