Assessment of Apple Blotch ( <i>Diplocarpon coronariae</i> ) Symptom Progression on <i>Malus × domestica</i> Leaves

ABSTRACT Apple blotch is a fungal disease of increasing importance in European apple cultivation. Infection by Diplocarpon coronariae leads to premature leaf abscission in summer, weakening tree vigour and exacerbating harvest losses. Developing resistant cultivars represents a promising preventive strategy. Traditional apple cultivars could potentially serve as resistance donors, but their susceptibility to apple blotch has not been systematically evaluated. To address this knowledge gap, 535 apple cultivars from the German Fruit Genebank collection were assessed through conidial inoculation of detached leaves to analyse the development of acervuli, necrosis and chlorosis 13‐days post‐inoculation. Subsequent greenhouse trials were conducted on a subset of 151 cultivars to validate these results and to include leaf abscission timing as an additional disease symptom. The results reveal that none of the tested cultivars exhibited complete resistance. However, eight cultivars with reduced symptom development in laboratory trials were identified that also showed a delayed leaf abscission compared to highly susceptible cultivars. These findings highlight potential candidates for cultivating robust cultivars and breeding programmes aimed at improving apple blotch resistance.