Media playback is unsupported on your device Media caption Environment Secretary Owen Paterson: "The scientific advice is that it won't be possible to eradicate this disease"

Environment Secretary Owen Paterson has acknowledged that ash dieback, a disease that threatens the UK's ash trees, will not be eradicated.

Announcing the government's action plan to tackle the disease, Mr Paterson said efforts would focus on slowing its spread through the countryside.

Diseased young trees would be removed and destroyed, he added.

But the action plan stated that there were no plans to remove mature trees, which are important for wildlife.

It added that efforts would also focus on developing resistance to the disease.

Image caption Mature ash trees will not be removed from the landscape, the government's action plan says

Conservation groups welcomed the government's assurance that no diseased mature tree would be cut down.

"The scientific advice is that it won't be possible to eradicate this disease now that we have discovered it in mature trees in Great Britain," Mr Paterson said.

"However, that does not necessarily mean the end of the British ash.

"If we can slow its spread and minimise its impact, we will gain time to find those trees with genetic resistance to the disease and to restructure our woodlands to make them more resilient."

He told reporters that the arrival of ash dieback meant his department would have to reconsider its priorities.

"If we are going to really do something radical on the way we handle our forestry in the future and change the priorities, we are going to have to shift resources within Defra.

Regulation must be put in place to ensure this does not happen again Martin Harper, RSPB

"There will be some things we do in Defra now that we are going to have to stop doing."

Speaking at a briefing, Defra chief scientific adviser Prof Ian Boyd said: "By next season, we could potentially have resistant forms of ash growing in this country.

"We need to put in scientific research to try and get genetic markers for resistance so that we can go out into our current woodlands that have not been infected and identify trees that might survive relative to those who might not survive."

Survey results on Wednesday confirmed that the disease was present at 115 sites in England, Wales and Scotland.

The action plan outlined a number of areas where government efforts would be focused. These included:

reduction of the rate of spread of the disease,

development of resistance to the disease in the native UK ash tree population,

encourage the public and landowners to help monitor trees for signs of ash dieback.

It explained that newly-planted diseased trees and diseased trees in nurseries would be traced and destroyed because once young trees become infected, they succumb to the pathogen.

Ecology of ash trees Referred to as the common or European ash, the tree - the UK's only native ash species - is scientifically known as Fraxinus excelsior. It is the UK's third most abundant species of broadleaved tree (after oak and birch), covering 129,000 hectares of woodland. Ash is deemed to be a very important species within the UK's hedgerows and accounts for about 10% of the nation's estimated 123 million "non-woodland" trees. The common ash is a large deciduous tree that can reach heights in excess of 40 metres (130ft). It can live up to 400 years but coppiced trees can live for centuries longer. The species is wind-pollinated, and the seeds (known as keys) are primarily dispersed by the wind. As the species is long-lived, it is important for wildlife. It supports specialist deadwood species such as the lesser stag beetle and hole-nesting birds including owls and woodpeckers. Ash woodlands have light, open canopies, so are an important habitat for flora such as bluebells and ramsons. Birds such as bullfinches feed on the trees' seeds. Also, ash provides an important habitat for more than a quarter of Britain's lichen, including nearly 14% of the nationally rare/scarce species. Upland mixed ash woodlands are listed as a priority habitat under the UK Biodiversity Action Plan. Farming Today: Ash dieback

However, there was no intention to fell mature trees because as well as being important to wildlife, they take longer to die and could provide vital clues for scientists looking for genetic strains that might be resistant to the disease.

The latest data showed that infection, caused by the fungus Chalara fraxinea, did not occur directly from tree to tree, only via leaf litter.

Over the coming weeks, the government said it would work with scientists and other "interested groups to further develop and implement the measures in the plan, and to set a longer term approach to tackling Chalara".

Aerial assault

A ban on ash imports and the movement of trees from areas with confirmed cases of dieback came into force on 29 October, just days after government scientists confirmed that the disease had reached the UK's natural environment - at sites in East Anglia.

Dieback, caused by the Chalara fraxinea fungus, was first recorded in eastern Europe in 1992 and spread over two decades to infect most of the continent.

The first confirmed cases in UK woodlands were at sites in Norfolk and Suffolk. Scientists said the infections were most likely to have been the result of spores from the fungus being blown across from mainland Europe.

Prof Boyd explained that computer modelling showed that there were days when meteorological conditions existed that would have seen a "plume of spores across the UK".

RSPB conservation director Martin Harper welcomed the publication of the government's action plan, saying it was a "vital part of stopping the spread of this disease".

Symptoms of Chalara dieback Diseased saplings typically display dead tops and side shoots.

Lesions often found at base of dead side shoots.

Lesions on branch or stem can cause wilting of foliage above.

Disease affects mature trees by killing off new growth. How to spot the signs

"Upland ash woodland and ash in the UK's lowland woods and hedges are important habitats for a wide range of plants and animals," he said.

"We are pleased that protecting this wildlife has been enshrined in the Government's approach to tackling ash dieback."

But he added that the arrival of Chalara Fraxinea in the UK should act as a "wake-up call".

"It is essential that we change the way we move animals and plants around the country and across international borders," he warned.

"Regulation must be put in place to ensure this does not happen again - our natural environment is too precious."

Woodland Trust chief executive Sue Holden echoed the RSPB's "wake-up call" warning, but added that she supported the view that no mature ash trees should be felled.

"This underlines widespread concerns that rapid and ill-considered action in our mature and ancient woods could do more harm than good," she said.

"We do not want to remove small populations of resistant ash that may hold the key to the survival of the species across the country."

The most visible sign that a tree is infected with Chalara ash dieback is bleeding sores and cankers on the bark, and discolouration of the underlying sapwood.

The sores often surround branches in the infected area of the tree, causing the dieback of shoots, twigs, branches and smaller stems.

The disease has also been shown to infect ash tree leaves, appearing as blemishes.

There are numerous other diseases that display similar symptoms, making it difficult to identify for most people.

The Forestry Commission has issued guidance on how to identify the disease.

How ash dieback could threaten Britain's wildlife