Six years after the first case of Ash Dieback was identified in Britain the disease has spread to all parts of the UK as well as Ireland. It is estimated that we have 80 million ash trees in this country all of which could become infected. The disease is transmitted by microscopic fungal spores which can be blown for miles on the wind, making prevention an impossibility. Once infected, there is no cure and most ash trees die.

Back in 2012, when I saw the first symptoms of the disease in Norfolk ancient woodlands, the wilting leaves reminded me of a house plant that really needed watering. Dieback then progressed to twigs and branches and finally whole trees. Looking at those woodlands now I cannot see any ash trees that have escaped infection, with many dead and many more dying.

Honey fungus at NWT Lower Wood, Ashwellthorpe, photo by Martin Hionstridge

The rate at which infected trees die varies considerably, due to factors such as genetics and environmental conditions, taking anything from two to twenty years. Smaller trees tend to succumb quicker but I have seen many large trees, weak from fighting Ash Dieback, be rapidly killed by honey fungus.

It’s the same case across all of mainland Europe. The Ash Dieback Fungus, Hymenoscyphus fraxineus, harmless in its native home of Asia, appeared in Poland in the early 1990s and has since spread to wipe out up to 95% of European Ash trees.

The spread of Ash Dieback from Asia is thought to be a result of human activity. But have we learned our lesson? Global movements of plant products from seedlings to green timber and woodland waste, such as bark being used as packing material in shipping, continue to threaten our shores with exotic pests and diseases. The emerald ash borer, an Asian beetle, has decimated stands of Ash in North America after being accidently imported. This is one to watch out for as its range is thought to be expanding at a rate of 25 miles per year. And we must not forget the first modern cases of Dutch Elm Disease, in this country, were from timber bought over from France after the First World War.

Dead coppice, photo by Steve Collin

Ash trees have evolved to cope well with most soil types especially the heavy clay or peat soils associated with much of East Anglia and can make up as much as 40% of the woodland trees. There are also very many in hedges, farmland, parks and gardens. The impact of this disease is likely to be widespread, devastating and extremely costly. Woodland trees are often left to stand as deadwood, eventually to fall over and decay as a natural process. This is an important part of a woodland ecosystem and benefits bats, woodpeckers, invertebrates and fungi. However dead trees likely to cause damage or harm to people or property often need felling or even tree surgery to make them safe. This can be very expensive and increased disruptions, such as temporary road closures likely in the near future.

But the future of our ash trees is not altogether hopeless. Scientists believe there are certain genetic traits that give a few individual trees a tolerance or resistance to Ash Dieback. There is a lot of genetic variation in the UK ash population, thought to be linked with how the ash first colonised Britain after the last ice age. Due to this it is likely that some areas will have more disease tolerant trees than other areas, with East Anglia unfortunate in having fewer than the North. Much research in the laboratory, woods and in the tree nurseries has been aimed at identifying these resistant trees. The hope is that once identified and propagated, these trees can be planted in places where there is only low resistance. Ash trees are prolific seed producers and if seedlings inherit disease tolerance they will freely establish in woods and hedgerows to regain their place in our landscape once again.

Steve Collin, Reserves Manager for Woods and Heaths.
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