F. excelsior is affected by dieback in the major part of its natural geographical range in Europe, which results in economic and ecological losses. The disease is caused by the ascomycetous fungus Hymenoscyphus fraxineus, a pathogen introduced to Europe most probably from East Asia. This paper presents data on fungi identified on F. excelsior trees representing two different stages of ash dieback in Poland. Fungal communities were identified in initial necrotic lesions on living stems and twigs using the classical method of isolation on malt extract agar and morphological and molecular analyses. In dead apical parts of stems and twigs, fungi were identified by microscopic analyses of fruit bodies formed in situ. Seventy-one fungal taxa were found in 720 samples with symptoms of initial or advanced necrosis. The most common fungus detected in initial necrotic lesions in each forest site was Hymenoscyphus fraxineus (59.2 % of analysed samples). Other frequently isolated fungi included Alternaria alternata, Diaporthe eres, Diplodia mutila, Fusarium avenaceum, F. lateritium and Phomopsis spp. Fruit bodies on dead apical parts of stems and twigs were produced mostly by Diaporthe eres, Diplodia mutila, Lophiostoma corticola, Phomopsis spp., Sirodothis sp. and Valsa cypri. Fungal communities from different sites were similar, as shown by high Sørensen similarity index values. Greatest variation in fungal community structure at the initial necrotic stage was realized by D. mutila and F. avenaceum, and at the advanced necrotic stage by D. eres, D. mutila and Phomopsis spp. Data show a close affinity of certain fungi to site, suggesting geographical relatedness. The ecological effects of distribution of the invasive H. fraxineus and of other fungi sporulating on diseased ash trees in Europe are discussed.

Studies on fungal communities in the previous year’s leaf petioles of Fraxinus excelsior found in litter in five ash stands in southern Poland were made in 2017. Fungi were identified on the basis of isolation from 300 surface sterilized leaf petioles and by in situ inventory of fruit bodies (on 600 petioles, in spring and autumn). Identification was based on morphology of colonies and fruit bodies, and sequencing of ITS region of the rRNA gene cluster. In total, 2832 isolates from 117 taxa (Ascomycota—100; Basidiomycota—15; Mucoromycota—2 taxa) were obtained with the isolation method. The most frequent taxa (with frequency >10%) were: Nemania serpens, Hymenoscyphus fraxineus, Alternaria sp. 1, Boeremia sp., Helotiales sp. 1, Epicoccum nigrum, Venturia fraxini, Fusarium sp., Fusarium lateritium, Nemania diffusa, Typhula sp. 2 (in descending order). In total, 45 taxa were detected with the in situ inventory method. Eleven taxa were classified as dominant: Hymenoscyphus fraxineus, Venturia fraxini, Leptosphaeria sp. 2, Cyathicula fraxinophila, Typhula sp. 2, Hypoderma rubi, Pyrenopeziza petiolaris, Cyathicula coronata, Hymenoscyphus scutula, Leptosphaeria sclerotioides and Hymenoscyphus caudatus. Among 202 leaf petioles colonized by H. fraxineus, 177 petioles also showed fructification of 26 other fungi. All the isolated saprotrophs were tested in dual-culture assay for antagonism to two strains of H. fraxineus. Three interaction types were observed: type A, mutual direct contact, when the two fungi meet along the contact line (occurred with 43.3% of test fungi); type B, with inhibition zone between colonies (with 46.9% of test fungi); type C, when the test fungus overgrows the colony of H. fraxineus (with 9.8% of test fungi). The possible contribution of the fungal saprotrophs in limiting of the expansion of H. fraxineus in ash leaf petioles, which may result in reduction in the inoculum of ash dieback causal agent, is discussed.

Two new species of Phialocephala were isolated from the periderm of living branches of forest trees in Germany. In culture, Phialocephala compacta Kowalski & Kehr sp.nov., found on Alnus, is characterized by crowded conidiogenous heads that become deeply pigmented with age. Phialocephala scopiformis Kowalski & Kehr sp.nov., isolated from Picea and other conifers, develops elongated, complex conidiogenous heads with up to 15 series of branches in culture. The taxonomic placement of both new species and their role in endophytism and in natural pruning of branches is discussed. Key words: Picea abies, Alnus glutinosa, Phialocephala compacta, Phialocephala scopiformis, branch pruning fungi.