<italic>Cryptostroma corticale</italic> and fungal endophytes associated with <italic>Fraxinus excelsior</italic> affected by ash dieback.

While assessing the filamentous fungi associated with woody tissues of stem collar rots, necroses, and lesions of European ash trees (<italic>Fraxinus excelsior</italic>) presenting symptoms of ash dieback in Germany, <italic>Cryptostroma corticale</italic> was recovered from three different ash trees. These isolated strains were the first report of <italic>C. corticale</italic> on ash and the first proof of an association of this plant pathogen with woody tissues of other tree species than <italic>Acer</italic> spp. in Germany. To test the pathogenicity of <italic>C. corticale</italic> against <italic>F. excelsior</italic> and to fulfil Koch’s postulates, inoculation tests <italic>in planta</italic> with strains isolated from <italic>Acer pseudoplatanus</italic> and <italic>F</italic>. <italic>excelsior</italic> were conducted according to Henle–Koch’s postulates in a greenhouse located in Göttingen. The pathogenicity tests were performed with apparently healthy ash saplings from June 2021 until January 2022. After three and seven months, neither necroses or lesions due to <italic>C. corticale</italic> nor disease symptoms were observed. Mostly, the inoculation wounds healed over, and <italic>C. corticale</italic> could not be re-isolated from the ash woody tissue. In an attempt to re-isolate the inoculated strains, the filamentous fungal endophytes of the ash woody tissues were isolated and identified. A total of 32 taxa of the Ascomycota were found, where the most common species was <italic>Boeremia exigua</italic>. Most frequently observed orders were <italic>Pleosporales</italic> (58.4%), followed by <italic>Sordariales</italic> (13.5%), <italic>Hypocreales</italic> (9.4%), and <italic>Diaporthales</italic> (8.7%). On average, 3.7 endophytic species were recorded on each sapling. [ABSTRACT FROM AUTHOR]