BRITAIN’S horse chestnut trees are falling victim to an aggressive disease that has spread throughout the country, striking more than 40,000 last year.

Ancient avenues of trees have been devastated. In parks and historic gardens, groundsmen have seen dark sores where trees have shed their bark and bled a glutinous resin. The tissue beneath dies, and if the sores form a ring around the trunk the tree withers.

Tree pathologists had assumed it to be the work of a species of Phytophthora fungus related to a disease commonly called sudden oak death. “Now we realise it’s something different,” said Professor Clive Brasier, of Forest Research, part of the Forestry Commission. “We don’t know what it is. It’s more aggressive and it’s being found all over the country.”

Chris Prior, head of horticultural sciences at the Royal Horticultural Society, first came across the disease on a lunchtime break last summer. “I was walking through the arboretum at Wisley Gardens in Surrey,” he said. “On one of the chestnut trees a branch of leaves had turned yellow. I looked at the trunk below and saw the dark sore. We took samples but we couldn’t find the fungus that usually causes this disease. Fortunately only two trees had been affected.”

While British tree pathologists received news of fresh outbreaks, Dutch scientists were also trying to isolate the cause of the disease and applying to their Government for emergency funding. “The trouble is it’s not a tree of commercial importance so it’s hard to drum up funding,” Mr Prior said. “The British Forestry Commission would probably like to but it’s already occupied with sudden oak death. The Environment Department has been working to eradicate Phytophthora fungus but this is something else entirely. English Heritage have estimated that 40,000 trees were affected last year, but that’s just on their sites. We don’t know how many more trees have got it.”

Alan Cathersides, a senior landscape manager for English Heritage, has been receiving similar reports from managers of historic sites across Britain. “I’ve had reports from Audley End in Essex, and at Hailes Abbey,” Mr Cathersides said. “With the other disease, trees would often recover, with this trees get struck down far more quickly. There will be a hint of yellow in the leaves one summer, the whole tree will be dead the next.”

Several colossal trees have been killed outright at Marble Hill Park in Twickenham, southwest London, raising fears for Bushey Hill Park two miles away. There, four colonnades of horse chestnut trees range above the Princess Diana Memorial Fountain. Each summer on Horse Chestnut Sunday, their flowers attract large crowds; each autumn the conker fighters arrive to harvest their crop.

Joan Webber, a tree pathologist for Forest Research, has led efforts to identify the new disease. “Like our Dutch colleagues, we feel this is a new problem altogether,” she said.

She recalls a recent trip to the ancient stone circle of Avebury in the West Country. The colonnade of trees leading to the monument had been hit and half had succumbed.

“The Phytopthora fungus was limited to London and the South East,” she said. “This is occurring across England and Wales, even in Scotland, where previously cases were unheard of.”

She could not compare it with Dutch elm disease. “We just don’t know enough about it yet,” she said.

OTHER KILLER IN THE WOODS

• Dutch elm disease is caused by two related species of fungi, which are spread by elm bark beetles
  
  
• They block the tree’s water-conducting vessels so that leaves wither on the upper branches. Starved of nutrients, the roots also perish
  
  
• Research into the disease began in 1919, led by a group of Dutch women scientists
  
  
• In the late 1960s an aggressive form of the disease arrived in Britain and wiped out 20 million of the country’s 30 million elms in a decade

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