Death is inevitable if the fungus strikes (Image: Forestry Commission)

A further six trees in Cornwall were confirmed on Monday as having sudden oak death which is causing experts concern.

They are in addition to three trees in the county and one in Sussex already infected with the disease said the Department for Environment, Food and Rural Affairs (Defra) and the Forestry Commission.

In another development, officials revealed that rhododendrons and another tree at a site in Cornwall had been infected with a previously unknown strain of the phytophthora fungus.

How it gets into a particular site is very difficult to say Dr David Slawson, Defra

The fungus, known as phytophthora ramorum, has killed 80% of one oak species in the western US.

There is no known cure for the disease, which affects the bark.

It was first confirmed in the UK in April 2002, but until November last year it was mostly confined to rhododendrons in nurseries and garden centres.

It is thought that at each of the three disease sites in Cornwall - two very close together - the infection has spread from nearby rhododendrons.

Experts are still no closer to finding out how the disease infected the rhododendrons.

The new trees infected are three holm oaks, a turkey oak, a beech and a sweet chestnut.

Dr David Slawson, principal plant health and seeds inspector at Defra, said they were no closer to finding out how phytophthora ramorum spread.

"Ramorum was first detected in Europe, in Germany and the Netherlands, in 1993 so it has been around for about 10 years," he said.

"How it gets into a particular site is very difficult to say unless we find it very soon after its arrival."