Evaluation of scab (Venturia inaequalis) severity on 54 cultivars of apple in an unsprayed common planting

Seventy apple (Malus x domestica Borkh.) genotypes were evaluated in a common planting to identify clones with partial resistance to apple scab [Venturia inaequalis (Cke.) Wint.] that could be used in a durable scab resistance breeding program. Fifty-four cultivars and 14 breeding selections of cultivated apple, and one clone each of Malus floribunda and Malus sikkimensis were evaluated. Two to ten replicate trees per genotype were propagated on dwarf rootstocks and planted into raised beds covered with black plastic at Agassiz, BC, Canada, a site with severe annual scab epidemics. No fungicides were applied. Field resistance to natural scab infection was rated on a 0 to 5 scale for each plant for three consecutive years. Scab-susceptible standards such as 'Delicious', 'McIntosh', 'Royal Gala' and 'Idared' had severe annual infection. Genotypes carrying V,, V. or V, scab resistance genes were free of leaf and fruit scab, as was Malus floribunda 821, indicating that scab races 5, 6 and 7 were absent from the plot. 'Regine', 'Rebella', 'Resi', 'Rewena' were notable for the absence of all visible leaf disease symptoms. Among cultivars with presumed polygenic resistance, 'Akane', 'Anis Aily','Antonovka','Bramley's Seedling', 'Chehalis', 'Discovery', 'Generos', 'Golden Reinette'. 'Golden Russett'. 'Margil', Peypring Cerueuko' and 'Wolf River' were free of sporulating scab every year. Trumos de Voinesti', 'Lord Lambourne', 'Spartan', 'Gravenstein' and 'Lodi' were less susceptible than 'McIntosh', but some trees had moderate to high infection (ratings 3 to 4) in some years. 'Goldgelbe' and 'Gladstone' initially appeared resistant, but became infected in the third year, and may not have durable resistance. Summerland apple introductions 'Sinta', 'Stirling', 'Silken', 'Chinook', '8S6923'(Aurora Golden Gala (TM)), and advanced selection BC 8S-26-50 were highly susceptible to scab, 'Creston' was moderately susceptible, and 'Golden Sentinel' and 'Scarlett Sentinel' were resistant. Several breeding selections of 'Akane' x 'Discovery' parentage originally selected for their fruit quality also appeared to be resistant to scab.