By Alex Kirby
BBC News Online environment correspondent
A disease which has destroyed many thousands of Californian oaks has been found for the first time in several well-loved British tree species.
The fungus strikes: Death is inevitable (Image: Forestry Commission)
It is a fungus called sudden oak death, and till now it had been found only in UK shrubs and a tree native to the US.
But the Forestry Commission says the disease has now struck beech, horse chestnuts and holm oaks in Cornwall.
There is no known cure for the disease, which kills the trees' bark and is thought likely to affect other species.
The fungus, known as Phytophthora ramorum, has killed 80% of one oak species in the western US.
It was discovered last year in viburnum plants in British garden centres, and there have been more than 300 subsequent outbreaks in plant nurseries, and in some wild rhododendrons.
Natives in danger
But the disease did not appear in a British tree until November, when it was identified in a southern red oak in Sussex - a tree imported from the US.
The news that it has now surfaced several hundred kilometres to the west, in Cornwall, is disturbing enough.
Worse, though, is the fact that its latest victims include a native British species, the beech, which is common across much of the UK.
One of the other infected species, the holm oak, was brought here from Europe, and the horse chestnut is also an introduced species.
That could mean disappointment for thousands of British children who look forward to a plentiful conker crop each year.
Tracking the infection
Ben Bradshaw, the Minister for Nature Conservation at the Department for Environment, Food and Rural Affairs (Defra), said: "At this stage we are unsure of the implications of this disease for our native trees.
"These findings are a matter of concern and we urgently need to assess... the best way forward to limit the disease's impact."
The Forestry Commission hopes by April next year to have completed a woodland survey to find out how far the fungus has spread.
Colin Morton of the commission told BBC News Online: "We know it spreads from shrubs like rhododendron and viburnum to trees, but we don't think it spreads from tree to tree.
Oaks are among the species at risk
"It seems to be spread by rain splashing off the shrubs' foliage onto the trees. Once on them, the fungus rings the bark all the way round, letting sap ooze out and cutting off the tree's lifeline so it will eventually die.
"There's no known treatment, but at the moment we're not killing healthy trees near infected ones to create a barrier. It may come to that, but we hope it won't.
"It seems that trees near rhododendrons and camellias are at greatest risk. We think we know the climatic conditions the fungus likes.
"We believe sweet chestnut trees are at high risk of infection, and others with a lower risk are sitka spruce, Douglas fir and yew."