Evaluation of Malus gene bank resources with German strains of Marssonina coronaria using a greenhouse-based screening method

Marssonina coronaria is considered a threat to organic apple production in Central Europe. Since the application of fungicides is limited in organic production, breeding for resistance seems to be a promising strategy to manage the disease in the future. In this study, an artificial inoculation method similar to procedures used in apple scab greenhouse screenings was developed using German strains of the pathogen M. coronaria for evaluating 110 Malus domestica cultivars. The strains were morphologically and molecularly characterized and confirmed as M. coronaria. Symptom development was significantly influenced by incubation method, conidia concentration and point of inoculation, but not by leaf position and different water types used for inoculation. The success of inoculation and spread of the fungus on infected leaves were confirmed by conventional PCR. Moreover, there was a difference in development of disease symptoms between inoculations with conidia from in vitro grown strains (in vitro inoculum) and conidia from diseased leaves from the orchard (field inoculum) in a time dependent manner. These differences which were first found with the susceptible cultivar Topaz' after artificial inoculation with in vitro and field inoculum were confirmed on 20 more apple cultivars inoculated with different in vitro grown strains. In summary, all tested cultivars, including 21 which are scab-resistant, developed symptoms of this disease. Results from three years of investigation indicate a decrease in virulence of M. coronaria strains, when cultivated on artificial culture (growth)-media. Hence field inoculum is recommended for artificial greenhouse screenings for the evaluation of disease resistance in Malus genetic resources.