British woodland owners and garden centres were warned yesterday that a deadly new fungus found last week in two oak trees in a Cornish wood possibly threatened large numbers of Britain's 200m oaks and other species of shrub and tree.
The fungus, too new to science to have a formal name, is closely related to Phytophthora ramorum, known in the US as "sudden oak death" because of the devastation it has caused to American oak trees in the past decade.
But the Forestry Commission said yesterday that the new fungus was only known in Britain and "far more aggressive and faster spreading than the US version". It is believed to be capable of killing trees in a few weeks, rather than just debilitating them.
"The latest discovery raises fears over the pathogen's potential impact on Britain's 200m oak trees, as well as other native trees species that may now prove to be susceptible", said a commission spokesman yesterday.
The fungus was found by scientists to have jumped from common rhododendron bushes to several tree species in an old wood near the village of Carharrack, Redruth.
It had killed all the rhododendrons and was found in 25 beech trees, an American tulip tree and, worryingly, in two native oaks. Two other oaks are now being closely monitored.
The fungus's host in Britain is known to be rhododendron ponticum, one of the most common shrubs in British woods. Its spores are spread in the air and need rainwater to bounce them onto the bark of trees. It then penetrates the inner bark and causes the tree to start bleeding, like an ulcer. After a serious infection the tree will come into leaf in the spring but die in June.
Local people were warned yesterday of the disease and those with rhododendrons were asked to prune them back away from trees, said a spokeswoman for the Department of Environment, Food and Rural Affairs (Defra).
"There is no parallel with foot and mouth. We are not going to shut the countryside down. Some footpaths may be closed while we sort out the rhododendrons, but there will be no 'keep out' signs", she said.
Professor Clive Brasier of the Forestry Commission's research agency, who discovered the pathogen, yesterday said that the new fungus belonged to the genus phytophthora, "one of the most dangerous organisms on earth."
He said it may have been in Britain for 5-10 years and was an entirely separate species from sudden oak death and ramorum, possibly evolving in Taiwan and the eastern Himalaya before being introduced here through the horticulture trade.
"I'm fairly sanguine about the ultimate prognosis if we can take rhododendron out of the equation", said Professor Brasier. "Although tests show the fungus is aggressive on beech, it is not that aggressive on oak. Some genetic individuals are more susceptible than others and most have some resistance", he said.
Apart from quarantining infected trees and shrubs and destroying the fungus where infections occur, the only way to control the problem was to control the movement of plants, he said.
"Because of the high volume of plants in the [horticultural] trade and the use of fungicide which masks the effects of these infections, it is difficult to spot infected plants.
"Ideally, less volume in the movement of plants around the world and limited regulation on certain plants to produce locally propagated plant material will control the spread of these fungi."
"We are asking all our members to be as vigilant as possible," said Alison Lee, a spokeswoman for the Horticultural Trades Association which rep resents all Britain's major garden centres.
"Obviously there is more concern, because this new pathogen actually kills the trees. If anything was found, the infected plants would be destroyed, as would others around it and others would be put in quarantine", she said.
The Forestry Commission yesterday appealed to anyone suspecting the presence of the disease on trees to contact its local offices. "Our hope was that phytophthora ramorum and this more virulent pathogen would not spread to native species. This new evidence indicates that this is not the case," said Roddie Burgess, head of plant health at the commission.