title: Hymenoscyphus fraxineusvs.Hymenoscyphus albidus- A comparative light microscopic study on the causal agent of European ash dieback and related foliicolous, stroma-forming species.
abstract: Five species ofHymenoscyphusthat 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 theHymenoscyphus-type represent two reliable morphological characteristics that, together with molecular data, provide clear evidence for their placement in the genusHymenoscyphus(Helotiaceae). Among the species treated isHymenoscyphus fraxineus(=Hymenoscyphus pseudoalbidus), the causal agent of the European ash dieback disease. Since 1992 this species started within Europe to replace the rather uncommonHymenoscyphus albidus, which is likewise confined to leaves ofFraxinus. Hy. fraxineushas been recorded already since 1990 in Eastern Asia (Japan, Korea, northeast of China), where it had been initially misidentified asLambertella albida(≡Hy. albidus). In these regions, it occurs as a harmless saprotroph onFraxinus mandshuricaandFraxinus rhynchophylla, 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 JapaneseHy. fraxineusin contrast to EuropeanHy. fraxineussupports this view. Genetic similarities between JapaneseHy. fraxineusand EuropeanHy. albidussuggest that alsoHy. albidusmight be a descendant of AsianHy. fraxineus, though having invaded Europe much earlier. However, consistent genetic deviation between European and AsianHy. fraxineusat 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.Hy. fraxineuswas described from Europe as a cryptic species that differed fromHy. albidusby molecular data alone. However, theHy. albidus/Hy. fraxineusspecies 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 resembleHy. albidusandHy. fraxineusform pseudosclerotia inAesculusleaves and again differ from each other mainly in the ascus base:Hymenoscyphus aesculionAesculus hippocastanumfrom Europe lacks croziers, whereasHymenoscyphus honshuanusfrom Japan onAesculus turbinatapossesses croziers. Other taxa treated here includeHymenoscyphus vacini, a European species growing on stromatized net veins of skeletonized leaves ofAcer, andHymenoscyphus torquatus, a Chinese species on unidentified herbaceous stems. An equivalent stroma-forming North American species on leaves ofFraxinus, Rutstroemia longipes(Rutstroemiaceae), is discussed and compared. A key to theHymenoscyphusspecies that form a dark stroma on leaves ofAcer, Aesculus, Fraxinus,andPicrasmais provided.
authors.affiliation: Blaihofstraße 42, D-72074 Tübingen , Germany.
authors.forename: Hans-Otto
authors.initials: HO
authors.lastname: Baral
authors.affiliation: Kleingemünderstraße 111, D-69118 Heidelberg , Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Bemmann
keywords: Acer; Aesculus; Fraxinus; Helotiaceae; Hy. honshuanus nom. nov. (for Lambertellinia scutuloides); Hy. torquatus comb. nov. (for Lambertella torquata); Hymenoscyphus aesculi comb. nov. (for Helotium/Lanzia aesculi); Rutstroemiaceae; croziers; homothallism; invasive species; molecular markers; morphology; pseudosclerotium; simple septa