A virulent plant disease which has ravaged forests in north America has spread across Britain, raising fears that the country's forests are facing an ecological disaster.
Experts at the Forestry Commission fear that, if it catches hold, the fungus known as Sudden Oak Death, could cause a more serious environmental catastrophe than the Dutch Elm disease epidemic which killed off 30 million trees after it re-emerged in the 1960s.
The first British case of Sudden Oak Death, an organism closely related to the fungus that causes potato blight, was discovered in a shrub at a garden centre in April.
But the Forestry Commission revealed yesterday that since then, more than 70 cases have been found across the UK and the Channel Islands, in rhododendron and viburnum bushes being sold in nurseries.
Worried by the spate of cases, Forestry Commission experts have now ordered urgent laboratory tests on ash, lime, chestnut, beech and birch trees, as well as native oaks, to find out which are susceptible.
Two days ago, the European Commission also began preparations to ban all imports of potentially risky American trees and bark to the EU, and ordered health checks on garden nurseries across Europe. Most EU members have so far failed to test for the disease.
There is no known case of the fungus, Phytophthora ramorum, infecting a wild oak in Britain, but commission scientists were alarmed to discover that chestnut trees and native oaks were infected in their lab tests.
Roddie Burgess, head of plant health for the Forestry Commission in Edinburgh, said the tests and import bans were a necessary precaution because they did not yet know how dangerous the disease could be.
"It's a damn sight easier to do something before we get the disease rather than try and manage it once it's here. Once it does get in, we can't eradicate it. The probability is that if it does get in and gets established, it will do a Dutch Elm disease-type thing."
The north American strain of the fungus has struck forests in 12 areas of California and a town in Oregon, where every tree at risk of infection has been cut down and burnt. The fungus has killed 80 to 90 per cent of the oaks it has infected. It causes lesions and cracks in the bark, which ooze an unsightly tarry, black-red substance.
Mr Burgess said early tests suggested that native British oaks are more resistant to the fungus than American varieties but they still do not know if they are totally resistant. The disease is incurable if a tree becomes affected.
The fungus was first detected in Germany and the Netherlands in 1993 but was not recognised as a new disease until late last year. By then, it had spread across California, affecting more than a dozen types of tree and shrub, including several species of north American oak and cranberry.
Trevor Dynes, an expert on British plants from the conservation charity Plantlife, said the commission's findings were "horrific". Losing large numbers of Britain's 203 million oaks would severely damage other plants and animals which rely on the oak forests.
"Oak woodlands are probably one of the most important habitats we've got in Britain," he said. "If this got in, and started decimating them, we would be worried about all the other species which depend on them."