Differences in apple cultivar susceptibility to apple blotch disease

Apple blotch is a disease caused by the fungus Diplocarpon coronariae. Typical symptoms are brown spots on leaves and fruits resulting in impaired fruit quality and fruits that cannot be marketed. A promising strategy to reduce the impact of this disease is the cultivation of more robust cultivars. There has been limited research about the susceptibility of apple cultivars to the disease. In this study, 154 apple cultivars from the German Fruit Genebank (GFG) collection were evaluated for their susceptibility to apple blotch disease in the greenhouse. Initial resistance screenings were carried out using detached leaf assays in petri dishes in the laboratory and revealed differences in susceptibility between apple cultivars. In particular, the GFG collection comprised cultivars that exhibited reduced symptom development compared to cultivars with heavy symptom expression. In this study, the cultivars were further tested in greenhouse trials to validate these findings and to monitor the time point of leaf drop. Variations in the time point of leaf drop were observed. These results provide valuable insight about the identification of apple cultivars with reduced apple blotch symptom development.