Screening of Venturia inaequalis virulence and resistance of Malus genotypes to apple scab using in vitro methodology

The apple scab induced by Venturia inaequalis is an economically significant disease of apples worldwide and is predominantly controlled by multiple fungicide applications. Therefore, resistant apple cultivation is important for long-standing disease control. The knowledge about cultivar resistance is mainly founded on their evaluation in orchard conditions and testing in a greenhouse that is laborious and requires a large space. This study evaluated apple cultivar resistance and the virulence of V. inaequalis strains by inoculating detached leaves and immature fruits. Nine V. inaequalis strains originating from different apple-growing regions and host genotypes were tested on eight apple genotypes. Microscopic and macroscopic symptom development and host tissue reactions were monitored during the experiments. The tested V. inaequalis strains and cultivars showed different levels of virulence and resistance, respectively. Cultivar ‘Lobo’ was scored as partially susceptible, with almost all strains tested. The incompatible interaction with or without host tissue reactions was observed only on the cultivar ‘Priscilla’ with all the strains and field populations tested. The results of this study using detached leaves and immature fruits were concordant with the data obtained in the cultivar evaluation trials in the orchards.