vs. - A comparative light microscopic study on the causal agent of European ash dieback and related foliicolous, stroma-forming species.

Five species of that fruit on black stromatized parts of dead leaves of deciduous trees are presented, giving details on their morphological and ecological characteristics. Several of these species have previously been misplaced in rutstroemiaceous genera because of the presence of a substratal stroma. However, the heteropolar, scutuloid ascospores with an often hook-like lateral protrusion at the rounded apex and the ascus apical ring of the -type represent two reliable morphological characteristics that, together with molecular data, provide clear evidence for their placement in the genus (Helotiaceae). Among the species treated is (=), the causal agent of the European ash dieback disease. Since 1992 this species started within Europe to replace the rather uncommon , which is likewise confined to leaves of has been recorded already since 1990 in Eastern Asia (Japan, Korea, northeast of China), where it had been initially misidentified as (≡). In these regions, it occurs as a harmless saprotroph on and , suggesting that those populations are native while the European ash dieback disease has a recent Eastern Asiatic origin. The distinctly higher genetic diversity found in Japanese in contrast to European supports this view. Genetic similarities between Japanese and European suggest that also might be a descendant of Asian , though having invaded Europe much earlier. However, consistent genetic deviation between European and Asian at two nucleotide positions of the ITS region indicates that the European ash disease originates from a region different from the presently known areas in Eastern Asia. Our results underline the importance of detailed morphological studies in combination with molecular work. was described from Europe as a cryptic species that differed from by molecular data alone. However, the / species complex represents one of many examples within the ascomycetes in which subtle microscopic differences between closely related species, in this case the presence or absence of croziers at the ascus base, are strictly correlated with molecular characteristics. Two species that closely resemble and form pseudosclerotia in leaves and again differ from each other mainly in the ascus base: on from Europe lacks croziers, whereas from Japan on possesses croziers. Other taxa treated here include , a European species growing on stromatized net veins of skeletonized leaves of , and , a Chinese species on unidentified herbaceous stems. An equivalent stroma-forming North American species on leaves of (Rutstroemiaceae), is discussed and compared. A key to the species that form a dark stroma on leaves of and is provided.