Evaluation of Venturia inaequalis virulence on apple immature fruits and detached leaves by an in-vitro methodology

Abstract: Apple scab caused by Venturia inaequalis (Cooke) G. Winter is an economically
important disease in apple-growing areas worldwide. The disease is predominantly controlled
by application of fungicides; however, growing resistant varieties is critical to the long-term
control of the disease. In this study, the initial screening of apple genotype resistance and
virulence of V. inaequalis strains was performed by inoculating detached immature fruits and
incubating in moist chambers. Eleven monoconidial strains of V. inaequalis originating from
different geographic regions and host cultivars were tested on seven apple genotypes.
Microscopic and macroscopic symptom development and host tissue reactions were monitored during the experiments. The interaction phenotypes from susceptible to resistant were observed, indicating divergent virulence of the tested V. inaequalis strains. Most of the apple genotypes were scored as partially susceptible. Cv. ‘Lobo’ was scored as partially susceptible with almost all strains tested. Fully compatible interactions without host tissue reactions were observed in almost all genotypes with one to three V. inaequalis strains. The incompatible interaction with or without host tissue reactions was observed only on cv. ‘Priscilla’ with all of the strains and field populations tested. The results on cultivar resistance or susceptibility obtained in this study were similar to those reported from cultivar evaluation trials in the orchards.