BBC Radio 4 ‘Ash To Ashes’ with Adam Hart, Sept 27 2013. An interesting programme on the threat to our native Ash (Fraxinus excelsior) from Ash Die-Back. The evidence looks rather grim…
http://www.bbc.co.uk/programmes/b03bg4vh
Ash Die-Back, was first found on wild ash, in Norfolk in October 2012. There are now 550 sites containing diseased trees in the UK, mainly on the eastern side of the country. Depending a trees size they are likely. The fungus responsible – Chalara fraxinea originally came from Japan and has spread across Europe at a rate of some 20kms per year. 90% of ash has been killed in Denmark for example.
The threads of the fungus – the mycelium, commences to grow after the arrival of a spore carried on the wind. The mycelium infects a leaf, then the stalk and then into the woody tissue of the tree. With the autumn leaf-fall, infected leaves fall to the ground where fungi are produced which in turn shoot more spores into the air and so multiply the infection on the host tree and its ash neighbours. Death of mature trees is a long process; taking into account effects on the European mainland, death will not occur until some 10-25 years of becoming infected. Trees try to fight the disease by growing more leaves but no woody stem tissue. Slowly, the tree starves allowing in the process, other pathogens such as honey fungus to attack the ailing tree.
Scientists from across Europe are collaborating to find answers to this devastating disease. This is difficult because ash has a complicated sex life, with male, female, hermaphrodite trees and with all stages in between! Work has been carried out on a self tree, one that has arisen from self-pollination, as the genome sequencing is far easier. At a plantation planted 19 years ago in Sweden to promote the production of ash trees with good growth characteristics; here, these are now being studied as which show any resistance to ash die-back.
However, the outlook for Europeís ash looks grimmer than the above facts. Step forward the Emerald Ash Borer. This ash killer is spreading west across Europe and at the present time is 250kms west of Moscow. In North America, where it arrived in 2002 in the Detroit area on dunnage or packing wood from China, it has now killed 99% of the native white and green ashes (and specimens of European ash). After females have laid eggs on the tree’s bark, large numbers of the larvae bore into the conducting tissue beneath, killing it and thus girdling the tree and leading to its death. In the Far-East, the Manchurian ash has co-evolved over thousands of years with both these pests, they having little effect on it. Scientists are now looking into a long-term solution involving creating a hybrid between this and the European ash, to which it looks very similar.
As with other tree diseases such as Dutch Elm Disease, these diseases are being spread outside of their geographic areas facilitated by the speedy transportation networks of globalisation.