Fungal diversity and foliar disease dynamics in urban horse chestnut trees: A transect study across southern to northern European cities

Horse chestnut trees ( Aesculus hippocastanum ) play a vital role in European urban landscapes, offering ecological, cultural, and aesthetic benefits. However, their health is increasingly affected by biotic stressors, including fungal pathogens and insect pests, within the context of urbanization and climate change. This study assessed leaf health in 659 horse chestnut trees across five European countries, using visual symptom scoring and fungal identification from necrotic leaf tissue. The results revealed a diverse fungal community associated with leaf blotch symptoms, indicating that multiple pathogens besides Guignardia aesculi are involved. Patterns of fungal symptoms and leaf miner ( Cameraria ohridella ) presence varied across geographic regions suggesting influence by broader environmental conditions. Fungal diversity was highest in both, the southern and the northern areas as compared to central European ones, with several identified fungal species having implications for leaf damage and public health. Street trees generally exhibited more severe foliar symptoms (higher Leaf Health Score) compared to park trees, an observation likely to more constrained growing conditions and increased exposure to urban stress. This study emphasizes the importance of integrating pathogen monitoring and species diversity into urban forest management to support the long-term resilience of urban tree populations. • Leaf blotch in horse chestnut trees ( Aesculus hippocastanum ) is caused by a diverse and competitive fungal community, with Guignardia aesculi playing only a minor role. • Fungal diversity was highest in Southern and Northern European cities, with distinct fungal species dominating in different geographic regions along the south-north transect. • Pathogenic and allergenic fungi made up over 50% of all fungal isolates from symptomatic urban tree leaves, indicating significant dual risks to tree health and public health. • Street trees showed significantly poorer leaf health than park trees, showing higher Leaf Health Scores (LHS) that indicate more severe foliar damage. • Adaptive urban forestry that integrates fungal pathogen monitoring and tree species diversification is crucial for mitigating biotic threats and enhancing resilience in urban environments.