European beech decline in Slovenia is caused by a complex disease

In recent decades, the average crown defoliation of European beech ( Fagus sylvatica ) in Central Europe has been steadily increasing, resulting in a decline in tree vitality. This study aimed to identify the key factors contributing to this deterioration. Forty healthy and 40 damaged European beech trees were felled on a systematic 16 × 16 km grid, and all tree parts were sampled for fungi and insects. Additionally, soil samples were collected for Phytophthora testing. Of 6400 cultured samples, 5828 fungal cultures were classified into 251 morphotypes. The twenty most frequent morphotypes from each tree part were selected for further molecular identification, revealing 44 different fungal taxa. The most frequently isolated fungal species were Neonectria coccinea , Neohendersonia kickxii , Apiognomonia errabunda and Aureobasidium pullulans —all well-known and common endophytes. Surprisingly, Phytophthora species were detected in only three of the 80 soil samples. The most frequent insect species were Orchestes fagi , Phyllaphis fagi , Psilocorsis reflexella and Phyllonorycter maestingella . The results indicate that the decline of European beech in Central Europe is driven by a multifaceted interplay of biotic and abiotic factors, with fungi playing the most significant role. Analysis revealed distinct differences in fungal and insect communities across sampled tree parts, but not between healthy and damaged trees. This finding is crucial, as it shows that healthy trees host endophytes that can exhibit pathogenic traits under external stress factors. Therefore, resilience and sustainability of beech will depend on mitigation of stressors and implementation of adaptive management strategies that address the evolving environmental challenges. • Fungal and insect communities did not differ between healthy and damaged trees. • Healthy trees host opportunistic pathogens that activate under external stress. • Resilience and sustainability of beech will depend on mitigation of stressors.