Thursday, 1 December 2016

Chalara Ash Dieback Reaches The New Forest

At tonight's Consultative Panel, the Deputy Surveyor announced the first laboratory confirmed case of ash dieback within the New Forest National Park.  This was discovered in trees near Picket Post.

Chalara Ash Dieback is a disease caused by fungal infestation of Hymenoscyphus fraxineus.  This fungus originated in Asia, where it is benign to the native Ash species.  The disease was first identified in Europe as Chalara Fraxinea in Poland in 1992. It is devastating to European species of Ash, and is now firmly entrenched across mainland Europe.  2012 saw the first confirmed cases in the UK in a Buckinghamshire nursery in imported plants from the Netherlands.   East Anglia, Kent and Essex have had the highest concentration of cases so far, but the outbreak is spreading to the west, with cases in the wild in Wales, and past the Forest to Cranbourne Chase and further west in southern England.

The fungus produces tiny fruiting bodies on the leaf stalks of infected trees.  By the following summer these produce spores which spread to other trees via their leaves.  A slightly different form of the fungus then migrates into the branches and trunk where its mycellium interrupts the flow of water and nutrients, slowly starving the tree.

Little can be done about it, there is no treatment.  It kills small trees very quickly.  Mature trees may be severely weakened, then killed by secondary pathogens.  Some survive indefinitely in a weakened state, and there may be various degrees of resistance in these, although they remain infected carriers. The only active practical measure that may be taken, as the spores are spread in the leaf litter of infected trees, is basic biosecurity, clean your boots off between walks in different woodlands, limiting transport of, or treating wood harvested from infected trees, etc.

Small comfort, but the Forest landscape will be less impacted than much of the countryside, as Ash is less common on acid forest soils, typically present here in wet/riverine woodlands.  That does not reduce its threat to the overall biodiversity of the country, nor the potential impact on the forest's habitat assemblies that include Ash.

One resistant tree has been identified in the UK, and several on the continent, which may support future propagation and DNA fingerprint tests for other resistant trees.  Panel Chair and botanist Clive Chatters observed that this is not as bad as Dutch Elm disease. That outbreak was exacerbated by the lack of genetic diversity in Elm (once intensively nursery produced), whereas in Ash in the wild "there is a vast amount of diversity".  This diversity is important as the likelihood of extant resistant plants is increased. While the vector for the disease is in the leaves, on a typical Ash plantation it would be a nonsense to hoover them up, Clive noted that "in our wood pastures, where the Commoners turn out their stock, the stock hoover up all those leaves, particularly in the wet woodlands where they get in there this time of year, they're absolutely hoovering up that fallen green. And I think the forest will be very interesting to monitor as a model for how things may cope in the future."

Much more information about Chalara Ash Dieback, including how to report possible sightings, is available from this Forestry Commission page:
A 2012 Episode of the BBC Radio 4 Programme The Long View contrasts Dutch Elm Disease and Ash Dieback .  And their programme from nature writer Richard Mabey, Mabey in the Wild of 3rd July 2013, featured a discussion of New Forest trees including Elm, Holly and Beech with Clive Chatters.