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European Ash dieback

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European Ash dieback causes leaf loss and crown dieback in affected trees. It started around 1992 in Poland and by 2010 the disease had been recorded in 22 European countries.

 

A fungal disease

The disease-causing agent was identified only in 2006. It was first described as a new fungal species, Chalara fraxinea. In 2009, based on morphological and DNA sequence comparisons, C. fraxinea was suggested to be the asexual stage (different stage of same biological species) of the ascomycete Hymenoscyphus albidus. This was a major surprise, as H. albidus is a widely spread and regionally common, saprotrophic fungus with a long history of records across Europe since 19th century. It has previously only been known as harmless decomposer of shed Fraxinus leaves.

However, in 2010 more detailed molecular investigations led to the conclusion that the disease agent belongs to a biologically different entity than H. albidus, and consequently, the ash dieback pathogen was described as H. pseudoalbidus – a species entirely new to science.

A highly invasive disease

Taking the pattern of successive geographic spread of the disease across Europe into account, H. pseudoalbidus seems to be an alien, highly invasive and aggressive fungus, the origin of which remains unknown. Since the first observations in central-east Europe, the disease spread across the continent in all geographic directions, and recently the disease almost simultaneously invaded Finland in the north, Belgium in the west, and Italy in the south thus demonstrating the wide climatic adaptability of H. pseudoalbidus.

Another intriguing feature of H. pseudoalbidus is the occurrence of this fungus in different parts of declining trees, as leaves, petioles, buds, bark, wood and roots, where it causes wilt, necroses, cankers, wood discoloration and decay. This makes H. pseudoalbidus unique, as no other fungus is known to cause such wide range of symptoms in such a broad range of physiologically, biochemically and physically different plant tissues. Moreover, the fungus has also been detected in symptomless Fraxinus leaves. The latter raises question if the observed were the initial yet latent infections, or H. pseudoalbidus is also a natural endophyte of Fraxinus. In addition, the possibility cannot be excluded that the fungus is a natural endophyte of certain Fraxinus species outside Europe, which would provide implications regarding origin of the disease.

Will ash survive ?

Preliminary evaluations of clonal seed orchards of Fraxinus excelsior (which is not the only susceptible Fraxinus species in europe) in Denmark and Sweden, and progeny trials in Denmark provided strong evidence for genetic variation in resistance against the disease. The potential may therefore be available for breeding resistant ash varietes, if the genetic background is understood. Currently, tests are ongoing aimed to check if the observed patterns suggest gene variation of qualitative or quantitative nature, and how the initially observed variation between F. excelsior clones and progenies evolve over time.

 

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