AbstractHyrcanian forests comprise a long strip along the southern coast of the Caspian Sea in northern Iran. Oriental beech (Fagus orientalis) is one of the most valuable forest tree species in the Hyrcanian forests. Bark necrosis and bleeding spots of oriental beech were observed in Savadkuh forests in Mazandaran province during summer 2021. Ten samples were collected from symptomatic oriental beech trees. The bark canker symptoms included stem tissue necrosis, dark brown to black spots on the trunk, bleeding with dark exudates and dark lesions in the inner bark. Twenty‐six strains of culturable bacteria were isolated from the samples which in culture were predominantly round, convex, smooth with entire margins on nutrient agar and a metallic green sheen pigment on eosin‐methylene blue (EMB) agar. Fourteen strains induced necrosis and browning of the tissues around the inoculation sites on acorns after 2 weeks. Five representative strains also caused dieback in twigs of oriental beech 3 weeks after inoculation. These bacteria were Gram‐negative, catalase positive and oxidase negative. All strains produced colonies with a green metallic sheen on EMB agar. Strains were negative in tests for starch and gelatine hydrolysis or production of levan. A phylogenetic tree based on concatenated sequences of gyrB and infB genes, of representative strains well as in each of the single gene‐based phylogenetic trees, indicated that these strains clustered with Brenneria alni NCPPB 3934T with high bootstrap values. To the best of our knowledge, this is the first report of B. alni associated with bark necrosis and bleeding spots on oriental beech.