A mysterious and deadly tree disease has spread across southern England, raising fears that Britain's woodlands are facing the biggest threat since Dutch elm disease changed the landscape 30 years ago.
Government scientists confirmed yesterday that sudden oak death, a fungal infection that has killed 80 per cent of American oaks in California, had been found in trees at two sites in Cornwall.
The first tree in Britain to be stricken by the disease - a 100-year-old southern red oak, a species imported from North America - was discovered last month at a private estate in Sussex.
Stephen Hunter, head of plant health at the Department for Environment, Food and Rural Affairs, said the disease had struck three species of tree at the two Cornish sites - beech, horse chestnut and holm oak.
Little is known about sudden oak death. The disease is caused by a microbe called Phytophthora ramorum, which was isolated and named in June 2000 by scientists at the University of California. Mr Hunter said surveillance was being stepped up at garden centres, nurseries and ports through which exotic plants are imported. Rhododendrons, camellias and viburnum plants are particularly susceptible, Mr Hunter said.
Since April 2002, when sudden oak death was identified for the first time in Britain, the disease has been reported in shrubs and rhododendrons at about 330 sites in the country. Most of the sites are garden centres and nurseries, but about 35 are established gardens such as the Royal Horticultural Society's Wisley estate in Surrey, Mr Hunter said.
The greatest fear is that the infection will become endemic in cultivated and wild woodlands, where it could be passed on to established native trees such as the English white oak.
"It's obviously of considerable concern that we've found this disease in mature trees. Our knowledge of it is fairly limited and we cannot say what will be the full impact," Mr Hunter said. "We're still on a rather steep learning curve in terms of understanding the disease's biology and of how it will spread."
Symptoms of the infection include swelling or cankers on tree trunks that ooze a sticky brown sap. The disease appears to be spread by water droplets, but its spores can also be carried in mud that sticks to people's boots, Mr Hunter said.
Horticulturists in the Netherlands and Gewrmany first reported the disease in rhododendrons in 1993. It later turned up in California, where it has attacked native tanoaks, coast live oaks and black oaks, killing eight out of 10 trees.
"Probably this disease did come into Britain on some material from elsewhere, we don't know where from," Mr Hunter said. "We are proposing to increase checks at ports." Laboratory tests have shown that beech, sweet chestnuts, sitka spruce and Douglas fir are the native species most susceptible to infection, although it is not possible to rule out risks to other seemingly less-susceptible species, such as the English oak, he said.
Infected rhododendrons are being either uprooted and destroyed or pruned back. But the infected trees will be left standing so that scientists can study how the disease progresses. "Fungicides do not destroy this disease, but they do suppress it. It's a very uncertain threat and that's the problem. We're working in an area of great uncertainty," Mr Hunter said.
Roger Herbert, of the Forestry Commission, said scores of tree specialists will be inspecting trees throughout Britain between now and April.
Mr Herbert said: "This is the most effort we've put into one particular pest at any one time for quite a number of years."