Resistance to mildew in apple progenies of the scab-resistant clone U 211 and mildew susceptible cultivars

Progenies of scab (Venturia inaequalis) resistant (V-f) clone U 211 ('Primula' open-pollinated, bred at the Department of Pomology, Warsaw Agricultural University) and cultivars with different degree of mildew susceptibility and others progenies were evaluated for natural mildew (Podosphaera leueotricha) incidence for up to 6 years in the field. Mildew infection of seedlings in the field greatly varied with weather conditions. U 211 transmitted mildew resistance in a considerably higher proportion compared to susceptible cultivars. In progenies obtained from the crosses of susceptible cultivars such as 'Jonared', 'Blackjon', 'Idared' and 'Granny Smith' with U 211, the percentage of non-and low infected seedlings ranged from 38% to 67%. In two other hybrid families (without U 211 as a parent), the percent of non-and low infected seedlings was 7% and 15%. Screening for mildew incidence during the second year in the nursery was not always effective; under Polish climatic conditions further screening should be continued in successive years. These studies have confirmed our earlier observations that the scab resistant U 211 is a good source for breeding of scab resistant apple cultivars combined with the field resistance to mildew, particularly when crossed with cultivars sensitive to mildew. Partially resistant cultivars increased the resistance of seedlings in progenies of U 211. Durability of mildew resistance should be increased by crossing U 211 with different sources of resistance.