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 of Hymenoscyphus 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 Hymenoscyphus-type represent two reliable morphological characteristics that, together with molecular data, provide clear evidence for their placement in the genus Hymenoscyphus (Helotiaceae). Among the species treated is Hymenoscyphus fraxineus (=Hymenoscyphus pseudoalbidus), the causal agent of the European ash dieback disease. Since 1992 this species started within Europe to replace the rather uncommon Hymenoscyphus albidus, which is likewise confined to leaves of Fraxinus. Hy. fraxineus has been recorded already since 1990 in Eastern Asia (Japan, Korea, northeast of China), where it had been initially misidentified as Lambertella albida (≡Hy. albidus). In these regions, it occurs as a harmless saprotroph on Fraxinus mandshurica and Fraxinus 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 Japanese Hy. fraxineus in contrast to European Hy. fraxineus supports this view. Genetic similarities between Japanese Hy. fraxineus and European Hy. albidus suggest that also Hy. albidus might be a descendant of Asian Hy. fraxineus, though having invaded Europe much earlier. However, consistent genetic deviation between European and Asian Hy. fraxineus 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. Hy. fraxineus was described from Europe as a cryptic species that differed from Hy. albidus by molecular data alone. However, the Hy. albidus/Hy. fraxineus 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 Hy. albidus and Hy. fraxineus form pseudosclerotia in Aesculus leaves and again differ from each other mainly in the ascus base: Hymenoscyphus aesculi on Aesculus hippocastanum from Europe lacks croziers, whereas Hymenoscyphus honshuanus from Japan on Aesculus turbinata possesses croziers. Other taxa treated here include Hymenoscyphus vacini, a European species growing on stromatized net veins of skeletonized leaves of Acer, and Hymenoscyphus torquatus, a Chinese species on unidentified herbaceous stems. An equivalent stroma-forming North American species on leaves of Fra...

Detection of Airborne Inoculum of Hymenoscyphus fraxineus and H. albidus during Seasonal Fluctuations Associated with Absence of Apothecia

“…A major concern in our results is the coexistence of H. fraxineus and H. albidus in this forest, especially when some authors had pointed out that H. albidus appeared to be extinct in the Czech Republic [15]. H. albidus has been infrequently recorded in Europe over the last 150 years [15].…”

“…H. albidus has been infrequently recorded in Europe over the last 150 years [15]. However, lastly, several authors have reported the presence of H. albidus in forests affected by H. fraxineus.…”

“…fraxineus and H. albidus overlap their niches. Both species may colonize living leaves of F. excelsior [15]. However, H. fraxineus fructifies earlier, which may enhance its colonization to the detriment of H. albidus [13].…”

Fungi on stems and twigs in initial and advanced stages of dieback of European ash (Fraxinus excelsior) in Poland

“…Further research based on DNA sequences of isolates from Switzerland and other European countries revealed that H. albidus represents an aggregate of two clearly delimited species (Queloz et al 2011). One was related to the non-pathogenic H. albidus sensu stricto, which had been known in Europe since 1850 (Kowalski and Holdenrieder 2009a;Baral and Bemmann 2014). The second was described as a new cryptic species, Hymenoscyphus pseudoalbidus, with Ch.…”

Fraxinus sogdiana, a Central Asian ash species, is susceptible to Hymenoscyphus fraxineus

“…F. rhynchophylla Hance). Also, the range of H. fraxineus may enfold only the eastern part of the ranges of its hosts, where by today this fungus has been recorded (see Figure 24 in Baral & Bemmann 2014). Conversely, in Europe the still easternmost point of occurrence of H. fraxineus was established in eastern Ukraine (Davydenko et al 2013).…”