The formation of collar rots in association with ash dieback was studied under different site conditions. The fungal com-munity associated with lesions, necroses and stem collar rots, especially the occurrence of Hymenoscyphus fraxineus at these symptomatic plant tissues, was investigated. Filamentous fungi and Phytophthora spp. were isolated from affected tissues of stem collar rots of various developmental stages. Tissue samples of collar rots were collected from 32 ash trees in seven different forest plots located in Northwest Germany. Obtained isolates were assigned to morphotypes and identified based on mycelial morphology and by molecular methods. Primary agents causing collar rots were identified and the influence of site conditions was derived. The studied stem collar rots were assigned to five symptomatic categories: (0) without collar rots or lesions, (1) emerging collar rots, (2) larger collar rots without visible wood decay, (3) advanced collar rots with visible wood decay and (4) collar rots, necroses or lesions associated with dark sap oozing. In most of the studied collar rots that were collected in Schles-wig-Holstein and Lower Saxony, H. fraxineus was isolated and assumed to be the primary agent. From samples of category 0 neither H. fraxineus nor other fungi or Phytophthora species were isolated. From collar rots of the other symptomatic categories, varying quantities of endophytic, saprotrophic and pathogenic species had been isolated. Overall, the number of isolated species was higher in advanced stages of collar rot. Most common species were H. fraxineus and Neonectria punicea, followed by Di-aporthe eres, Botryosphaeria stevensii, Gibberella sp., Fusarium solani and Cadophora sp. However, collar rots in early stages were only associated with H. fraxineus and N. punicea. Armillaria or Phytophthora species were only isolated from advanced collar rots or occurred under special side conditions.

Keywords:Ash dieback, Fraxineus excelsior, Collar rot, Wood decay, Hymenoscyphus fraxineus, Neonectria punicea, Armillaria, Phytophthora, Endophytes, Plant pathogens, Fungi.