Acute Oak Decline in pedunculate oak forests in Croatia: first record and assessment of disease occurrence and severity

Key message Acute Oak Decline was confirmed at 22 sites across Croatian pedunculate oak forests; among 2,580 trees assessed at confirmed sites, 21.0% had stem bleeding, site-level incidence ranged from 0.83% to 58.33%, and individual trees had up to 19 bleeds, demonstrating widespread but strongly heterogeneous disease incidence and severity. Context Acute Oak Decline (AOD) has emerged as a major threat to European oak forests. As a complex decline syndrome shaped by interactions among bacterial pathogens, Agrilus spp. beetles and environmental stress, understanding its distribution and epidemiological status is essential for disease monitoring and forest management. However, its occurrence and epidemiological status in pedunculate oak ( Quercus robur L.) forests of south-eastern Europe remain poorly understood. Aims This study assessed AOD in Croatian pedunculate oak forests to determine disease occurrence and severity and provide a reference point for future monitoring and epidemiological research. Methods During 2024–2025, symptomatic pedunculate oaks were surveyed at 72 locations across Croatia. Trees with stem bleeding lesions were analysed by bacteriological isolation and multiplex real-time PCR for AOD-associated bacteria. Disease occurrence and severity were assessed from the proportion of symptomatic trees and the number of stem bleeding lesions per tree. Results AOD was confirmed across multiple Croatian pedunculate oak forests, demonstrating that the syndrome is established within the country rather than confined to isolated outbreaks. Brenneria goodwinii Denman et al. 2012 was detected in 22 samples, while Gibbsiella quercinecans Brady et al. 2010 was identified in 11 samples and always co-occurred with B. goodwinii . Disease occurrence ranged from 0.8% to 58.3%, with the highest severity recorded in eastern Croatia. Conclusion AOD is already geographically widespread in Croatian pedunculate oak forests, but its incidence and severity are strongly heterogeneous. Because this cross-sectional survey cannot determine whether the affected range or disease severity is increasing, repeated standardised surveillance is needed to identify possible expansion, detect changes at high-incidence sites and guide risk-based forest management.