Testing scab-resistance stability of new resistant cultivars within the apple breeding programme

World-wide most apple breeding programmes for scab-resistance are based on the monogenic resistance from Maims floribunda 821, V-f. In different tests V-f resistant cultivars have shown scab symptoms with sporulation. Therefore, since 1994 for examination of seedlings and cultivars we used highly virulent inoculum from the infected wild species M. floribunda of the Fruit Genebank Dresden-Pillnitz. The susceptibility of various cultivars carrying the V-f gene was very high after inoculation with this scab inoculum compared to the results of the conventional inoculum. V-f resistant cultivars like 'Jolana', 'Vanda', 'Rosana', 'Rubinola', 'Otava', 'Delorina', 'Baujade', and 'Liberty', the parent 'M. floribunda 821' and the accessions of M. floribunda were heavily infected by scab and showed strong sporulation. The Pillnitz 'Re-cultivars (R)' showed a hypersensitive reaction to scab without sporulation. The presence of the V-f gene in several tested cultivars was proved by marker analysis which should be used further in breeding for stable resistance. V-r resistant cultivars did not show attack by scab. The V-f resistance seems unlikely to be durable in future. Therefore the strategy must be to develop new resistant cultivars with a different genetic basis for resistance against scab.