Cryptostroma corticale and fungal endophytes associated with Fraxinus excelsior affected by ash dieback

While assessing the filamentous fungi associated with woody tissues of stem collar rots, necroses, and lesions of European ash trees (Fraxinus excelsior) presenting symptoms of ash dieback in Germany, Cryptostroma corticale was recovered from three different ash trees. These isolated strains were the first report of C. corticale on ash and the first proof of an association of this plant pathogen with woody tissues of other tree species than Acer spp. in Germany. To test the pathogenicity of C. corticale against F. excelsior and to fulfil Koch's postulates, inoculation tests in planta with strains isolated from Acer pseudoplatanus and F. excelsior were conducted according to Henle-Koch's postulates in a greenhouse located in Gottingen. 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 C. corticale nor disease symptoms were observed. Mostly, the inoculation wounds healed over, and C. corticale 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 Boeremia exigua. Most frequently observed orders were Pleosporales (58.4%), followed by Sordariales (13.5%), Hypocreales (9.4%), and Diaporthales (8.7%). On average, 3.7 endophytic species were recorded on each sapling.