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.
The causal agent is the fungus Chalara fraxinea, the anamorph (asexual form) of an ascomycete. 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.
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 pseudoalbidus. Whether the fungus has been introduced or became a pathogen by mutation is not known to date. Analyses of herbarium specimens showed that the fungus was present in Switzerland already in 1978 without causing any symptoms. 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. pseudoalbidus. 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.
Queloz V, Grünig CR, Berndt R, Kowalski T, Sieber TN & Holdenrieder O. Cryptic speciation in Hymenoscyphus albidus. Forest Pathology, published online 30 March 2010. doi:10.1111/j.1439-0329.2010.00645.x
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