The apple pathogen Marssonina coronaria (teleomorph: Diplocarpon mali) has recently become a significant problem in Central European organic apple production, causing dark spots on both the leaves and fruit, and early leaf fall. Field observations and resistance testing under controlled conditions indicate that there are differences in resistance to M.coronaria between cultivars. We screened 39 apple cultivars, selected from a large collection of genetic resources, for their susceptibility or resistance to M.coronaria. 1 year old saplings were artificially inoculated under semi-controlled conditions. The development of disease symptoms was observed and assessed several times over more than two months. Symptoms varied largely between cultivars, from small spots on which acervuli immediately developed to large round brown necrotic spots, often with one acervulus in the centre. Leaf fall often started in the middle of the shoot and in parallel on the lower, older leaves. Even though important differences in susceptibility were observed, no cultivar with complete resistance was identified. The observations suggest that resistance to the disease is complex, and involves a number of mechanisms
