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Scientists believe that the arrival of just one more ash dieback spore could kill off Europe's remaining ash trees.
The dieback problem could have been caused by just one or two mushroom-like fruiting bodies of a fungal pathogen, according to a comprehensive genome sequence study published in Nature Ecology & Evolution.
Analysis suggests the spore came to Europe from Asia, where it grows on Asian ash trees without causing disease. But the pathogen has been devastating European varieties.
The study suggests that even the least susceptible ash trees could be at threat from the introduction of just one more spore from East Asia.
Dieback threatens 95% of all European ash trees and has already killed or severely damaged a quarter of the species in southern Sweden, and destroyed more than 80% of young ash trees in Norway.
A large team of scientists worked on the study, including senior author Matthew Clark, a research leader in plant interactions at the Museum.
The study's lead author, Mark McMullan from the Earlham Institute (EI), says, 'It's incredible that from such limited genetic diversity the ash dieback fungus has already devastated trees across Europe.
'Now that the disease is established, the introduction of genes from outside of Europe would tremendously increase the genetic diversity of the pathogen and seriously threaten the remaining ash trees.'
The disease is caused by a fungus called Hymenoscyphus fraxineus.
It was first spotted in Europe in Poland in 1992, where it probably arrived on commercially imported ash from East Asia.
It steadily moved west and was found in the UK in 2012, where spores may have landed from the continent. The fruiting bodies, each about the size of a match head, fire thousands of tiny spores that spread in the wind.
The disease causes dark brown or orange lesions on leaves, followed by wilting, lesions of dead cells on shoots and then diamond-shaped lesions on the stems. Finally, the crown of the tree dies back and the pathogen either kills the tree or weakens it to such an extent that it succumbs to other pests or pathogens.
In its native Asia, the fungus is a leaf pathogen and has little impact on its host tree. It is widespread and extremely diverse, but relatively harmless to Asian ash species.
However, in Europe it is killing trees at an alarming rate and is replacing native fungus.
The EI is a leading research institute focusing on the development of genomics and computational biology.
Researchers have sequenced the genome of the deadly fungus. They found that in samples from the UK, Norway, France, Poland and Austria, there is low genetic diversity.
The diversity of the fungus found across Europe is just an eighth of that found in a single Japanese woodland, so is likely to be a just a tiny fraction of that found across Asia.
Allowing some of that Asian genetic diversity to come into Europe, for example by bringing in fungi on wood, has the potential to increase the severity of the disease in Europe.
Matthew Clark says, 'The risk is that if the pathogenic fungus gets the chance to mate and reproduce with just one new individual, the resulting offspring could have the ability to kill the remaining ash trees that have survived the disease so far.'
Elizabeth Orton, a co-author on the paper from the John Innes Centre, says, 'Trees have such a long lifetime, and their populations take so long to recover from attacks by invasive pathogens, that it is vital to restrict the movement of potentially infected plants into and around Europe in accordance with the precautionary principle.'