A lethal killer is on the loose in our woods

Forty years after Dutch elm disease wreaked havoc on the countryside, Britain's trees are again under attack. The Forestry Commission has highlighted 15 pests and diseases - ranging from the Asian longhorn beetle and pine tree lappet moth to chestnut blight and Dothistroma needle blight - that are infesting our woodland.

Acute oak decline, which can kill within four or five years, is reported to be on the rise. And the latest high-profile invader, ash dieback (caused by the fungus Chalara fraxinea), now affects 17 counties across the nation; last month, thousands of trees had to be destroyed in Devon alone.

As if all this wasn't bad enough, an even more lethal scourge is on the loose. It is killing larch, sycamore, beech, bay laurel and sweet and horse chestnut trees in Britain, and has already destroyed millions of trees in the United States.

The cause of destruction is the fungus-like Phytophthora ramorum, commonly known as sudden oak death. Its name is misleading on every count. The disease does not always result in death. Nor are the effects sudden - although it can kill trees within a single season. But most importantly, it is not restricted to oak.

That does not mean, however, that this disease is anything less than a menace. In the past four years, it has claimed hundreds of thousands of acres of commercially grown Japanese larch in South Wales, Devon and Cornwall.

"The landscape," says Matt Elliot of the government agency Forest Research, "has been changed for ever in certain parts of the country." Ramorum blight, as scientists are renaming sudden oak death, can infect and colonise around 150 different species of shrubs and trees, producing a significant knock on effect on insects, birds and other wildlife.

Since the pathogen - which probably comes from Asia - appeared in Scotland in 2002, it has been the subject of emergency EU measures, regulatory action and, in England and Wales, containment and eradication policies. But how to control such diseases?

In the case of ash dieback, British scientists have succeeded in sequencing the genome of a disease-resistant type of ash tree - but it could be years before it results in the creation of a resilient strain of ash to replace those that have died.

In the case of sudden oak death, however, experts are thinking laterally and turning to an invasive species whose exotic blooms win gold medals at the Chelsea Flower Show: the rhododendron.

First introduced from Asia by the Victorians, the invasive Rhododendron ponticum is extremely good at spreading Ramorum blight. So Scottish scientists are hoping a recently published rhododendron map will help control its spread.

"We've made maps of where the rhododendron likes," says Dr Bethan Purse from the Centre for Ecology and Hydrology in Penicuik. "We looked at how temperature and moisture conditions vary over time in each square kilometre of Scotland on a daily basis, and made a map of where the suitable climate conditions are best for Phytophthora infections."

The map shows that more than half of Scotland's native woodland has good conditions for rhododendrons, and would therefore have a higher risk of spreading the disease. In woodlands around 10 miles outside Edinburgh, I am taken to an infected rhododendron bush tagged for removal. Its leaves are either brown, withered and drooping, or tipped black - symptoms of Ramorum blight.

"It has spores that can enter the leaves and stems of plants via lenticels - small holes in leaves that plants breathe through," says Elliot. "The plant dies because it can no longer take in the water and nutrients that it needs. It's a devastating pathogen."

Nearby, several stumps remain from previously infected bushes, treated with herbicides to prevent regrowth. One infected plant stands within 10 metres of ash, beech, larch and cherry trees, as well as being immediately below a sycamore tree. It's easy to see how spores could blow from one species to another as their branches intertwine.

"Infection is by direct contact and rain splash," says Purse. "Also, Phytophthora can live in the soil for a long time and was recently found to infect the roots of rhododendrons as well, so there are all these ways in which these pathogens can be maintained on a site, which makes them really difficult to control."

Rhododendrons can grow equally well in open habitats (as witnessed on the slate hills of North Wales) or under partial shade. Despite their beauty, they have a Marmite effect on conservationists. "I'd love to get rid of every single rhododendron bush," says Colin Edwards of Forestry Commission Scotland. "It's been my desire for the past 20 years."

Edwards is a rhododendron expert, who first came across Ramorum blight in Cornwall in the mid-Nineties. He knows this area of Scottish woodland well and has followed the disease's progression in this particular plant. "It's gone from full health, to where only about 10 per cent of the leaves are alive in a year," he says.

"So it's a very rapid progression from healthy to unhealthy. Hopefully it will be removed before it transfers its spores on to something else." The problem is that rhododendrons are being infected by Ramorum blight but not being killed.

"Unlike when it gets on to trees and kills the host, with rhododendrons it infects the leaves," says Edwards. "They then produce spores that could go on to infect more rhododendron bushes or other species."

The Natural Environment Research Council and the Scottish Government, which funded the research, are now extending it to larch plantations and heathland, making maps of susceptible species and climate suitability. But it's not just Scotland, or indeed rhododendrons. Four years ago in Staffordshire, Ramorum blight was found in the heathland plant bilberry.

Defra also set up a five-year Phytophthora disease-management programme, a review of whose progress was published this month. It concluded that the programme's "ambitious, but achievable" aim of reducing the spread of the disease to epidemiologically insignificant levels "will not now be met within the time scale".

Larch infections, it found, "remain the most significant issue affecting risk to other plants". The external review team recommended that it was important to consider the future impact of Japanese larch and rhododendron infections on the nursery trade.

"We don't have sufficient funds to tackle every single rhododendron bush," says Edwards, "so this mapping will help prioritise areas where we concentrate effort to remove the rhododendron, reducing the risk of spread into commercial species or into the native flora that's on the sites.

"To put it into perspective, we did a survey on Argyll and Bute, where we identified 4,500 hectares [11,119 acres] of rhododendrons. The clearance cost for that would be pounds 12 million. Multiply that across the whole of Scotland and you're looking at somewhere around pounds 70-pounds 90 million - if the money was available, if the resources were there, and if every eradication technique worked first time.

That's just completely unrealistic." The experts are, of course, doing their best. The review praised the science involved in the programme as "excellent", and supported the use of the rhododendron modelling to inform decision-making.

"There has been a lot of joined-up science here," says Prof Chris Gilligan of the University of Cambridge, who worked with Rothamsted Research Institute to produce a government-funded report that modelled the control, spread and infection of Ramorum blight in England and Wales last year.

Still, the task they face is weighty. Dutch elm disease killed 20 million trees in Britain within 10 years. Ash dieback, acute oak decline and sudden oak death are also permanently reshaping our countryside. Nature adapts and changes all the time - but if we are to preserve our trees, then scientists must be given the funding to fight these diseases on the front line. If not, then Britain's green and pleasant land could end up without its backbone.