Gene study highlights threat of ash dieback
A disease that has devastated ash trees across Europe developed from just one or two sources of fungus on imported ash trees, a large-scale genetic study shows.
Analysis of the DNA of dieback fungus samples from the UK, Norway, France, Poland and Austria showed little diversity - showing that they probably all came from just two spores.
This limited mix of genes in the fungus across Europe would be expected to curb its impact – but instead ash dieback threatens 95% of all European ash trees.
The finding suggests that the arrival of a third fungal spore from overseas would be enough to intensify the disease and wipe out Europe’s remaining ash population.
In its native Asia, the fungus Hymenoscyphus fraxineus is widespread and extremely diverse, but relatively harmless to the native Asian ash species.
Dieback disease in European ash is caused by the same fungus, which has jumped to a new host.
The disease begins with dark brown or orange lesions on leaves. It either kills the tree directly or makes it vulnerable to other pests or pathogens.
The fungal bodies, each about the size of a match head, produce thousands of tiny spores that spread on the wind.
Ash dieback was first observed in European ash trees in Poland in 1992, where it probably arrived on commercially imported trees from East Asia.
It has already killed or severely damaged a quarter of ash trees in southern Sweden and destroyed more than four-fifths of young ash in Norway.
It was discovered in the UK in 2012 and is now found throughout the country.
Tree populations take a long time to recover from disease, so it is vital to restrict the movement of potentially infected plants into and around Europe, researchers say.
Movement of ash trees into and around the UK is currently prohibited.
European ash trees are under increasing threat from invasive pests such as ash dieback. Other pests and diseases, such as the Emerald ash borer beetle, have been found in Russia and could push the remaining European ash trees to the brink.
Ash borer beetle has already caused five ash tree species in the US to be threatened with extinction.
Researchers recommend creating seed orchards to breed ash trees that are less susceptible to dieback and other pests. Initial research in the UK and Denmark is enabling selection of trees for development.
The research was carried out by researchers at the Earlham Institute, the University’s genome facility Edinburgh Genomics, and other partners.
It was funded by the Biotechnology and Biological Sciences Research Council, Defra, the Economic and Social Research Council, the Forestry Commission, the Natural Environment Research Council, the Scottish Government and the French National Research Agency.
Ash dieback is already recognised as a serious threat to plant life, but our study of its genetic family tree underscores its potential to wipe out ash trees across Europe. Advanced genomic tools such as we used in this study must be applied to other invasive pathogens.