Ministers are considering restricting imports of plants from America to protect Britain’s oaks from a disease that could depopulate ancient woodlands.

“Sudden oak death” is a fungus-like disease initially brought in by plants imported from Asia. It can kill oaks within a few weeks of infection.

Concerns are now being raised about imports from America where a second form of the disease exists, for fear the two could interbreed. The disease also attacks ash, beech and other species including bilberry and Scottish heather.

Outbreaks of the disease, also called phytophthora ramorum, have now been found at 625 sites in England and Wales and many others in Scotland, Northern Ireland and the Channel Islands. Oaks account for 10% of all woodland trees.

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Background

• Order after the storm

• Plan to thwart foreign threat to British oaks

The infestation starts with brown and black blotching on the leaves and twigs and then develops into oozing cankers on the trunk, leading to rotting and death. Experts fear it is spreading so fast that it could have the same impact as Dutch elm disease, which killed 25m elms from the start of the outbreak in the late 1960s to the 1990s.

This week Lord Rooker, the environment minister, will launch a report and consultation on what to do about the disease. He will also release a scientific study from the Royal Horticultural Society (RHS), which will warn that the number of disease outbreaks on all types of plant is up 60% on last year.

It says this is linked with the surge in imports of exotic foreign plants. British consumers spent £870m on them in 2005.

The RHS report adds: “Alien pests and diseases, inadvertently imported on exotic plants, are threatening the plants in our gardens and across the countryside. Increased global plant trade, coupled with evidence of rapid climate change, suggests that the problem will only get worse.”

Rooker wants plant health specialists, nursery owners and expert gardeners to submit ideas on monitoring imports and spotting outbreaks.

One option to deal with sudden oak death is to extend the existing “plant passport” system controlled by the European Union, which restricts the import of vulnerable species, especially rhododendrons, from infected parts of America.

Simon Thornton-Wood, the director of science and learning at the RHS, said: “This disease is hard to control. It has killed a lot of trees in America and we need to take it very seriously.”

Heart of Britain

— Oaks have grown in the UK since the last ice age. There are two native species: English and Sessile oaks

— The country’s oldest oak tree, at Bowthorpe Park farm in Lincolnshire, is thought to date back more than 1,000 years

— A single tree can contain up to 350 insect species and up to 30 species of lichen

— About 2,000 oaks were used to build HMS Victory, Nelson’s flagship

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