It is the greatest threat to Britain's trees since Dutch elm disease 40 years ago, and a major campaign will be launched today to stamp out the killer fungus phytophthora.
Two forms of the disease, from the same family as the mould which caused the Irish potato famine, now affect more than 700 sites across Britain. Plant health experts consider it a serious danger to woodlands and heathlands, especially to heritage gardens of the sort maintained by the National Trust.
So today the Government will announce a five-year, £25m programme to manage and contain the risks of both pathogens, Phytophthora ramorum, which in the US has been responsible for the phenomenon known as "sudden oak death", and Phytophthora kernoviae, which is regarded as a considerable danger to beeches as well as other trees.
Until recently, the main host of phytophthora in Britain has been the wild Rhododendron ponticum, the purple-flowering shrub often found in woodlands and country estates. Once established, the phytophthora fungus sends out spores, which is how it spreads. On other trees and plants it forms lesions on the stems and trunks, which eventually can kill them.
However, last year phythophthora was found to have developed a new host, infecting wild bilberry plants on heaths both in Cornwall and Scotland, where it has spread rapidly and killed off infected plants completely.
In December, the National Trust, which has seen 19 of its historic gardens affected by phytopthora, wrote to the Environment Secretary, Hilary Benn, warning that "the threat is now very real to native plant life in the wider countryside" and that the impact could be "devastating".
Mr Benn's response, after a Government consultation exercise, is the campaign to be announced today, and the £25m funding – in the middle of a recession – is an indication of how seriously the Government takes the threat.
The campaign is to be launched by the Environment Minister, Jane Kennedy, at one of the National Trust's historical gardens, Nymans near Haywards Heath in Sussex (so far phytophthora-free). "These lethal diseases are having a detrimental effect in pristine locations, which in turn could have a detrimental effect on our local tourism industry, and our own lifestyles," Ms Kennedy said yesterday.
A typical National Trust property which has been badly affected is Arlington Court near Barnstaple in Devon, where large areas of rhododendrons have had to be cleared. "It's a very serious threat, and tackling it is an expensive operation," said the Trust's gardens adviser for Devon and Cornwall, Ian Wright.
Roddy Burgess, head of plant health for the Forestry Commission, said phytophthora was "the most serious threat to plant communities in Britain since Dutch elm disease", adding: "We are very glad that this action is being taken."
Fungus the bogeymen
Phytophthoras make up a genus of hundreds of separate but related fungus-like plant-damaging organisms (the name, from the Greek, means "plant- destroyer"). One of them has already had devastating human, cultural and environmental consequences in the British Isles: Phytophthora infestans was the infective agent of the potato blight that caused the Great Famine in Ireland between 1845 and 1849. In successive years, the blight destroyed most of the potato crop, on which a large part of the population depended for sustenance. As many as a million people died, while perhaps another million emigrated, mainly to America. The population of Ireland was reduced by a quarter, and the famine, and the lasting bitterness over the laggardly response of Ireland's then British rulers, became the defining event in Irish history and the development of the new Irish national consciousness.