Forest Pathology and Dendrology
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Hymenoscyphus fraxineus


A new lethal disease of ash trees (Fraxinus excelsior) was detected in 2008 in Switzerland after having caused significant damage previously in other countries. Symptoms are necrotic bark lesions (Fig. 1, 2) and wilting (Fig. 3).

Kowalski, T., Holdenrieder, O. (2008): Eine neue Pilzkrankheit an Esche in Europa. Schweiz. Z. Forstw. 159 (3): 45-50.
Kowalski, T., Holdenrieder, O. (2009): Pathogenicity of Chalara fraxinea. Forest Pathology 39: 1-7.
Pautasso, M., Aas, G., Queloz, V. and Holdenrieder O. (2013): European ash (Fraxinus excelsior) dieback - A conservation biology challenge. Biological Conservation 158: 37–49.


The causal agent is the fungus Chalara fraxinea, the anamorph (asexual form) of the ascomycete Hymenoscyphus fraxineus. The fungus forms slow growing cultures (Fig. 4, reverse Fig. 5) and distinct conidiophores on which sticky, colorless, ellipsoid conidia (spores) are produced (Fig. 6). The conidia of C. fraxinea probably are not infectious.

Kowalski, T. (2006): Chalara fraxinea sp. nov. associated with dieback of ash (Fraxinus excelsior) in Poland. Forest Pathology 36(4): 264 - 270.
Gross, A., Zaffarano, P.L., Duo, A. and Grünig C.R. (2012): Reproductive mode and life cycle of the ash dieback pathogen Hymenoscyphus pseudoalbidus. Fungal Genetics And Biology 49: 977–986.


Later on, the teleomorph (sexual form or ascus form) of the fungus was detected. It is formed during summer on ash petioles in the litter (Fig. 7, 8; diameter of fruitbodies up to 3 mm). The ascospores are produced in asci (Fig. 9) and are transmitted by wind and this can explain the rapid spread of the disease. Using molecular genetic methods, the fungus was recognized as a new species and named Hymenoscyphus fraxineus. This pathogen has been introduced from East Asia to Europe. There is a second, morphologically almost indistinguishable species, Hymenoscyphus albidus, which is known from Europe since 1851 and which is not regarded as pathogenic. Previously, H. albidus was confused with the ash dieback pathogen H. fraxineus. We are studying the biology of the pathogen and his sister species, aiming at the understanding of pathogenicity mechanisms and the development of control strategies.

Gross, A., Holdenrieder, O., Pautasso, M., Queloz, V. and Sieber T.N. (2013): Hymenoscyphus pseudoalbidus, the causal agent of European ash dieback. Molecular Plant Pathology 15: 5–21.
Queloz, V., Grünig, C.R., Berndt, R., Kowalski, T., Sieber, T.N. & Holdenrieder O. (2011): Cryptic speciation in Hymenoscyphus albidus. Forest Pathology 41: 133-142.
Queloz, V., Grünig, C.R., Berndt, R., Kowalski, T., Sieber, T.N. & Holdenrieder O. (2012) Corrigendum. Forest Pathology 42: 352-352


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