Under threat: Britain's horse chestnut trees
Blight 'as bad as Dutch elm disease' is ravaging Britain's horse chestnut trees
Thursday 24 August 2006
Britain's horse chestnut trees, providers of conkers for generations of schoolboys, are dying in their thousands in the worst case of tree blight since Dutch elm disease 30 years ago.
The horse chestnuts, which often stand in majestic rows in city streets, are being hit by a "triple whammy" of drought, pest attack and disease. On many, the leaves have already withered and shrunk, and conkers, the fruits of the tree, are not being produced at all.
Stands of horse chestnuts in the streets around the Royal Botanic Gardens in Kew, west London have no conkers this year. This time last year they had a carpet of conkers underneath them - as they have had for decades. For many boys, looking forward to the age-old game, 2006 will be the Autumn Of No Conkers - but the situation is far more serious than that.
The trees are being severely hit in many parts of Britain and according to the Forestry Commission between 40,000 and 50,000 of them may already be affected - about 10 per cent of all the horse chestnuts in Britain - and that figure may be even higher.
Since branches tend to drop off the weakened trees, thousands of them on the fringes of urban streets may have to be cut down for safety reasons, a situation likely to cause traffic chaos and play havoc with council budgets - as well as causing great sadness to people who for years have enjoyed their summer shade, and their beautiful "Roman candle" flowers in the spring. "I think you could compare this to Dutch elm disease," said Tony Kirkham, the head of the arboretum at the Royal Botanic Gardens. "The last thing we would want is another epidemic that wiped out a common British tree species."
The trees have already been stressed by three winters of drought. But now in their weakened state they are suffering from simultaneous attack by a pest, the leaf miner moth, whose larvae eats the leaves, and a disease known as bleeding canker.
This causes a dark liquid to ooze from spots on the trunk of a tree, which can quickly develop into large damaged patches, spreading all the way around the branch or trunk until limbs fall off, or the tree falls over.
Chris Howkins, a botanist and author of a recent book on the horse chestnut, said the public had not yet woken up to the extent of the disease.
"At the weekend, here in Runnymede in Surrey, we drove round looking, and we couldn't find a horse chestnut that wasn't infected, and there are thousands of them," he said.
"They've all gone brown, and they show up in the green landscape. All the roadside plantings, and what we think of as screening trees, are going to go. And we've adopted this tree to our hearts in England, and used it extensively for commemorative plantings, and that is going to go too. It's spreading like wildfire. I was speaking to a botanist in Norwich a couple of weeks ago and she said all the Norwich ones are going."
Mr Howkins added: "It's tragic. People don't realise the scale of it yet. They see these trees turning brown and they just think autumn is coming early. A lot of people won't have realised that this is actually death."
According to the Forestry Commission, of those trees which are seriously affected, having already developed large open cankers, about around half will die.
But the threat is exacerbated by the fact that many of the nation's 400,000 to 500,000 conker trees are planted in public places. "Even if it doesn't kill the tree, it can make it dangerous and require that the tree is cut down," said David Rose, a scientist from Forest Research, the Forestry Commission's research arm. "If you look at the number of cases and the likelihood of tree death, certainly it's a significant tree disease."
What is making the problem worse is that the majority of new cases of bleeding canker are not related to the fungus that was originally thought to be the cause of the problem. "We and the Dutch have isolated a bacterium which seems to be suddenly causing the damage," Mr Rose said. "We have our prime suspect, we now have to do our own version of an identity parade. We hope to know by the end of the year whether we've identified the causal agent."
If it is eventually proven that the bacterium is the cause, there will then be a long process of finding an effective treatment and then getting it approved for use.
Again, this will be made more difficult because so many horse chestnuts stand in parks or on residential streets, meaning the use of potentially dangerous chemicals is problematic. The disease appears to be having its strongest effect in southern England. Organisers of the World Conker Championships, which take place on Sunday 8 October in Ashton, Northamptonshire, are watching the situation closely.
"At the moment we're all right, because we haven't had the moth up here, or the canker, as far as I'm aware," said John Hadman, the secretary of the Ashton Conker Club, the championship organisers. "But if we do go short of conkers, we will have to get them in from elsewhere."
Dutch Elm Disease
The UK is still blighted by this devastating disease caused by two related species of fungi in the genus Ophiostoma, which are disseminated by various elm-bark beetles. They block the tree's water-conducting vessels so that leaves wither on the upper branches. It was first recorded in the UK in the 1920s and by the 1940s had caused the loss of 10 to 40 per cent of elms in Europe. There was a major outbreak in the late 1960s and within a decade about 20 million elms out of an estimated UK population of 30 million were dead. In the mid-1990s many young trees were infected. All our four main native elms are susceptible.
Sudden Oak Death (Phytophthora ramorum)
This fungal disease, first seen in the UK in 2002, is most commonly found in potted shrubs such as rhododendrons, viburnums, lilac and camellia. Hundreds of garden centres have been affected and many were forced to destroy their stocks when the outbreak was at its greatest. The disease is named after damage done to Californian oaks. It has appeared in only a few trees so far, including American oaks and European holm oaks. The most vulnerable native tree is thought to be the beech, and trees on three sites in southern England have been infected. British oaks seem resistant.
Gypsy Moth (Lymantria dispar)
A serious pest of trees and shrubs, their caterpillars are active from April to July and feed on a wide range of deciduous and coniferous trees but prefer the leaves of oak (Quercus) and poplar (Populus). They grow up to three inches long and are hairy with coloured "warts" on the back. The UK does not have a resident population of the moth but the south is often visited by migrants. There was an outbreak in London in the late-1990s, when forests in north-west London were affected, probably by eggs transported from continental Europe.
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